Patent Publication Number: US-2022233882-A1

Title: Radiation shields for brachytherapy

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority on U.S. Patent Application No. 62/861,052 filed Jun. 13, 2019, the entire content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosure relates generally to apparatus and methods for radiation therapy, more particularly, to brachytherapy. 
     BACKGROUND 
     Radiation therapy is known to be an effective treatment for cancer. New technology developments in external beam radiation therapy during the last decades have led to improvements in tailoring the dose distribution to the shape of the tumour and minimizing the dose to organs at risk (OAR). However, location of the tumour within the organ, errors in treatment delivery because of incorrect patient positioning, large margins and tumour/patient movement during the treatment can result in excessive doses to organs at risk. Delivery of a specified dose requires more monitor units and, as a consequence, the total body dose due to leakage radiation can be increased. This may lead to dose escalation, conformation and sharp dose gradients on one hand, while conversely they may increase the integral dose exposure of healthy tissues, since larger volumes are exposed to low doses. 
     The use of external radiation therapy may be limited in cases where the proximity of the tumour to radiation sensitive normal tissues makes it difficult to obtain an optimal absorbed dose distribution in the tumours area. Brachytherapy is thus desirable in these circumstances. High dose rate brachytherapy is a form of radiation therapy where radiation is administered from radiation sources (radionuclides) directly into or near the tumour, giving a high radiation dose to the target volume while sparing the surrounding radiation sensitive healthy tissues. The radionuclides are administered using an afterloading technique, where the applicator is first placed into the tumour and the radioactive sources are loaded later by a machine known as an afterloader. 
     Magnetic resonance imaging (MRI) guided brachytherapy provides good dose distributions in the tumour, with decreased margins and toxicity, due to excellent delineation of the tumour and surrounding tissue. The steep dose gradient from brachytherapy sources results in improved therapeutic ratio compared with external beam radiation therapy for selected tumour sites. However, optimal dose distribution in the tumour is limited in many tumour sites due to the proximity of the tumour to organs at risk, such as the urethra, rectum, urinary bladder and neurovascular bundle for prostate cancer. Other organs at risk are skin and axilla for breast cancer, bladder, rectum, sigmoid, and vagina for cervical and uterine cancer, urethra, rectum, bone, and skin for vaginal and vulvar cancer, salivary glands and mandible for head and neck cancer, and lung and spine for oesophageal cancer. 
     IMBT, designed and delivered with accurate anatomic reference, has the objective to tailor treatments to each individual patient by treating all parts of the tumour without needlessly irradiating large regions of organs at risk. With MRI guided IMBT the oncologist can identify where the cancer has spread, and instead of treating a large area around the tumour, they can fit the cell-killing treatments to the tumour. 
     High dose rate brachytherapy (HDR-BT), especially with magnetic resonance (MR) image guidance, is one of the most effective and precise radiation delivery modalities with major impact on gynecological, genitourinary and prostate cancers. HDR-BT has been adopted rapidly in Canada, USA and Europe and is cost efficient compared to other treatment modalities. About 60% of the radiation oncology clinics across Canada offer this treatment modality with gynecologic, genitourinary and prostate cancers as the most commonly treated cancer sites. Due to the growing evidence of the benefits of HDR-BT and increasing number of cancer patients, the treatment modality has been successfully used on other cancer sites as well. In HDR-BT, encapsulated high activity radiation sources are temporary placed directly into or near localized tumours using needles, plastic catheters or other specialized applicators giving a high radiation dose to the tumour while the dose to sensitive organs at risk (OARs) surrounding the tumour is lower. However, radiation sources used in BT conventionally provide rotationally symmetric dose distributions, and deliver high dose to tumours with often poor target conformity due to the non-symmetrical shape of the tumours resulting in dose spillage to OARs. 
     SUMMARY 
     Brachytherapy is a targeted high-dose radiation treatment that can be delivered via radioactive seeds that are placed directly in or near the tumor or via an applicator device placed at the tumor site. Brachytherapy is an integral component of the treatment regimen of several localized and locally advanced cancers. Outcome in terms of disease control and radiation associated complications is a function of dose to the tumor and sensitive organs at risk (OARs). By utilizing an intensity modulated brachytherapy (IMBT) technique, dose delivery could be optimized to better conform to the tumor, enabling dose escalation while maintaining or reducing OARs doses. Herein, rotating shields, compatible with modern MRI-compatible intracavitary applicators, are disclosed. In certain embodiments these may be used, as an example only, for cervix and endorectal brachytherapy. 
     Three shield models are shown. In a particular embodiment, these shields are composed of platinum or tungsten, however other suitable materials may also be used. These shields might be used in conjunction with an  192 Ir HDR source. Additionally,  75 Se and  169 Yb may be used as alternative gamma-ray emitting sources for IMBT. 
     In another aspect, different designs of flexible shields for brachytherapy are presented. 
     By incorporating metallic shields inside BT catheters, IMBT might open the possibility to escalate the dose to the tumour while more effectively shielding OARs by dynamically directing the radiation towards the target and away from the OARs, i.e., the dose distributions might better conform to the shape of the tumour. Dose escalation of the tumour might significantly improve the potential of HDR-BT while simultaneously reducing toxicity will increase quality of life of the patients and lead to improved therapeutic ratio and clinical outcomes. 
     Shields with an angular cut in the radial axis (and flexible longitudinally) are disclosed and might be used for vaginal, rectal cervix or any other intracavitary and intraluminal applicators. 
     The shields can either be solid, with cuts along the longitudinal axis making them flexible, or they can be flexible with cuts along the longitudinal axis and have an angular cut radially. 
     The shield can be standalone, i.e. a solid (no angular cuts in it, not connected to a rotating system) flexible, bendable rod that can be placed in an applicator or flexible, bendable with cut in any degree combined with a rotating system. The rotating shields can be controlled through a moving panel driven by a stepper motor. Sensors in the rotating mechanism will detect the position of the shield. 
     There is accordingly provided an intracavitary shield configured to be inserted inside a patient for brachytherapy, comprising: an elongated body having a central axis, the elongated body composed of a radiation-shielding material that is MRI-compatible; and a radioactive seed receiving cavity extending through or along the elongated body for slideably receiving a radioactive seed therein. 
     In the intracavitary shield as defined above, a center of the radioactive seed receiving cavity may be radially offset from the central axis of the elongated body. 
     In the intracavitary shield as defined above, the radiation-shielding material may be tungsten. 
     In the intracavitary shield as defined above, the radiation-shielding material may be platinum. 
     There is also provided an assembly configured to be inserted inside a patient for brachytherapy, comprising: an elongated body having a central axis, the elongated body composed of a radiation-shielding material that is MRI-compatible; and a radioactive seed receiving cavity extending through or along the elongated body for slideably receiving a radioactive seed therein, a center of the radioactive seed receiving cavity being radially offset from the central axis of the elongated body 
     There is further provided an intracavitary shield configured to be inserted inside a patient for brachytherapy, comprising a flexible shield body formed by a plurality of shield sections interconnected to one another via flexible joints permitting relative movement between respective ones of the plurality of shield sections, each of the shield sections defining a cavity therein that is aligned with a corresponding cavity in an adjacent one of the plurality of shield sections, the cavities collectively forming a passage through which a radionuclide seed is slideably received. 
     In accordance with a first aspect, there is provided a radiation shield for an intensity modulated brachytherapy (IMBT) system, the radiation shield extending along a central axis and defining at least two radiation shield sections interconnected to one another and pivotable one relative to the other about an axis normal to the central axis, the at least two radiation shield sections defining radionuclide-receiving passages being circumferentially aligned with one another relative to the central axis for slideably receiving therein a radionuclide. 
     In accordance with the first aspect, the radiation shield may include a peripheral wall extending around the central axis, the peripheral wall defining a slit pattern. 
     Still in accordance with the first aspect, slits of the slit pattern may have a jigsaw shape or a helicoid shape. 
     Still in accordance with the first aspect, the radiation shield may include a monolithic body defining a flexible section, the flexible section including a plurality of slits, each of the slits extending from an outer face of the monolithic body toward the central axis, the slits ending at a core, discs defined between the slits and being axially spaced apart from one another to allow bending of the core. 
     Still in accordance with the first aspect, the at least two radiation shield sections may be interconnected to one another by a joint. 
     Still in accordance with the first aspect, the joint may include a ball protruding axially from an axial end face of one of the at least two radiation shield sections and a rounded cavity extending from an axial end face of the other of the at least two radiation shield sections, the ball received within the rounded cavity. 
     Still in accordance with the first aspect, the joint may include a tab protruding from an axial end face of one of the at least two radiation shield sections and a recess extending from the axial end face of the other of the at least two radiation shield sections, the tab pivotably received within the recess, the tab locked within the recess via a pin extending through a first aperture defined through the tab and through a second aperture defined through the other of the at least two radiation shield sections. 
     Still in accordance with the first aspect, the at least two radiation shield sections may be connected to one another via a flexible rod extending through registering apertures defined by the at least two radiation shield sections. 
     Still in accordance with the first aspect, the radionuclide-receiving passages may be grooves, bores, or cavities. 
     In accordance with a second aspect, there is provided a shield assembly for an intensity modulated brachytherapy (IMBT) system, comprising: a tubular applicator engageable to a rotating mechanism of the IMBT system, the tubular applicator having a peripheral wall enclosing an internal cavity extending longitudinally along a central axis; a radiation shield extending axially along the central axis and received within the internal cavity, the radiation shield made of an MRI-compatible and radiation attenuating material; and a radionuclide-receiving passage within the internal cavity of the tubular applicator, the radionuclide-receiving passage extending axially and being radially offset from the central axis. 
     The shield assembly and/or radiation shield thereof as described herein may further include, in whole or in part, one or more of the following additional features. 
     Still in accordance with the second aspect, the central axis may be free of intersection with the radionuclide-receiving passage. 
     Still in accordance with the second aspect, the radionuclide-receiving passage may be a groove defined by the radiation shield and extending axially along the central axis, the groove extending radially from an outer face of the radiation shield toward the central axis. 
     Still in accordance with the second aspect, the outer surface of the radiation shield may be convex but for the groove. 
     Still in accordance with the second aspect, the radionuclide-receiving passage may be defined between a cylindrical outer face of the radiation shield and the peripheral wall. 
     Still in accordance with the second aspect, a ratio of a diameter (Da) of the radiation shield to an internal diameter (D 8 ) of the tubular applicator may be about 0.75. 
     Still in accordance with the second aspect, the radionuclide-receiving passage may be a bore extending through the radiation shield. 
     Still in accordance with the second aspect, the radiation shield may define apertures extending from an outer surface of the radiation shield to the bore, the apertures axially spaced-apart from one another along the central axis and being circumferentially aligned with one another. 
     Still in accordance with the second aspect, a ratio of a distance (D 3 ) between a center of the bore and a center of the radiation shield to an internal diameter (D 8 ) of the tubular applicator may range from 0.06 to 0.2. 
     Still in accordance with the second aspect, a ratio of a diameter (D 7 ) of the bore to a the internal diameter (D 8 ) of the tubular applicator may range from 0.09 to 0.3, wherein a ratio of a distance (d 3 ) taken along the central axis between two adjacent ones of the apertures to the internal diameter of the tubular applicator may range from 0.6 to 2, and wherein a ratio of a diameter (D 4 ) of the apertures to the internal diameter (D 8 ) of the tubular applicator may range from 0.06 to 0.2. 
     Still in accordance with the second aspect, the MRI-compatible and radiation attenuating material may be tungsten. 
     Still in accordance with the second aspect, the MRI-compatible and radiation attenuating material may be platinum. 
     Still in accordance with the second aspect, the radiation shield may include at least two radiation shield sections interconnected to one another and pivotable one relative to the other about an axis normal to the central axis. 
     Still in accordance with the second aspect, the peripheral wall may define a slit pattern. 
     Still in accordance with the second aspect, slits of the slit pattern may have a jigsaw shape or a helicoid shape. 
     Still in accordance with the second aspect, the radiation shield may include a monolithic body defining a flexible section, the flexible section including a plurality of slits, each of the slits extending from an outer face of the monolithic body toward the central axis, the slits ending at a core, discs defined between the slits and being axially spaced apart from one another to allow bending of the core. 
     Still in accordance with the second aspect, the at least two radiation shield sections may be interconnected to one another by a joint. 
     Still in accordance with the second aspect, the joint may include a ball protruding axially from an axial end face of one of the at least two radiation shield sections and a rounded cavity extending from an axial end face of the other of the at least two radiation shield sections, the ball received within the rounded cavity. 
     Still in accordance with the second aspect, the joint may include a tab protruding from an axial end face of one of the at least two radiation shield sections and a recess extending from the axial end face of the other of the at least two radiation shield sections, the tab pivotably received within the recess, the tab locked within the recess via a pin extending through a first aperture defined through the tab and through a second aperture defined through the other of the at least two radiation shield sections. 
     Still in accordance with the second aspect, the at least two radiation shield sections may be connected to one another via a flexible rod extending through registering apertures defined by the at least two radiation shield sections. 
     In accordance with a third aspect, there is provided an intensity modulated brachytherapy (IMBT) system, comprising a rotating system and a shield assembly drivingly engaged to the rotating system for rotating the shield assembly about a central axis thereof, the shield assembly having: a tubular applicator including a peripheral wall enclosing an internal cavity extending axially along the central axis; a radiation shield within the internal cavity and extending axially along the central axis, the radiation shield made of an MRI-compatible and radiation attenuating material; and a radionuclide-receiving passage within the internal cavity of the tubular applicator, the radionuclide-receiving passage being eccentric relative to the internal cavity of the tubular applicator. 
     The intensity modulated brachytherapy (IMBT) system and/or radiation shield thereof as described herein may further include, in whole or in part, one or more of the following additional features. 
     Still in accordance with the third aspect, the shield assembly may be pivotably connected to the rotating system via a joint. 
     Still in accordance with the third aspect, the joint may be a U-joint. 
     Still in accordance with the third aspect, the rotating system may include a motor in driving engagement with a shaft, the shaft drivingly engaged to the shield assembly. 
     Still in accordance with the third aspect, the central axis may be free of intersection with the radionuclide-receiving passage. 
     Still in accordance with the third aspect, the radionuclide-receiving passage may be a groove defined by the radiation shield and extending axially along the central axis, the groove extending radially from an outer face of the radiation shield toward the central axis, the groove sized to slidably receive therein a radionuclide. 
     Still in accordance with the third aspect, a depth of the groove taken in a radial direction relative to the central axis may be greater than a diameter of the radionuclide. 
     Still in accordance with the third aspect, the radionuclide-receiving passage may be defined between a cylindrical outer face of the radiation shield and the peripheral wall. 
     Still in accordance with the third aspect, a ratio of a diameter (Da) of the radiation shield to an internal diameter (D 8 ) of the tubular applicator may be about 0.75. 
     Still in accordance with the third aspect, the radionuclide-receiving passage may be a bore extending through the radiation shield axially along the central axis. 
     Still in accordance with the third aspect, the radiation shield may define apertures extending from an outer surface of the radiation shield to the bore, the apertures axially spaced-apart from one another along the central axis and being circumferentially aligned with one another. 
     Still in accordance with the third aspect, a ratio of a distance (D 3 ) between a center of the bore and a center of the radiation shield to an internal diameter (D 8 ) of the tubular applicator may range from 0.06 to 0.2. 
     Still in accordance with the third aspect, a ratio of a diameter (D 7 ) of the bore to a the internal diameter (D 8 ) of the tubular applicator may range from 0.09 to 0.3, wherein a ratio of a distance (d 3 ) taken along the central axis between two adjacent ones of the apertures to the internal diameter of the tubular applicator may range from 0.6 to 2, and wherein a ratio of a diameter (D 4 ) of the apertures to the internal diameter (D 8 ) of the tubular applicator may range from 0.06 to 0.2. 
     Still in accordance with the third aspect, the MRI-compatible and radiation attenuating material may be tungsten. 
     Still in accordance with the third aspect, the MRI-compatible and radiation attenuating material may be platinum. 
     Still in accordance with the third aspect, the radiation shield may include at least two radiation shield sections interconnected to one another and pivotable one relative to the other about an axis normal to the central axis. 
     Still in accordance with the third aspect, the peripheral wall may define a slit pattern. 
     Still in accordance with the third aspect, slits of the slit pattern may have a jigsaw shape or a helicoid shape. 
     Still in accordance with the third aspect, the radiation shield may include a monolithic body defining a flexible section, the flexible section including a plurality of slits, each of the slits extending from an outer face of the monolithic body toward the central axis, the slits ending at a core, discs defined between the slits and being axially spaced apart from one another to allow bending of the core. 
     Still in accordance with the third aspect, the at least two radiation shield sections may be interconnected to one another by a joint. 
     Still in accordance with the third aspect, the joint may include a ball protruding axially from an axial end face of one of the at least two radiation shield sections and a rounded cavity extending from an axial end face of the other of the at least two radiation shield sections, the ball received within the rounded cavity. 
     Still in accordance with the third aspect, the joint may include a tab protruding from an axial end face of one of the at least two radiation shield sections and a recess extending from the axial end face of the other of the at least two radiation shield sections, the tab pivotably received within the recess, the tab locked within the recess via a pin extending through a first aperture defined through the tab and through a second aperture defined through the other of the at least two radiation shield sections. 
     Still in accordance with the third aspect, the at least two radiation shield sections may be connected to one another via a flexible rod extending through registering apertures defined by the at least two radiation shield sections. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures in which: 
         FIG. 1 a    is a schematic partially transparent view of an intensity modulated brachytherapy (IMBT) system in accordance with one embodiment; 
         FIG. 1 b    is an enlarged view of zone  1   b - 1   b  on  FIG. 1   a;    
         FIG. 1 c    is a schematic cross-sectional view of a shield assembly of the IMBT system of  FIG. 1 a    taken along line  1   c - 1   c  on  FIG. 1   b;    
         FIG. 2 a    is a schematic top three dimensional view of an intracavitary shield in accordance with one embodiment; 
         FIG. 2 b    is a schematic cross-sectional view of the intracavitary shield of  FIG. 2   a;    
         FIG. 2 c    is a schematic front three dimensional view of the intracavitary shield of  FIG. 2   a;    
         FIG. 3 a    is a schematic top three dimensional view of an intracavitary shield in accordance with another embodiment; 
         FIG. 3 b    is a schematic cross-sectional view of the intracavitary shield of  FIG. 3   a;    
         FIG. 3 c    is a schematic front three dimensional view of the intracavitary shield of  FIG. 3   a;    
         FIG. 4 a    is a schematic top three dimensional view of the intracavitary shield of  FIG. 1   a;    
         FIG. 4 b    is a schematic cross-sectional view of the intracavitary shield of  FIG. 4   a;    
         FIG. 4 c    is a schematic front three dimensional view of the intracavitary shield of  FIG. 4   a;    
         FIGS. 5 a  to 5 l    are axial cross-sectional views of Monte Carlo (MC) calculated doses for a non-shielded tandem ( FIGS. 5 a  to 5 c   ) and for the rotating shields of  FIG. 2 a    ( FIGS. 5 d  to 5 f   ),  FIG. 3 a    ( FIGS. 5 g  to 5 i   ), and  FIG. 4 a    ( FIGS. 5 j  to 5 l   ) using  192 Ir ( FIGS. 5 a , 5 d , 5 g , 5 j   ),  75 Se ( FIGS. 5 b , 5 e , 5 h , 5 k   ), and  169 Yb ( FIGS. 5 c , 5 f , 5 i , 5 l   ) as radionuclides; 
         FIGS. 6 a  to 6 l    are longitudinal cross-sectional views of Monte Carlo (MC) calculated doses for a non-shielded tandem ( FIGS. 6 a  to 6 c   ) and for the rotating shields of  FIG. 2 a    ( FIGS. 6 d    to  60 ,  FIG. 3 a    ( FIGS. 6 g  to 6 i   ), and  FIG. 4 a    ( FIGS. 6 j  to 6 l   ) using  192 Ir ( FIGS. 6 a , 6 d , 6 g , 6 j   ),  75 Se ( FIGS. 6 b , 6 e , 6 h , 6 k   ), and  169 Yb ( FIGS. 6 c , 6 f , 6 i , 6 l   ) as radionuclides; 
         FIGS. 7 a  to 7 l    are polar anisotropy graphs taken at a radial distance of 1 cm and normalized a radial distance of 1 cm, a polar angle of 90 degrees and an azimuthal angle of 0 degree, for a non-shielded tandem ( FIGS. 7 a  to 7 c   ) and for the rotating shields of  FIG. 2 a    ( FIGS. 7 d  to 7 f   ),  FIG. 3 a    ( FIGS. 7 g  to 7 i   ), and  FIG. 4 a    ( FIGS. 7 j  to 7 l   ) using  192 Ir ( FIGS. 7 a , 7 d , 7 g , 7 j   ),  75 Se ( FIGS. 7 b , 7 e , 7 h , 7 k   ), and  169 Yb ( FIGS. 7 c , 7 f , 7 i , 7 l   ) as radionuclides; 
         FIGS. 8 a  to 8 l    are azimuthal anisotropy graphs taken at a radial distance of 1 cm and normalized a radial distance of 1 cm, a polar angle of 90 degrees and an azimuthal angle of 0 degree, for a non-shielded tandem ( FIGS. 8 a  to 8 c   ) and for the rotating shields of  FIG. 2 a    ( FIGS. 8 d    to  80 ,  FIG. 3 a    ( FIGS. 8 g  to 8 i   ), and  FIG. 4 a    ( FIGS. 8 j  to 8 l   ) using  192 Ir ( FIGS. 8 a , 8 d , 8 g , 8 j   ),  75 Se ( FIGS. 8 b , 8 e , 8 h , 8 k   ), and  169 Yb ( FIGS. 8 c , 8 f , 8 i , 8 l   ) as radionuclides; 
         FIG. 9  is a graph showing radial dose in function of various brachytherapy sources normalized at 1 cm; 
         FIGS. 10 a -10 b    are axial and longitudinal views of 100% and 25% isodose lines for the intracavitary shield of  FIG. 3 a    made of platinum and with  192 Ir (solid),  75 Se (dashed) and  169 Yb (dotted) radionuclides; 
         FIGS. 10 c -10 d    are axial and longitudinal views of 100% and 25% isodose lines for the intracavitary shield of  FIG. 2 a    made of platinum and with  192 Ir (solid),  75 Se (dashed) and  169 Yb (dotted) radionuclides; 
         FIGS. 10 e -10 f    are axial and longitudinal views of 100% and 25% isodose lines for the intracavitary shield of  FIG. 4 a    made of platinum and with  192 Ir (solid),  75 Se (dashed) and  169 Yb (dotted) radionuclides; 
         FIGS. 11 a  to 11 d    are schematic views of flexible intracavitary shields shown in different positions in accordance with possible embodiments; 
         FIGS. 12 a  and 12 b    are schematic front and three dimensional views of a flexible intracavitary shield in accordance with another embodiment; 
         FIGS. 13 a  and 13 b    are schematic three dimensional views of a flexible intracavitary shield in accordance with another embodiment shown in two different positions; 
         FIGS. 14 a  and 14 b    are schematic three dimensional views of a flexible intracavitary shield in accordance with another embodiment shown in two different positions; 
         FIG. 15  is a schematic three dimensional view of a flexible intracavitary shield in accordance with another embodiment; 
         FIG. 16  is a schematic three dimensional view of a flexible intracavitary shield in accordance with another embodiment; and 
         FIGS. 17 a  and 17 b    are cross-sectional views of flexible intracavitary shields in accordance with yet other embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In interstitial high dose rate brachytherapy (HDR-BT), a radionuclide, also referred to as a radioactive seed, is temporarily placed via thin catheters inside or in close proximity of the tumor (for example, prostate, breast, head &amp; neck or cervix cancers). The catheters are then connected to an HDR afterloader, which is a machine that contains a single highly radioactive (e.g.,  192 Ir) source at the end of a wire. This technique is referred to as a remote afterloading technique. The source may be pushed into each of the catheters, one by one under computer control and guided to the tumour site. The computer controls how long the source stays in each catheter (dwell time), and where along the catheter, inside or close to the tumour, it should pause to expose its radiation (dwell positions). With a few well-placed catheters in the tumor, HDR-BT may provide a precise treatment taking only a few minutes. After the desired dose is delivered, the radiation source is returned back to the afterloader and the catheters are removed. 
     HDR-BT with remote afterloading is also performed with intracavitary or intraluminal BT. In intracavitary BT the radionuclide is placed in a special applicator inside a body cavity (for example for treatment of gynecological and rectal cancers). In intraluminal BT the radionuclide is placed in a special applicator inside a body passage and guided to the tumour site (esophageal or lung cancers). 
     HDR-BT has proven to increase overall survival for cervical and rectum cancer patients with even further overall survival benefit when moving to image-guided BT. However, optimal dose distribution in the tumour is limited for many tumour sites treated with HDBT due to the proximity of the tumour to organs at risk (OARs) such as urethra, rectum, urinary bladder and neurovascular bundle for prostate cancer, skin and axilla for breast cancer, bladder, rectum, sigmoid, and vagina for cervical and uterine cancer, urethra, rectum, bone, and skin for vaginal and vulvar cancer, salivary glands and mandible for head &amp; neck cancer and lung and spine for oesophageal cancer. These cancer sites may benefit from intensity modulated brachytherapy (IMBT) since the radiation dose may be directed towards the target and away from OARs. In BT, the dose is prescribed to an isodose encompassing a small target volume. 
     Again, clinical studies have shown that dose escalation for locally advanced cervical cancer under image-guided HDR-BT can lead to increased local control. However, for conventional intracavitary HDR-BT dose to bladder, rectum and sigmoid may limit the maximum dose deliverable to the tumour due to the fixed geometry of the applicators and the symmetrical shape of the dose distribution. The tumour is not symmetrically shaped, all parts of the planning target volume will not receive the prescribed dose, while large volume of bladder, rectum and sigmoid may be overdosed. The ability to produce anisotropic dose distributions from individual dwell positions might allow for protection of OARs from excessive dose without compromising target coverage in many cancer sites treated with HDR-BT. IMBT might allow for the placing of dwell positions very close to the OARs, irradiating larger volume and escalating the dose inside the tumour while shielding the OARs. 
     Intracavitary and intraluminal applicators can be straight or bend with up to 45 degrees to effectively treat a variety of anatomies. Different configurations of straight and flexible shields prototypes are discussed herein below. 
     IMBT Delivery System 
     Referring to  FIGS. 1 a  to 1 c   , an IMBT delivery system is shown generally at  10 . While the system  10  may be adapted for cervix cancer applications, it is to be understood that its feature and the system as whole may alternately be used for other cancer applications. The system  10  includes an afterloader connection  12  connectable to an afterloader AL containing a source of an isotope for treating tumors. A needle  14  is connected to the afterloader connection  14  and is used to receive therein a radionuclide from the afterloader. The system  10  includes a rotating mechanism  16 , a joint assembly  18 , and a shield assembly  20 . 
     The rotating mechanism  16  is used for rotating the shield assembly  20  about its central axis A. As will be described herein below, different shields may be used. The shields are used to allow radiation to radiate from the radionuclide toward a tumor in a certain orientation and to attenuate radiation in other orientations to limit radiation from radiating organs at risk (OAR). 
     In the depicted embodiment, the rotating mechanism  16  includes a casing  16   a  enclosing a motor  16   b  and a gearbox  16   d  having a plurality of gears  16   c . The gears  16   c  are drivingly engaged by the motor  16   a , which is herein an electric motor, and are engaged to the shield assembly  20  for rotating the same about the central axis A. An angular sensor  16   e  is located within the casing  16   a  and operatively connected to shield assembly  20  for determining an angular position of the shield assembly  20 . 
     The shield assembly  20  is in driving engagement with the electric motor  16   a  via a driving shaft  22 . The driving shaft  22  is connected to the shield assembly  20  via the joint assembly  18 , which is herein a U-joint. More details about the delivery system  10  are presented in Famulari G, Enger S A. A novel intensity modulated high dose rate brachytherapy delivery system with 169 Yb. Int J Radiat Oncol 2017; 99(2):E657; and in Famulari G, Enger S A. Intensity modulated brachytherapy system for dynamic modulation of shielded catheters. Radiother Oncol 2018; 127:S90, and in U.S. patent application Ser. No. 16/471,703, the entire contents of which are presented herein by reference. 
     Referring now to  FIGS. 1 b -1 c   , the shield assembly  20  includes a rotating shield  300  received within a tube  20   a , also referred to as a tandem or a tubular applicator. The tandem  20   a  has a peripheral wall  20   b  enclosing an internal cavity  20   c  extending longitudinally along the central axis A. The shield  300  is received within the internal cavity  20   c  and is made of an MRI-compatible material, such as platinum or tungsten. Although the shield assembly  20  is shown with the shield  300  described below with reference to  FIGS. 4 a  to 4 c   , the shield assembly  20  may include any of the shields described below with reference to  FIGS. 2 a  to 3 c    and  FIGS. 11 a    to  17   b.    
     In the embodiment shown, the MRI/CT-compatible tandem and ring applicator (Elekta Brachytherapy, Veenendaal, The Netherlands) was redesigned to enable rotating shield IMBT. Brachytherapy cervix applicators are typically built such that they have a tandem/ring or tandem ovoids. The ring extends annularly around the tandem  20   a . The ovoids are two oval volumes on each side of the tandem  20   a . In one particular embodiment, the clinical tandem  20   a  has a 6 mm outer diameter D 1  and an inner diameter close to 3 mm. To maximize the amount of shielding material in the tandem  20   a , the applicator was redesigned with an inner diameter D 8  of 5.4 mm. The ring is unshielded and is left unchanged. For the purpose of this study, the tandem casing material having a thickness of 0.3 mm is considered to be water equivalent. 
     Straight Shields Prototypes 
     Shields were modeled to fit inside the redesigned MRI/CT-compatible tandem  20   a , which has an inner diameter D 8  of 5.4 mm, and connect to the IMBT delivery system  10  described above via the joint assembly  18 , which enables the transfer of rotational force while maintaining the bend required for the angled tandem. The shields have a single channel that may maximize an amount of attenuating material within the tandem  20   a  and is possible due to the rotational IMBT delivery system  10 . All shield designs disclosed herein below are based on a solid cylinder. 
     Tungsten is used herein as a shield material due its relative high density and low magnetic susceptibility. Tungsten and its non-iron alloys may strike a balance between affordability and manufacturability. It will be appreciated that any suitable shielding material may be used without departing from the scope of the present disclosure. For instance, the shielding material may be platinum. The shield is a radiation shield made of an MRI-compatible material that is also radiation attenuating (i.e. it substantially prevents radiation from being able to pass therethrough). 
     Referring to  FIGS. 2 a  to 4 c   , three rotating intracavitary shields are designed. The shields are designed to fit inside the intrauterine tandem  20   a  described above and having an external diameter D 1  of about 6 mm. This tandem is commonly used in the treatment of locally advanced cervical cancer. As will be described below, each of the shield assemblies defines a radionuclide-receiving passage C, or channel, within the internal cavity of the tandem  20   a . In all cases, the radionuclide-receiving passages C are radially offset from the central axis A. In other words, those passages C are eccentric with the internal cavity  20   c  tandem  20   a . This may allow the dose to be anisotropic such that a higher dose is delivered to the tumor while shielding the OARs. In the depicted embodiments, the central axis A is free of intersection with the radionuclide-receiving passage C. 
     All three shields may be compatible with a rotating IMBT delivery system as disclosed in international patent application published under number WO 2018/112625, the entire content of which is incorporated herein by reference. 
     The disclosed IMBT shields may enable radial dose modulation with a single channel in the shield. When compared to static multichannel shields, the disclosed shields might permit more shielding material to fit inside the intracavitary applicator, thereby enhancing the modulation capacity. 
     The radionuclide-receiving passage diameter D 7  for all shields  100 ,  200 ,  300  is 1.33 mm. All shields are 8 cm long for compatibility with the longest tandem size. For the conventional, non-shielded tandem, the radionuclide-receiving passage was placed in the center. 
     Referring to  FIGS. 2 a  to 2 c   , a shield in accordance with one embodiment is shown generally at  100 . This shield is referred to herein below as the type-A shield  100 . The type-A shield  100  is a solid cylinder having a diameter Da of about 4.07 mm. In the embodiment shown, a ratio of a diameter (Da) of the radiation shield to an internal diameter (D 8 ) of the tubular applicator is about 0.75. 
     The shield  100  is radially offset from the radionuclide-receiving passage C. In the embodiment shown, the radionuclide-receiving passage C is located radially outwardly of an outer surface  100   a  of the type-A shield  100  relative to the central axis A. The radionuclide-receiving passage C is defined between the outer face  100   a  of the shield  100  and the peripheral wall  20   b  of the tandem  20   a ; the wall  20   b  is shown in  FIG. 2 b    for easier understanding. The shield  100  and the tandem  20   a  are spaced-apart from one another such that an annular gap G between the shield  100  and the peripheral wall  20   b  of the tandem  20   a  has a radial thickness being non-axisymmetric. The radionuclide-receiving passage C is herein located at a circumferential position where the radial thickness of the annular gap G is maximal. In the present case, upon rotation of the shield assembly  20  via the rotating mechanism  16 , the type-A shield  100  and the radionuclide rotate around each other within the tandem  20   a  ( FIG. 1 a   ). 
     Referring to  FIGS. 3 a  to 3 c   , a shield in accordance with another embodiment is shown generally at  200  and is referred to as the type-B shield herein below. The type-B shield  200  is a solid cylindrical shield having a diameter db of 5.4 mm to fit inside the tandem  20   a  and has a single groove  202  extending radially from an outer face  200   a  of the shield  200  toward the central axis A. The radionuclide-receiving passage C is defined by the groove  202 . The groove  202  extends along the central axis A and parallel thereto. A depth H of the groove  202  taken in a radial direction relative to the central axis A is greater than a diameter of the radionuclide S. In the embodiment shown, the shield is free of another groove. The outer surface  200   a  is convex but for the groove  202 . 
     In the embodiment shown, the groove  202  has a straight portion that extends from the outer surface  200   a  of the shield  100  and a circular portion that extends from the straight portion. As shown, the radionuclide, when received in the groove  202 , do not protrude radially beyond the outer surface  200   a  of the shield  100 . The groove  202  may extend along the entire length L of the type-B shield  200  and along the central axis A. 
     It will be appreciated that the shield  200  may be used to treat other cancers, such as but not limited to rectal cancer. Dimensions of the shield  200  are adjusted in accordance with the type of cancer to treat. For rectal application, the shield  200  may have a diameter of about 15 mm. The groove  202  may have a depth extending radially relative to the central axis A of about 4.33 to about 4.45 mm and a width in a direction normal to the depth of about 1.33 mm. The length of the shield may be 80 mm. The radionuclide may be located 4.5 mm from the outer face  200   a  of the shield  200 . The shield used for rectal cancer may have a emission window of up to 45 degrees. The shield may be flexible and thicker than the above dimensions by about 5 mm. This shield may also be used for vaginal and endometrial cancers. 
     Three endorectal HDR-BT brachytherapy cases with high-risk-CTV (HR-CTV) ranging between 5.1-13.7 cm 3 , were planned and optimized in RapidBrachyMC-TPS, a MC-based research TPS. Shield rotations were limited to 10-degree increments. The contralateral, uninvolved rectal D50 was compared by normalizing the clinical and IMBT plans such that CTV D90 received 6.5 Gy per fraction. 
     Mean treatment times were 2.1±0.6 min and 15.1±3.3 min for clinical and IMBT, respectively. IMBT may reduce the uninvolved rectum D50 by 55.5±3.9% relative to conventional HDR-BT. Similarly, IMBT may improve CTV D98 by 12.4±5.5%. This may show that dynamic shield IMBT applicator for endorectal brachytherapy may be capable of improving healthy tissue sparing while improve target coverage. 
     Referring now to  FIGS. 4 a  to 4 c   , a shield in accordance with another embodiment is shown generally at  300  and is referred to below as the type-C shield. The type-C shield  300  defines a longitudinal bore  302  defining the radionuclide-receiving passage C. The bore  302  has a center radially offset from a center of the shield  300  by a distance D 3  ( FIG. 1 c   ) of 1 mm relative of the central axis A. In the embodiment shown, a ratio of a distance (D 3 ) between a center of the bore  302  and a center of the radiation shield  300  to an internal diameter (D 8 ) of the tubular applicator is about 0.19. The shield  300  is shown installed into the tandem  20   a  in  FIG. 1 c   . The type-C shield  300  is similar to a flute due to the beam collimations. The type-C shield  300  defines a plurality apertures or holes  300   b  having a diameter D 4  of 1 mm and being axially spaced apart from another relative to the central axis A by a distance d 3  of 10 mm. The diameter D 4  of the apertures  300   b  is less than that of the bore  302 . In the embodiment shown, all of the holes  300   b  are circumferentially aligned with one another relative to the central axis A. In the embodiment shown, the diameter D 4  of the spaced apart apertures  300   b  is less than the diameter D 7  ( FIG. 1 c   ) of the bore  302 . In the embodiment shown, a ratio of a diameter (D 7 ) of the bore  302  to a the internal diameter (D 8 ) of the tubular applicator  20   a  is about 0.25, a ratio of a distance (d 3 ) taken along the central axis A between two adjacent ones of the apertures  300   b  to the internal diameter (D 8 ) of the tubular applicator  20   a  is about is about 1.85, and a ratio of a diameter (D 4 ) of the apertures  300   b  to the internal diameter (D 8 ) of the tubular applicator is about 0.19. 
     In some cases, the shield  100 ,  200 ,  300  may have a diameter of from 5 to 15 mm. Therefore, a ratio of a distance (D 3 ) between a center of the bore  302  and a center of the radiation shield  300  to an internal diameter (D 8 ) of the tubular applicator may range from 0.06 to 0.2. In the embodiment shown, a ratio of a diameter (D 7 ) of the bore  302  to a the internal diameter (D 8 ) of the tubular applicator  20   a  may range from 0.09 to 0.3, a ratio of a distance (d 3 ) taken along the central axis A between two adjacent ones of the apertures  300   b  to the internal diameter (D 8 ) of the tubular applicator  20   a  may range from 0.6 to 2, and a ratio of a diameter (D 4 ) of the apertures  300   b  to the internal diameter (D 8 ) of the tubular applicator is about 0.06 to 0.2. The diameter D 1  of the tandem  20  ranges from 5 to 6 mm. 
     Each of the disclosed shields has only a single radionuclide-receiving passage C. This allows to maximize an amount of shielding material (e.g., tungsten) within the tandem  20   a , which has a fixed size. The rotating mechanism  16  allows to rotate the shield assembly to direct the radiation towards the tumors while having the shielding material to protect the OAR. The ability to rotate the shield assembly  20  and to shield the OAR may allow to use higher radiation dose for treating the tumor while minimizing damage to OAR. 
     For uterus applications, the length of the shields is selected such that it may match a length of a patient&#39;s uterus. For instance, a 7 cm long tandem is used for a 7 cm deep uterus. This matching length corresponds to that of a therapeutic portion of the tandem, that is, the portion received in the patient. For uterus application, the diameter of the tandem ranges from 5 to 6 mm. For cervix applications, the diameter of the shield is as large as possible and may be the same as that of the tandem  20   a  minus 0.5 to 0.9 mm. 
     Monte Carlo Simulations 
     The simulated HDR source was modeled after the Flexisource used in the Elekta Flexitron afterloader (Elekta Brachytherapy, Veenendaal, The Netherlands). The active core (radionuclide) is a cylinder with 0.6 mm diameter and 3.5 mm length. The active core material was set to  192 Ir,  75 Se or  169 Yb. The active core was encapsulated by stainless-steel-304 with outer dimensions of 4.6 mm length and 0.85 mm diameter. The drive cable is also composed of stainless-steel-304 and is modeled with a length of 5 mm. 
     Simulations were performed using a Monte Carlo based treatment planning software (RapidBrachyMCTPS with Geant4 MC toolkit). Herein, decay events were simulated for each radionuclide.  192 Ir which has approximately 2.3 photons per decay event would lead to a simulation with 2.3×108 primary photons. Similarly  75 Se and  169 Yb generate approximately 2.3 and 3.8 photons per decay, respectively. Penelope low-energy electromagnetic physics list was used to simulate electromagnetic interactions. Due to the low photon energies emitted from the simulated radionuclides, dose was approximated by the collisional kerma and scored using a track length estimator. Parallel world formalism implemented in Geant4 was used for scoring with the resolution of the scoring grid being 1 mm 3 . The source-shield geometry was placed in the center of a (50 cm)3 water phantom. A single dwell position located halfway up the 8 cm long intrauterine shield was simulated for all shield types and simulated radionuclides. The source was oriented in the positive-z direction. The tandem is centered at the origin. In the spirit of TG-43, the polar angle (also known as the zenith angle) measured from the z-axis is denoted as θ. We define ψ to be the azimuthal angle in the x-y plane and is the axis of rotation for dose modulation. The radius, r, is defined as the distance from the origin. To quantitatively evaluate the attenuation capacity of each shield-radionuclide combination, the transmission factor TF is calculated and defined as: 
     
       
         
           
             
               T 
               ⁢ 
               F 
             
             = 
             
               
                 D 
                 ⁡ 
                 ( 
                 
                   
                     r 
                     = 
                     
                       1 
                       ⁢ 
                           
                       cm 
                     
                   
                   , 
                   
                     θ 
                     = 
                     
                       90 
                       ⁢ 
                       ° 
                     
                   
                   , 
                   
                     ψ 
                     = 
                     
                       180 
                       ⁢ 
                       ° 
                     
                   
                 
                 ) 
               
               
                 D 
                 ⁡ 
                 ( 
                 
                   
                     r 
                     = 
                     
                       1 
                       ⁢ 
                           
                       cm 
                     
                   
                   , 
                   
                     θ 
                     = 
                     
                       90 
                       ⁢ 
                       ° 
                     
                   
                   , 
                   
                     ψ 
                     = 
                     
                       0 
                       ⁢ 
                       ° 
                     
                   
                 
                 ) 
               
             
           
         
       
     
     where TF is the ratio of the dose at 1 cm from the center of the tandem on the x-y plane of the shielded side to the unshielded side. The emission window is at an azimuthal angle of 0°. 
     Results 
     Referring now to  FIGS. 5 a  to 5 l    and to  FIGS. 6 a  to 6 l   , graphs of dose distributions are presented and are normalized at a radius of 1 cm from the central axis A, a polar angle of 90 degrees and an azimuthal angle of 0 degree. The transmission factors are summarized in the table below. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Type A 
                 Type B 
                 Type C 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                   192 Ir 
                 13.1% ± 0.4%  
                 12.9% ± 0.4%  
                 32.2% ± 0.4% 
               
               
                   
                   75 Se 
                 4.2% ± 0.7% 
                 4.0% ± 0.8% 
                 16.1% ± 0.6% 
               
               
                   
                   169 Yb 
                 1.8% ± 1.2% 
                 1.2% ± 1.5% 
                  6.4% ± 1.3% 
               
               
                   
                   
               
            
           
         
       
     
       FIGS. 5 a  to 5 c , 6 a  to 6 c , 7 a  to 7 c , and 8 a  to 8 c    show the dose distributions for a tandem devoid of a shielding material. Those graphs show that the dose distributions is substantially isotropic.  FIGS. 5 d  to 5 l , 6 d  to 6 l , 7 d  to 7 l , and 8 d  to 8 l    show that the shielding material allows to create an anisotropy in the dose distributions. 
     The normalized polar and azimuthal anisotropies are shown in  FIG. 5  and  FIG. 6 . Beam widths, defined as the full width at 80% maximum in the polar direction (evaluated at r=1 cm, =0°) are annotated on the subplots of  FIG. 7 . Similarly, beam widths in the azimuthal direction (evaluated at r=1 cm, =90°) are annotated in  FIG. 8 . 
     To quantify dose homogeneity, which is the ratio of the volume receiving at least 200% of the prescribed dose to the volume receiving at least 100% of the prescribed dose is calculated for all cases. Dose in voxels which are inside the tandem are excluded to assess dose homogeneity the patient is exposed to. Dose homogeneities for all shield-radionuclide combinations are summarized in Table 2. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 No Shield 
                 Type-A 
                 Type-B 
                 Type-C 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   192 Ir 
                 0.31 
                 0.33 
                 0.33 
                 0.29 
               
               
                   
                   75 Se 
                 0.30 
                 0.32 
                 0.34 
                 0.29 
               
               
                   
                   169 Yb 
                 0.31 
                 0.33 
                 0.35 
                 0.27 
               
               
                   
                   
               
            
           
         
       
     
     The results presented herein show that all three radionuclides are viable sources for the disclosed rotating shields IMBT  100 ,  200 ,  300 , achieving significantly anisotropic dose distributions, for all shield-radionuclide combinations evaluated. The disclosed rotating shields may maximize the amount of shielding material inside the tandem  20   a  since only single radionuclide-receiving passage is required. 
     Although a very low energy beam is more easily shielded for IMBT, too low an energy may compromise the ability to treat patients with disease that has spread in the parametrium and paravaginal tissue. As shown in  FIG. 9 , it was observed that the radial dose decreases radially away from the central axis A when using a 50 kVp electronic BT source for treatment. By comparison,  76 Se and  169 Yb may be better alternative lower energy sources since their radial doses are substantially constant with an increasing radial distance from the central axis A. 
     The dose homogeneity, defined as V 200 %/V 100 % was within 12% of the conventional tandem for all shield-radionuclide combinations. This may demonstrate that dose modulation may be achieved without introducing unacceptable hotspots in the patient when using the disclosed shields  100 ,  200 ,  300 . The type-A shield  100  may be capable of reducing the dose on the shielded side of the tandem, at 1 cm, to 13.1%, 4.2% and 1.8% for  192 Ir,  75 Se and  169 Yb, respectively. 
     As shown in  FIGS. 8 d  to 8 f   , the type-A shield  100  may exhibit a relatively large beam width (˜106°) in the azimuthal direction. This may limit its axial modulation capacity. 
     The type-B shield  200  having a single groove may improve upon the type-A shield  100  by filling the tandem with more attenuating material and simplifying the rotation mechanics as the shield rotation is concentric with the tandem. As shown in  FIGS. 8 g  to 8 i   , a key improvement is the narrower azimuthal beam width (from 71 to 91 degrees depending of the radionuclide) without sacrificing transmission on the posterior end. 
     The type-C shield  300  may achieve small beam widths in the azimuthal and polar directions as depicted in  FIGS. 8 j  to 8 l   . However, the resulting narrow beam may come at the expense of increasing posterior transmission. Indeed, since less shielding material is provided between the posterior side of the tandem and the radionuclide, less attenuation is achieved in this direction. A perusal of  FIGS. 6 g  and 6 j    show that a greater dose is provided on the posterior side of the tandem. 
     A tightly collimated beam in the azimuthal and polar directions has the potential benefit of treating distance parametrial disease without significantly increasing dose to nearby OARs in the superior-inferior direction. 
     Retrospective Planning Study 
     A retrospective planning study was performed on a patient with locally advanced cervical cancer. The original treatment plan was treated using a hybrid intracavitary and interstitial needle implant with a conventional  192 Ir brachytherapy source, which is current state-of-the-art treatment. Three IMBT plans were simulated and optimized; (1) Type-B shield with  192 Ir, (2) Type-A shield with  169 Yb and (3) Type-C shield with  169 Yb. All IMBT plans were performed with shields composed of tungsten. IMBT plans only used source positions within the intracavitary applicator. Interstitial needles were not used in IMBT treatment plans to highlight the benefits of intracavitary-only IMBT. Each dwell position within the shielded intrauterine tandem were simulated using 16 angles (22.5 degrees between each IMBT dwell). Dwells within the vaginal (ring) portion of the intracavitary applicator were unshielded. A brachytherapy prescription dose of 27.5 Gy in 5 fractions was used in all treatment plans. To evaluate dose to tumor and OARs, dose was converted to 2 Gy equivalents (EQD2) and added to a pre-brachytherapy 45 Gy in 25 fraction whole pelvis external beam irradiation, consistent with standard-of-care protocol. Total EQD2 dose metrics for tumor and OARs were optimized according to the American Brachytherapy Society&#39;s recommendations: (1) D90%, the minimum dose to 90% of the tumor volume to receive approximately 80 Gy10 or more. (2) D2cc, the minimum dose to the most irradiated 2 cc of OAR to be less than 90 Gy3 for bladder and less than 75 Gy3 for rectum or sigmoid. 
     Referring to  FIGS. 10 a  to 10 f   , normalized dose distributions at 1 cm from the center of the shielded applicator are shown for shields made of platinum. TFs are summarized in Table 3 and might be favorable for IMBT as TFs ranged between 4.1%-24.1% for  192 Ir, 0.7%-8.6% for  75 Se and 0.1-2.7% for  169 Yb. On average, Pt shields attenuated 34% (19%-67%) more than W, for all source/shield combinations. Shields had a TF of at least 50% over an average arc of 245 degrees, 208 degrees and 283 degrees for  192 Ir,  75 Se and  169 Yb, respectively. 
     In order to design a shield with a narrow emission window, the Type-C shields required the radionuclide-receiving passage to be placed closed to the applicator center, which increased the transmission relative to Type-B. Type-C shield did, however, exhibit a narrower beamlet when observed on longitudinal and axial planes. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Transmission factors Dose [1 cm, 180 deg]/Dose[1 cm, 0 deg] 
               
               
                 for shield models made of tungsten (W) and platinum (Pt). 
               
            
           
           
               
               
               
               
            
               
                   
                 Type-A 
                 Type-B 
                 Type-C 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 W 
                 Pt 
                 W 
                 Pt 
                 W 
                 Pt 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Ir-192 
                 5.6% 
                 4.1% 
                 12.4% 
                 9.5% 
                 24.1% 
                 19.5% 
               
               
                 Se-75 
                 1.2% 
                 0.7% 
                 3.4% 
                 2.1% 
                 8.6% 
                 5.7% 
               
               
                 Yb-169 
                 0.3% 
                 0.1% 
                 0.1% 
                 0.8% 
                 2.7% 
                 1.6% 
               
               
                   
               
            
           
         
       
     
     Table 4 displays the clinical dose metrics for the conventional hybrid intracavitary/interstitial brachytherapy. A D90 of 80.2 Gy 10  for the tumor and D2cc to the bladder, rectum and sigmoid of 68.4 Gy 3 , 59.0 Gy 3  and 59.5 Gy 3  were achieved for the clinical treatment plan. The dose metrics for the clinical treatment plan may be ideal in terms of tumor coverage and dose to OARs despite the locally advanced spread of the cancer. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Total EQD2 doses for conventional  192 Ir brachytherapy. 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Pre 
                 Brachy 
                 Total 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 TOTAL 
                 Prescription 
                 44.3 
                 35.5 
                 79.8 
                   
               
               
                 EQD2 
                 D90 
                 44.3 
                 35.9 
                 80.2 
               
               
                 (Gy) 
                 Bladder 
                 43.2 
                 25.2 
                 68.4 
                   &lt;90 Gy 
               
               
                   
                 Rectum 
                 43.2 
                 15.8 
                 59.0 
                 &lt;70-75 Gy 
               
               
                   
                 Sigmoid 
                 43.2 
                 16.3 
                 59.5 
                 &lt;70-75 Gy 
               
               
                   
               
            
           
         
       
     
     Optimized IMBT dose distribution metrics are summarized in Tables 5, 6 and 7. IMBT with Type-B shield and  192 Ir achieved a similar tumor and bladder dose to the conventional treatment plan. Rectum and sigmoid doses were approximately 10Gy 3  greater but are still below clinically acceptable dose constraints. IMBT with Type-B shield and  169 Yb achieved a similar tumor and bladder dose to the conventional treatment plan. Compared to the  192 Ir IMBT plan, the rectum and sigmoid received 7 Gy 3  less. The IMBT treatment plan using the Type-C shield and  169 Yb delivered approximately 2 Gy 10  more to the tumor while respecting dose constraints as shown in Table 5. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Total EQD2 doses for Type-A shield  192 Ir IMBT 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Pre 
                 Brachy 
                 Total 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 TOTAL 
                 Prescription 
                 44.3 
                 35.5 
                 79.8 
                   
               
               
                 EQD2 
                 D90 
                 44.3 
                 32.9 
                 77.2 
               
               
                 (Gy) 
                 Bladder 
                 43.2 
                 26.2 
                 69.4 
                   &lt;90 Gy 
               
               
                   
                 Rectum 
                 43.2 
                 26.9 
                 70.1 
                 &lt;70-75 Gy 
               
               
                   
                 Sigmoid 
                 43.2 
                 27.7 
                 70.9 
                 &lt;70-75 Gy 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Total EQD2 doses for Type-A shield  169 Yb IMBT 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Pre 
                 Brachy 
                 Total 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 TOTAL 
                 Prescription 
                 44.3 
                 35.5 
                 79.8 
                   
               
               
                 EQD2 
                 D90 
                 44.3 
                 32.9 
                 77.2 
               
               
                 (Gy) 
                 Bladder 
                 43.2 
                 20.1 
                 63.3 
                   &lt;90 Gy 
               
               
                   
                 Rectum 
                 43.2 
                 20.1 
                 63.3 
                 &lt;70-75 Gy 
               
               
                   
                 Sigmoid 
                 43.2 
                 19.2 
                 62.4 
                 &lt;70-75 Gy 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Total EQD2 doses for Type-C shield  169 Yb IMBT 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Pre 
                 Brachy 
                 Total 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 TOTAL 
                 Prescription 
                 44.3 
                 35.5 
                 79.8 
                   
               
               
                 EQD2 
                 D90 
                 44.3 
                 37.6 
                 81.9 
               
               
                 (Gy) 
                 Bladder 
                 43.2 
                 24.4 
                 67.6 
                   &lt;90 Gy 
               
               
                   
                 Rectum 
                 43.2 
                 24.4 
                 67.6 
                 &lt;70-75 Gy 
               
               
                   
                 Sigmoid 
                 43.2 
                 19.9 
                 63.1 
                 &lt;70-75 Gy 
               
               
                   
               
            
           
         
       
     
     Flexible Shields 
     Flexible/bendable metallic shields that can be placed inside applicators and rotated during the treatment with the shielded side toward the OARs and emission window towards the tumour are disclosed herein below. The metallic shields may be solid metal bars with cuts along its longitudinal axis enabling the metal bar to be flexible/bendable. The flexible shields disclosed herein below include shield sections that are secured to one another and that are pivotable one relative to the other while remaining attached to one another. Herein, “pivotable” implies that two consecutive section may become non-parallel one relative to the other such that a central axis of one of the two consecutive sections defines an angle with a central axis of the other of the two consecutive sections. The pivot motion is about an axis that may be normal to the central axis of the shield. 
     Referring to  FIGS. 11 a  to 11 d   , a plurality of flexible shields  400 ,  500 ,  600 ,  700  are shown. The shield  400  has a slit pattern  400   a  on its outer surface that may enable it to be bent/flexed. The slit patterns  400   a ,  600   a  may be similar to a jigsaw puzzle ( FIG. 11 a    and  FIG. 11 c   ). Alternatively, the slit patterns  500   a ,  700   a  may be a plurality of axially spaced apart slits being helicoid ( FIG. 11 b   ). Each slits is defined between two adjacent ones of shield sections  400   b ,  500   b ,  600   b  that are pivotable one relative to the other about an axis normal to the central axis A. 
     Referring now to  FIGS. 12 a -12 b   , another embodiment of a flexible shield is shown at  800 . The shield  800  extends along a central axis A and defines a groove  800   a  for receiving the radionuclide seed. The shield  800  is a monolithic body  800   e  having flexible section  800   f . The flexible section  800   f  includes a plurality of axially spaced apart slits  800   b  that extends from the outer surface  800   g  of the shield  800  toward a center thereof. Discs  800   c  are located between the slits  800   b  and are connected to one another via a core  800   d  of the shield  800 . As a cross-sectional area of the core  800   d  is less than that of the discs  800   c , the core  800   d  might allow the shield  800  to be bent. The spacing between the discs  800   c  allow for bending the core  800   d.    
     Referring now to  FIGS. 13 a -13 b   , another embodiment of a flexible shield is shown at  900 . The shield  900  includes a plurality of shield sections  902  that are interconnected to one another. Each of the shield sections  902  defines a groove  902   a  for receiving the radionuclide seed. The shield sections  902  defines two axial end faces, namely top axial end faces  902   b  and bottom axial end faces  902   c . Rounded cavities  902   d  extend from the top axial end faces toward the bottom axial end faces and balls  902   e  extend from the bottom axial end faces and away from the top axial end faces. The balls  902   e  are received within the rounded cavities  902   d . Articulations  904  are provided by a cooperation of the rounded cavities  902   d  and the balls  902   e . The articulations  904 , also referred to as flexible joints, allow the shield sections  902  to move one relative to the other to allow the shield  900  to bend. 
     In the embodiment shown, the axial end faces  902   b ,  902   c  slope toward one another away from the central axis A to avoid the top face of one sections  902  to abut against the bottom face of an adjacent section  902  before a suitable pivot angle between those two sections is achieved. 
     Referring now to  FIGS. 14 a -14 b   , another embodiment of a flexible shield is shown at  1000 . The shield  1000  includes a plurality of shield sections  1002  that are interconnected to one another. Each of the shield sections  1002  defines a groove  1002   a  for receiving the radionuclide seed. The shield sections  1002  defines two axial faces, namely bottom axial faces  1002   b  and top axial faces  1002   c . Recesses  1002   d  extend from the bottom axial faces toward the top axial faces and tabs  1002   e  extend from the top axial faces and away from the bottom axial faces. The tabs  1002   e  are received within the recesses  1002   d . Each of the shield sections  1002  defines an aperture  1002   f  for slidably receiving a pin  1002   g  therethrough. The apertures  1002   f  are in register with apertures  1002   h  through the tabs  1002   e  such that the pins  1002   g  extend through both of the apertures  1002   f ,  1002   h . The pins  1002   g  lock the tabs into the recesses. Articulations  1004  are provided by a cooperation of the recesses  1002   d , the tabs  1002   e , and the pins  1002   g . The pins  1002   g  define pivot axes P of two adjacent sections  1002  The articulations  1004 , also referred to as flexible joints, allow the shield sections  1002  to move one relative to the other to allow the shield  1000  to bend. 
     Referring now to  FIG. 15 , another embodiment of a flexible shield is shown at  1100 . The shield  1100  includes a plurality of interconnected shield sections  1102  each having a central aperture  1102   a  in register with one another. Each sections is cylindrically shaped. Flexible joints  1104  are defined using any suitable means to allow the shield sections  1102  to move one relative to the others. In the embodiment shown, a flexible rod  1106  extends through the apertures  1102   a  to connect the sections  1102  with one another and to allow each sections  1102  to pivot relative to its neighbouring sections  1102 . In the embodiment shown, the radionuclide-receiving passage and the flexible rod  1106  are concentric. The rod  1106  may be annular. The axial end faces of each sections  1102  are tapered to allow a pivot motion between each two adjacent ones of the sections  1102 . 
     Referring now to  FIG. 16 , another embodiment of a flexible shield is shown at  1200 . The shield  120  includes a plurality of interconnected shield sections  1202  each having an aperture  1202   a . Each sections  1202  is an half-cylinder. Flexible joints  1204  are defined using any suitable means to allow the shield sections  1202  to move one relative to the others. In the embodiment shown, a flexible rod  1206  extends through the apertures  1202   a  to connect the sections  1202  with one another and to allow each sections  1202  to pivot relative to its neighbouring sections  1202 . The axial end faces of each sections  1202  are tapered to allow a pivot motion between each two adjacent ones of the sections  1202 . 
     Referring now to  FIGS. 17 a  and 17 b   , example of possible cross-sections of the shield sections  1102  and  1202  of the shields  1100 ,  1200  are shown. Radionuclide-receiving passages C are defined by concave portions of the sections. An angle θ may be varied from 0 degree as shown in  FIG. 15  to 180 degrees as shown in  FIG. 16  and of about 45 to 90 degrees as shown in  FIGS. 17 a   ,  17   b.    
     In the present disclosure, the expression “about” means that a value may vary by plus or minus 10% of its value. For instance, a value of about 10 means that the value may range from 9 to 11. It will be appreciated that the dimensions disclosed herein are adjustable to cater to different applications. For instance, 1 to 3 mm may be added to the thickness of the shields, length can be chosen to cover the entire tandem for the applicator model used. For some dimensions, their value may be varied by plus or minus 1 to 2 mm. 
     The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.