bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐320: Dosimetric Evaluation of Intracranial Stereotactic Radiotherapy Plans Using Jaws‐Only Collimation On a LINAC
Authors: Mistry N , Kim A , Schaum J , Bermudez M , Driscoll K , Holowinski C , Yang C , Chen Y , Sheth N
Published: June 2015

Purpose:

To evaluate the dosimetry of cranial stereotactic radiotherapy (SRT) plans using jaws‐only collimation on linac that meets appropriate TG‐142 tolerances.

Methods:

Seventeen spherical targets were generated in the center of a head phantom with diameters ranging from 8 mm to 40 mm. Plans balanced treatment time with dose gradient and conformity using 13 static fields and 3 couch angles: 9 non‐opposed and coplanar fields and 4 non‐coplanar fields. The symmetrical jaws field size was target diameter plus 2 mm. The prescription (Rx) was 7 Gy per fraction to the 80% isodose line. Two plans were created for each target: one kept the collimator at 0° (C0), one adjusted the collimator angle 40° for each field to create a 360° sweep over the 9 coplanar fields (CR).Conformity of the Rx to the target was evaluated using a ratio of Rx to target volume (PITV). Heterogeneity was determined using a ratio of maximum dose to Rx dose. Falloff was scored using CGIg: the difference of effective radii of spheres equal to half and full Rx volumes.

Results:

All plans met RTOG SRS criteria for conformity and heterogeneity. The use of collimator rotation improved conformity by 3.2% on average, the mean PITV was 1.7±0.1 for C0 plans and 1.6±0.1 for CR. Mean heterogeneity was 1.25±0.0 for both C0 and CR. The mean CGIg was 75.9±16.4 for C0 plans and 74.4±17.0 for CR; with a mean dose falloff degradation of 2.5% by CR.

Conclusion:

Clinically acceptable SRT plans for spherical targets were created using jaws‐only collimation with static fields. The addition of sweeping collimator rotation improves conformity at the expense of gradient. This technique can expand the availability of SRT to patients especially to those who cannot travel to a facility with a dedicated stereotactic radiosurgery machine.
SU‐E‐T‐322: Dosimetric Evaluation of Rib Dose in Peripheral Lung Tumors Treated with X‐Ray Voxel Monte Carlo (XVMC) Based Lung Stereotactic Body Radiotherapy (SBRT) Following RTOG 0915 Guidelines
Authors: Pokhrel D , Sood S , Badkul R , Jiang H , Kumar P , Wang F
Published: June 2015

Purpose:

To evaluate XVMC computed rib doses for peripherally located non‐small‐cell‐lung tumors treated with SBRT following RTOG‐0915 guidelines.

Methods:

Twenty patients with solitary peripherally located non‐small‐cell‐lung tumors were treated using XVMC‐based SBRT to 50–54Gy in 5−3 fractions, respectively, for PTV(V100%)=95%. Based on 4D‐CT, ITV was delineated on MaximumIP images and organs‐at‐risk(OARs) including ribs were contoured on MeanIP images. Mean PTV(ITV+5mm uniform margin) was 46.1±38.7cc (range, 11.1–163.0cc). XVMC SBRT treatment plans were generated with a combination of non‐coplanar 3D‐conformal arcs/beams, and were delivered by Novalis‐TX consisting of HD‐MLCs and a 6MV‐SRS(1000MU/min) beam, following RTOG‐0915 criteria. XVMC rib maximum dose and dose to <1cc, <5cc, <10cc were evaluated as a function of PTV, prescription dose and 3D‐distance from tumor isocenter to the most proximal rib contour. Plans were re‐computed using heterogeneity‐corrected pencil‐beam (PB‐hete) algorithm utilizing identical beam geometry/MLC positions and MUs and subsequently compared to XVMC.

Results:

XVMC average maximum rib dose was 50.9±6.4Gy (range, 35.1–59.3Gy). XVMC mean rib dose to <1cc was 41.6±5.6Gy (range, 27.9–47.9Gy), <5cc was 31.2±7.3Gy (range, 10.6–43.1Gy), and <10cc was 21.2±8.7Gy (range, 1.1–36Gy), respectively. For the given prescription, correlation between PTV and rib doses to <5cc (p=0.005) and <10cc (p=0.018) was observed. 3D‐distance from the tumor isocenter to the proximal rib contour strongly correlated with maximum rib dose (p=0.0001). PB‐hete algorithm overestimated maximum rib dose and dose to <1cc, <5cc, and <10cc of ribs by 5%, 3%, 3%, and 3%, respectively.

Conclusion:

PB‐hete overestimates ribs dose relative to XVMC. Since all the clinical XVMC plans were generated without compromising the target coverage (per RTOG‐0915), almost all patient's ribs doses were higher than the protocol guidelines. As expected, larger tumor size and proximity to ribs received higher absolute dose to ribs. Prospective observation is needed to determine if XVMC delivered rib doses correlates with patient symptoms including chest wall pain and/or rib fractures.
SU‐E‐T‐325: Dosimetric Impact Due to FlexiShield in Electronic Brachytherapy (eBx) of Breast IORT: A Phantom Study
Authors: Kim Y , Huynh J , Ley M , Gonzalez V
Published: June 2015

Purpose:

To evaluate dosimetric change of eBx plan due to Flexishield.

Methods:

To simulate a clinically difficult case (skin spacing < 1 cm and touching chest wall), prostheses breast tissue phantom overlaid Xoft spherical balloon applicator. To minimize significant metal streak artifact, megavoltage CT (MVCT) scan was acquired using helical TomoTherapy HiART. Two sets of MVCT images were taken with/without FlexiShield for 15 cases: 4 for small (3–4 cm), 4 for medium (4–5 cm) and 7 for large (5–6 cm) balloon. Total 30 MVCT scans were obtained with 50 % contrast to improve image contrast of balloon relative to breast tissue phantom. Balloon deformation was measured in anterior‐posterior (AP) and lateral (LAT) dimensions in the middle of balloon. Skin spacing was also evaluated. Treatment plan was made based on each MVCT scan and two balloon surface doses (AP and LAT directions) and skin dose were compared between plans with/without FlexiShield.

Results:

The balloon was deformed due to pressure from both FlexiShield (skin side) and FlexiShield mini (chest wall side). Mean ± standard deviation (maximum) value was 1.5 ± 1.0 mm (3.3 mm) for AP compression and 0.4 ± 0.3 mm (1.1 mm) for LAT expansion. Balloon surface dose was increased by 1.8 ± 1.2 Gy (4.9 Gy) at AP point and decreased by 0.4 ± 0.4 Gy (1.3 Gy) at LAT point. Skin spacing was constantly reduced by 1.1 ± 0.8 mm (3 mm). Skin surface dose was increased by 1.5 ± 0.8 Gy (3.3 Gy) and its relative increase was 17.9 ± 9.3% (39.6%).

Conclusion:

FlexiShield deforms the balloon and reduces skin spacing, thereby resulting in higher dose in AP direction, lower dose in LAT direction, and elevated skin dose compared to the plan without FlexiShield. In the clinic, this balloon deformation and corresponding dose variation should be considered.
SU‐E‐T‐332: Dosimetric Impact of Photon Energy and Treatment Technique When Knowledge Based Auto‐Planning Is Implemented in Radiotherapy of Localized Prostate Cancer
Authors: Liu Z , Larsen E , Salmon H , Grow A , Hayes C , Balamucki C , Thompson M , Kennedy A
Published: June 2015

Purpose:

The aim of this study was to investigate the dosimetric impact of the combination of photon energy and treatment technique on radiotherapy of localized prostate cancer when knowledge based planning was used.

Methods:

A total of 16 patients with localized prostate cancer were retrospectively retrieved from database and used for this study. For each patient, four types of treatment plans with different combinations of photon energy (6X and 10X) and treatment techniques (7‐field IMRT and 2‐arc VMAT) were created using a prostate DVH estimation model in RapidPlan™ and Eclipse treatment planning system (Varian Medical System). For any beam arrangement, DVH objectives and weighting priorities were generated based on the geometric relationship between the OAR and PTV. Photon optimization algorithm was used for plan optimization and AAA algorithm was used for final dose calculation. Plans were evaluated in terms of the pre‐defined dosimetric endpoints for PTV, rectum, bladder, penile bulb, and femur heads. A Student's paired t‐test was used for statistical analysis and p > 0.05 was considered statistically significant.

Results:

For PTV, V95 was statistically similar among all four types of plans, though the mean dose of 10X plans was higher than that of 6X plans. VMAT plans showed higher heterogeneity index than IMRT plans. No statistically significant difference in dosimetry metrics was observed for rectum, bladder, and penile bulb among plan types. For left and right femur, VMAT plans had a higher mean dose than IMRT plans regardless of photon energy, whereas the maximum dose was similar.

Conclusion:

Overall, the dosimetric endpoints were similar regardless of photon energy and treatment techniques when knowledge based auto planning was used. Given the similarity in dosimetry metrics of rectum, bladder, and penile bulb, the genitourinary and gastrointestinal toxicities should be comparable among the selections of photon energy and treatment techniques.
SU‐E‐T‐335: Dosimetric Investigation of An Advanced Rotating Gamma Ray System for Imaged Guided Radiation Therapy
Authors: Ma C , Eldib A , Li C , Chibani O , Mora G , Li J , Chen L
Published: June 2015

Purpose:

Co‐60 beams have unique dosimetric properties for cranial treatments and thoracic cancers. The conventional concern about the high surface dose is overcome by modern system designs with rotational treatment techniques. This work investigates a novel rotational Gamma ray system for image‐guided, external beam radiotherapy.

Methods:

The CybeRT system (Cyber Medical Corp., China) consists of a ring gantry with either one or two treatment heads containing a Gamma source and a multileaf collimator (MLC). The MLC has 60 paired leaves, and the maximum field size is either 40cmx40cm (40 pairs of 0.5cm central leaves, 20 pairs of 1cm outer leaves), or 22cmx40cm (32 pairs of 0.25cm central leaves, 28 pairs of 0.5cm outer leaves). The treatment head(s) can swing 35° superiorly and 8° inferiorly, allowing a total of 43° non‐coplanar beam incident. The treatment couch provides 6‐degrees‐of‐freedom motion compensation and the kV cone‐beam CT system has a spatial resolution of 0.4mm. Monte Carlo simulations were used to compute dose distributions and compare with measurements. A retrospective study of 98 previously treated patients was performed to compare CybeRT with existing RT systems.

Results:

Monte Carlo results confirmed the CybeRT design parameters including output factors and 3D dose distributions. Its beam penumbra/dose gradient was similar to or better than that of 6MV photon beams and its isocenter accuracy is 0.3mm. Co‐60 beams produce lower‐energy secondary electrons that exhibit better dose properties in low‐density lung tissues. Because of their rapid depth dose falloff, Co‐60 beams are favorable for peripheral lung tumors with half‐arc arrangements to spare the opposite lung and critical structures. Superior dose distributions were obtained for head and neck, breast, spine and lung tumors.

Conclusion:

Because of its accurate dose delivery and unique dosimetric properties of C‐60 sources, CybeRT is ideally suited for advanced SBRT as well as conventional RT.

This work was partially supported by Cyber Medical Corp.
SU‐E‐T‐340: Dosimetry of a Small Field Electron Beam for Innovative Radiotherapy of Small Surface Or Internal Tumors
Authors: Reft C , Lu Z , Noonan J
Published: June 2015

Purpose:

An innovative small high intensity electron beams with energies from 6 to 12 MeV is being developed at Argonne National Laboratory to deliver an absorbed dose via a catheter to small malignant and nonmalignant lesions. This study reports on the initial dosimetric characteristics of this electron beam. These include output calibration, percent depth dose, beam profiles and leakage through the catheter.

Methods:

To simulate the narrow electron beam, the Argonne Wakefield Accelerator is used to produce high energy electron beams. The electron beam from the accelerator is monitored by measuring the current through a transmission coil while the beam shape is observed with a fluorescent screen. The dosimetry properties of the electron beam transmitting through bone and tissue‐like materials are measured with nanodot optically stimulated luminescent dosimeters and EDR radiographic film. The 6 MV photon beam from a Varian True beam linac is used to calibrate both the OSLDs and the film.

Results:

The beam characteristics of the 12 MeV beam were measured. The properties of the small diameter, 5 mm, beam differs from that of broad clinical electron beams from radiotherapy linacs. Due to the lack of scatter from the narrow beam, the maximum dose is at the surface and the depth of the 50% depth dose is 35 mm compared to 51 mm for a clinical 12 MeV. The widths of the 90% isodose measured at the surface and depths of 2, 6, 12, and 16 mm varied from 6.6 to 8.8 mm while the widths of the FWHM isodose varied from 7.8 to 25.5 mm.

Conclusion:

Initial beam measurements show favorable dosimetric properties for its use in treating either small surface or internal lesions, particularly to deliver radiation at the time of surgery to maximize the dose to the lesion and spare normal tissue.
SU‐E‐T‐341: DVH‐Based Comparison of True 3D Measurements to a Delta4 System
Authors: Crockett E , Oldham M , Ren L
Published: June 2015

Purpose:

Delta4 dosimetric software can be used to calculate DVH‐based metrics for patient‐specific quality assurance from measurements made by a Delta4 QA device. This study investigates the effectiveness of a novel transform method that transposes measurements made with a full‐density 3D dosimeter onto patient anatomy, enabling the calculation of DVHs. This allows for DVH comparisons from the transformed dose distribution, which are based on true 3D measurements, to those from the Delta4 system, which are based on semi‐3D measurements and interpolation.

Methods:

A double‐arc VMAT treatment for a head‐and‐neck case was delivered to a 1kg PRESAGE 3D dosimeter inserted into a polyurethane head phantom. The dosimeter was readout using an in‐house optical‐CT scanner to gather full‐density 3D dosimetric data. The transform method is achieved by multiplication of the measured doses with a “transformation matrix” which accounts for heterogeneities and differences in geometry between the patient and the phantom. The transformation matrix is a voxel‐by‐voxel division of the patient planned dose by the phantom planned dose, both calculated in the treatment planning system (Eclipse). The transformed distribution was then overlaid on the patient CT data, enabling the calculation of DVHs. The same VMAT treatment was delivered to the Delta4 phantom and DVH data was calculated using its associated software.

Results:

The transformed dose distribution showed good agreement with calculated patient values, determined by similarity in dose profiles between the two distributions and a 3D gamma index passing rate of 94.87% for 3%/3mm criteria. For every structure contained within the dosimeter volume, the transformed DVHs demonstrated better agreement than the Delta4 DVHs, when compared to the values calculated in the treatment planning system.

Conclusion:

The coupled technique of full‐density 3D dose measurements and the presented transform method enables clinical patient‐specific quality assurance data that is more accurate than the semi‐3D Delta4 system.

This work was supported by NIH R01CA100835.
SU‐E‐T‐343: Dynamic Collimator Rotation for Volumetric Modulated Arc Therapy
Authors: Fix MK , Frei D , Volken W , Terribilini D , Manser P
Published: June 2015

Purpose:

Volumetric modulated arc therapy (VMAT) utilizes dynamic gantry rotation, dynamic MLC and varying dose rate. However, in addition the collimator angle could be changed dynamically, thus, increasing the degrees of freedom for the optimization, which might lead to improved dose distributions. This work investigates the feasibility of including a dynamic collimator rotation for VMAT.

Methods:

In this work a 20×20×20 cm3 water phantom with a cigar shaped target volume and a spherical shaped critical structure was used. By means of the Eclipse Research Scripting a predefined collimator rotation was included in a not yet optimized treatment plan, consisting of one partial arc, by assigning for each dicom control point a different collimator angle. Thereby the collimator rotation takes into account the physical limitations for the delivery. This treatment plan was then imported into Eclipse using the Eclipse Research Scripting interface and VMAT PRO3 optimization was performed in a research version of Eclipse. For the dose calculation of the optimized treatment plan the Swiss Monte Carlo Plan (SMCP) was used. Similarly, a dose distribution was determined using a static collimator angle as applied in VMAT. The resulting DVHs for the target and the critical structure were compared for the two treatment plans.

Results:

A VMAT treatment plan with dynamically moving collimator angle was successfully generated. The comparison of the DVHs for the target volume shows a slight improvement of the coverage and dose homogeneity for the treatment plan using dynamic collimator rotation. Additionally, the dose to the critical structure could be reduced when using the moving collimator rotation.

Conclusion:

The usage of dynamic collimator rotation for VMAT is feasible and has the potential to improve the dose distribution for the target while reducing the dose to critical structures. This work was supported by Varian Medical Systems.

This work was supported by Varian Medical Systems.
SU‐E‐T‐344: Dynamic Electron Beam Therapy Using Multiple Apertures in a Single Cut‐Out
Authors: Rodrigues A , Yin F , Wu Q
Published: June 2015

Purpose:

Few leaf electron collimators (FLEC) or electron MLCs (eMLC) are highly desirable for dynamic electron beam therapies as they produce multiple apertures within a single delivery to achieve conformal dose distributions. However, their clinical implementation has been challenging. Alternatively, multiple small apertures in a single cut‐out with variable jaw sizes could be utilized in a single dynamic delivery. In this study, we investigate dosimetric characteristics of such arrangement.

Methods:

Monte Carlo (EGSnrc/BEAMnrc/DOSXYnrc) simulations utilized validated Varian TrueBeam phase spaces. Investigated quantities included: Energy (6 MeV), jaw size (1×1 to 22×22 cm 2; centered to aperture), applicator/cut‐out (15×15 cm2), aperture (1×1, 2×2, 3×3, 4×4 cm2), and aperture placement (on/off central axis). Three configurations were assessed: (1) single aperture on‐axis, (2) single aperture off‐axis, and (3) multiple apertures. Reference was configuration (1) with standard jaw size. Aperture placement and jaw size were optimized to maintain reference dosimetry and minimize leakage through unused apertures to <5%. Comparison metrics included depth dose and orthogonal profiles.

Results:

Configuration (1) and (2): Jaw openings were reduced to 10×10 cm2 without affecting dosimetry (gamma 2%/1mm) regardless of on‐ or off‐axis placement. For smaller jaw sizes, reduced surface (<2%, 5% for 1×1 cm2 aperture) and increased Bremsstrahlung (<2%, 10% for 1×1 cm2 aperture) dose was observed. Configuration (3): Optimal aperture placement was in the corners (order: 1×1, 4×4, 2×2, 3×3 cm2) and jaw sizes were 4×4, 4×4, 7×7, and 5×5 cm2 (apertures: 1×1, 2×2, 3×3, 4×4 cm2 ). Asymmetric leakage was found from upper and lower jaws. Leakage was generally within 5% with a maximum of 10% observed for the 1×1 cm2 aperture irradiation.

Conclusion:

Multiple apertures in a single cut‐out with variable jaw size can be used in a single dynamic delivery, providing a practical alternative to FLEC or eMLC. Future simulations will expand on all variables.
SU‐E‐T‐347: Effect of MLC Leaf Position Inaccuracy On Dose Distribution for Spinal SBRT with Different Energies and Dose Rates
Authors: You T , Dang J , Dai C , Yin F
Published: June 2015

Purpose:

To evaluate dosimetric impact of spinal SBRT when MLC leaf positions deviate from planning positions for different energies and doserates.

Methods and Materials:

18 localized spinal metastases patients were selected for SBRT using IMRT planning with 9 posterior beams delivered at gantry angles ranging between 100°–260°. A modern linear accelerator(Varian Turebeam STX with HDMLC 2.5 mm thick leaf at isocenter) IMRT plans were generated using both 6X and 6X_FFF(Flattening filter free) beams with a nominal prescription dose of 6 Gy/fraction to PTV. Doserates ranging from 200–600 MU/min for 6X and 400–1400 MU/min for 6X_FFF, with 200 increments were examined. A fixed amount(0.3, 0.5, 1, and 2 mm) of MLC‐leaf position deviation was simulated to each plan under following conditions: 1)only along X1 collimator; 2)with increments at both X1 and X2 collimator directions;3)with reductions at both X1 and X2 collimator directions. Dose was recalculated for each modified plans. Both original and modified plans were delivered using Turebeam STX machine and measured using both portal dosimetry and a 3D dosimeter(Delta4 of ScandiDos). Each field's Result were compared using following three parameters: the 95% iso‐dose level Conformal Index(95%CI), the spinal cord maximum dose(SCDmax), and the planned target volume(PTV) mean dose.

Results:

Dosimetric impacts on the 95%CI, SCDmax and the PTV mean dose are: 1)negligible if MLC‐leaf position deviation only along a single collimator direction ≥1.0 mm,2)substantial if MLC‐leaf position increment along both collimator directions ≥0.3 mm(95% CI decreases while SCDmax and PTV mean‐dose increase), 3)substantial if MLC‐leaf position reduction along both collimator directions ≥0.3 mm(95% CI first increases and then decreases while SCDmax and PTV mean‐dose decrease). Different energies and doserates demonstrated comparable dosimetric impacts.

Conclusion:

Substantial dose deviations could happen for spinal SBRT using IMRT plan with HD‐MLC if leaf position deviation ≥0.3 mm. The effects of different energy and doserate are negligible.
SU‐E‐T‐348: Effect of Treatment Table and Immobilization Devices On Surface Dose When Using a GRID Technique
Authors: Gajdos S , Donaghue J
Published: June 2015

Purpose:

To determine the increase of surface dose of MLC‐designed GRID therapy in the presence of immobilization devices and treatment table.

Methods:

To create a GRID field, our facility utilizes an MLC consisting of four millimeter wide leaves. The field is designed to have aperture sizes of 0.8 cm X 0.8 cm with inter‐aperture distance of 3.2 cm. Gafchromic EBT3 film was placed between the surface of a solid water phantom and the immobilization device. The treatment table was also present within the beam path. The devices consist of carbon fiber exterior shell. A piece of film was also placed at maximal depth for the photon energy of 10 MV. Image files were converted to dose per a calibration curve based on the selected red channel. The surface dose to maximum dose was established by comparing the ratio of seven centrally located aperture regions‐of‐interest and four adjacent inter‐aperture regions‐of‐interest were measured with the available software tools.

Results:

With no devices present in beam path, the ratio of surface dose to maximum dose was 11.5% ± 0.3% for aperture region and 7.0% ± 0.1% for inter‐aperture region. When devices are present, the ratio of surface dose to maximum dose was 45.2% ± 0.5% and 33.8% ± 1.1%, respectively. Due to the presence of devices, the surface dose increases in aperture region by 3.8 times or in the inter‐aperture region by 4.7 times.

Conclusion:

The purpose of using GRID technique is to deliver a single fractional dose in range of 15–20 Gy to a bulky lesion while also preserving skin tolerance. The increase of surface dose due to devices placed in beam path may increase the chance of skin toxicity in GRID therapy. Care should be used to determine best manageable patient immobilization while considering skin dose especially for posteriorly located lesions.
SU‐E‐T‐356: Efficient Segmentation of Flattening Filter Free Photon Beamsfor 3D‐Conformal SBRT Treatment Planning
Authors: Barbiere J , Beninati G , Ndlovu A
Published: June 2015

Purpose:

It has been argued that a 3D‐conformal technique (3DCRT) is suitable for SBRT due to its simplicity for non‐coplanar planning and delivery. It has also been hypothesized that a high dose delivered in a short time can enhance indirect cell death due to vascular damage as well as limiting intrafraction motion. Flattening Filter Free (FFF) photon beams are ideal for high dose rate treatment but their conical profiles are not ideal for 3DCRT. The purpose of our work is to present a method to efficiently segment an FFF beam for standard 3DCRT planning.

Methods:

A 10×10 cm Varian True Beam 6X FFF beam profile was analyzed using segmentation theory to determine the optimum segmentation intensity required to create an 8 cm uniform dose profile. Two segments were automatically created in sequence with a Varian Eclipse treatment planning system by converting isodoses corresponding to the calculated segmentation intensity to contours and applying the “fit and shield” tool. All segments were then added to the FFF beam to create a single merged field. Field blocking can be incorporated but was not used for clarity.

Results:

Calculation of the segmentation intensity using an algorithm originally proposed by Xia and Verhey indicated that each segment should extend to the 92% isodose. The original FFF beam with 100% at the isocenter at a depth of 10 cm was reduced to 80% at 4cm from the isocenter; the segmented beam had +/−2.5 % uniformity up to 4.4cm from the isocenter. An additional benefit of our method is a 50% decrease in the 80%‐20% penumbra of 0.6cm compared to 1.2cm in the original FFF beam.

Conclusion:

Creation of two optimum segments can flatten a FFF beam and also reduce its penumbra for clinical 3DCRT SBRT treatment.
SU‐E‐T‐360: End‐To‐End Dosimetric Testing of a Versa HD Linear Accelerator with the Agility Head Modeled in Pinnacle3
Authors: Saenz D , Narayanasamy G , Cruz W , Papanikolaou N , Stathakis S
Published: June 2015

Purpose:

The Versa HD incorporates a variety of upgrades, primarily including the Agility head. The distinct dosimetric properties of the head from its predecessors combined with flattening‐filter‐free (FFF) beams require a new investigation of modeling in planning systems and verification of modeling accuracy.

Methods:

A model was created in Pinnacle3 v9.8 with commissioned beam data. Leaf transmission was modeled as <0.5% with maximum leaf speed of 3 cm/s. Photon spectra were tuned for FFF beams, for which profiles were modeled with arbitrary profiles rather than with cones. For verification, a variety of plans with varied parameters were devised, and point dose measurements were compared to calculated values. A phantom of several plastic water and Styrofoam slabs was scanned and imported into Pinnacle3. Beams of different field sizes, SSD, wedges, and gantry angles were created. All available photon energies (6 MV, 10 MV, 18 MV, 6 FFF, 10 FFF) as well four clinical electron energies (6, 9, 12, and 15 MeV) were investigated. The plans were verified at a calculation point (8 cm deep for photons, variable for electrons) by measurement with a PTW Semiflex ionization chamber. In addition, IMRT testing was performed with three standard plans (step and shoot IMRT, small and large field VMAT plans). The plans were delivered on the Delta4 IMRT QA phantom (ScandiDos, Uppsala, Sweden).

Results:

Homogeneous point dose measurement agreed within 2% for all photon and electron beams. Open field photon measurements along the central axis at 100 cm SSD passed within 1%. Gamma passing rates were >99.5% for all plans with a 3%/3mm tolerance criteria. The IMRT QA results for the first 23 patients yielded gamma passing rates of 97.4±2.3%.

Conclusion:

The end‐to‐end testing ensured confidence in the ability of Pinnacle3 to model photon and electron beams with the Agility head.
SU‐E‐T‐362: Enhanced Dynamic Wedge Output Factors for Varian 2300CD and the Case for a Reference Database
Authors: Njeh C
Published: June 2015

Purpose:

Dose inhomogeneity in treatment planning can be compensated using physical wedges. Enhanced dynamic wedges (EDW) were introduced by Varian to overcome some of the short comings of physical wedges. The objectives of this study were to measure EDW output factors for 6 MV and 20 MV photon energies for a Varian 2300CD. Secondly to review the literature in terms of published enhanced dynamic wedge output factors (EDWOF) for different Varian models and thereby adding credence to the case of the validity of reference databases.

Methods:

The enhanced dynamic wedge output factors were measured for the Varian 2300CD for both 6 MV and 20 MV photon energies. Twelve papers with published EDWOF for different Varian Linac models were found in the literature.

Results:

The EDWOF for 6 MV varied from 0.980 for a 5×5 cm 10 degree wedge to 0.424 for 20×20 cm 60 degree wedge. Similarly for 20 MV, the EDWOF varied from 0.986 for 5×5 cm 10 degree wedge to 0.529 for 20×20 cm 60 degree wedge. EDWOF are highly dependent on field size. Comparing our results with the published mean, we found an excellent agreement for 6 MV EDWOF with the percentage differences ranging from 0.01% to 0.57% with a mean of 0.03%. The coefficient of variation of published EDWOF ranged from 0.17% to 0.85% and 0.1% to 0.9% for the for 6 MV and 18MV photon energies respectively. This paper provides the first published EDWOF for 20 MV photon energy. In addition, we have provided the first compendium of EDWOFs for different Varian linac models.

Conclusion:

The consistency of EDWOF across models and institution provide further support that, a standard data set of basic photon and electron dosimetry could be established, as a guide for future commissioning, beam modeling and quality assurance purposes.
SU‐E‐T‐373: Evaluation and Reduction of Contralateral Skin /subcutaneous Dose for Tangential Breast Irradiation
Authors: Butson M , Carroll S , Whitaker M , Odgers D , Martin D , Hinds S , Kader J , Ho K , Amos S , Toohey J
Published: June 2015

Purpose:

Tangential breast irradiation is a standard treatment technique for breast cancer therapy. One aspect of dose delivery includes dose delivered to the skin caused by electron contamination. This effect is especially important for highly oblique beams used on the medical tangent where the electron contamination deposits dose on the contralateral breast side. This work aims to investigate and predict as well as define a method to reduce this dose during tangential breast radiotherapy.

Methods:

Analysis and calculation of breast skin and subcutaneous dose is performed using a Varian Eclipse planning system, AAA algorithm for 6MV x‐ray treatments. Measurements were made using EBT3 Gafchromic film to verify the accuracy of planning data. Various materials were tested to assess their ability to remove electron contamination on the contralateral breast.

Results:

Results showed that the Varian Eclipse AAA algorithm could accurately estimate contralateral breast dose in the build‐up region at depths of 2mm or deeper. Surface dose was underestimated by the AAA algorithm. Doses up to 12% of applied dose were seen on the contralateral breast surface and up to 9 % at 2mm depth. Due to the nature of this radiation, being mainly low energy electron contamination, a bolus material could be used to reduce this dose to less than 3%. This is accomplished by 10 mm of superflab bolus or by 1 mm of lead.

Conclusion:

Contralateral breast skin and subcutaneous dose is present for tangential breast treatment and has been measured to be up to 12% of applied dose from the medial tangent beam. This dose is deposited at shallow depths and is accurately calculated by the Eclipse AAA algorithm at depths of 2mm or greater. Bolus material placed over the contralateral can be used to effectively reduce this skin dose.
SU‐E‐T‐376: Evaluation of a New Stereotactic Diode for Small Field Dosimetry
Authors: Kralik J , Kosterin P , Mooij R , Solberg T
Published: June 2015

Purpose:

To evaluate the performance of a new stereotactic diode for dosimetry of small photon fields.

Methods:

A new stereotactic diode, consisting of an unshielded p‐type silicon chip, and with improved radiation hardness energy dependence was recently developed (IBA Dosimetry, Schwarzenbruch, Germany). The diode has an active volume of 0.6 mm dia. × 0.02 mm thick. Two new diodes were evaluated, one which was pre‐irradiated to 100kGy with 10 MeV electrons and another which received no prior irradiation. Sensitivity, stability, reproducibility, and linearity as a function of dose were assessed. Beam profiles and small field output factors were measured on a CyberKnife (CK) and compared with measurements using two commercially available diodes.

Results:

The new diodes exhibit linear behavior (within 0.6%) over a dose range 0.02 – 50 Gy; a commercially available device exhibits excursions of up to 4% over the same range. The sensitivity is 4.1 and 3.8 nC/Gy for the un‐irradiated and pre‐irradiated diodes, respectively. When irradiated with 150 Gy in dose increments of 5, 20 and 35 Gy, both new diodes provide a stable response within 0.5%. Output factors measured with the two new diodes are identical and compare favorably with other commercially available diodes and published data. Similarly, no differences in measured field size or penumbra were observed among the devices tested.

Conclusion:

The new diodes show excellent stability and sensitivity. The beam characterization in terms of output factors and beam profiles is consistent with that obtained with commercially available diodes.
SU‐E‐T‐378: Evaluation of An Analytical Model for the Inter‐Seed Attenuation Effect in 103‐Pd Multi‐Seed Implant Brachytherapy
Authors: Safigholi H , Meigooni A Soleimani , Han D , Soliman A , Song W
Published: June 2015

Purpose:

Brachytherapy treatment planning systems based on TG‐43 protocol calculate the dose in water and neglects the heterogeneity effect of seeds in multi‐seed implant brachytherapy. In this research, the accuracy of a novel analytical model that we propose for the inter‐seed attenuation effect (ISA) for 103‐Pd seed model is evaluated.

Methods:

In the analytical model, dose perturbation due to the ISA effect for each seed in an LDR multi‐seed implant for 103‐Pd is calculated by assuming that the seed of interest is active and the other surrounding seeds are inactive. The cumulative dosimetric effect of all seeds is then summed using the superposition principle. The model is based on pre Monte Carlo (MC) simulated 3D kernels of the dose perturbations caused by the ISA effect. The cumulative ISA effect due to multiple surrounding seeds is obtained by a simple multiplication of the individual ISA effect by each seed, the effect of which is determined by the distance from the seed of interest. This novel algorithm is then compared with full MC water‐based simulations (FMCW).

Results:

The results show that the dose perturbation model we propose is in excellent agreement with the FMCW values for a case with three seeds separated by 1 cm. The average difference of the model and the FMCW simulations was less than 8%±2%.

Conclusion:

Using the proposed novel analytical ISA effect model, one could expedite the corrections due to the ISA dose perturbation effects during permanent seed 103‐Pd brachytherapy planning with minimal increase in time since the model is based on multiplications and superposition. This model can be applied, in principle, to any other brachytherapy seeds. Further work is necessary to validate this model on a more complicated geometry as well.
SU‐E‐T‐380: Evaluation of BEBIG HDR 60Co System for AccuBoost Therapy
Authors: Zehtabian M , Sina S , Rivard M , Meigooni A Soleimani
Published: June 2015

Purpose:

In this project, the possibility of utilizing the BEBIG 60Co HDR system for AccuBoostTM treatment has been evaluated.

Methods:

Dose distributions in various breast sizes have been calculated for both Co‐60 and Ir‐192 sources using the MCNP5 code. These calculations were performed in breast tissues with thicknesses of 4cm, 6cm, and 8cm. The initial calculations were performed with the same applicator dimensions as the existing applicators used with the HDR Ir‐192 system. The activity of the Co‐60 source was selected such that the dose at the breast center was the same as the values from 192Ir. Then, the applicator thicknesses were increased to twice of those used with HDR Ir‐192 system, for reducing skin and chest doses by Co‐60 system. Dose to breast skin and chest wall were compared for both applicators types, with and without inclusion of a focusing cone at the applicator center.

Results:

The results showed that loading HDR Co‐60 source inside the thin applicators impose higher doses to breast skin and chest wall compared to the 192Ir source. The area of the chest wall covered by 10Gy when treated by Co‐60 with the thin and thick applicators, or treated by Ir‐192 with thin applicator are 79cm2, 39cm2, and 3.8cm2, respectively. These values are reduced to 34cm2, 0cm2, and 0cm2 by using the focusing cone. It is worth noting that the breast skin areas covered by the 60Gy isodose line are 9.9cm2 and 7.8cm2 for Co‐60 with the thin and thick applicators, respectively, while it is 20cm2 for Ir‐192 when no focusing cone is present. These values are 0cm2, 0cm2, and 11cm2 in the presence of the focusing cone.

Conclusion:

The results indicate that using Co‐60 with the thicker applicators is beneficial because of the higher half‐life of Co‐60, and the reduced maximum skin dose when compared with Ir‐192.
SU‐E‐T‐381: Evaluation of Calculated Dose Accuracy for Organs‐At‐Risk Located at Out‐Of‐Field in a Commercial Treatment Planning System for High Energy Photon Beams Produced From TrueBeam Accelerators
Authors: Wang L , Ding G
Published: June 2015

Purpose:

Dose calculation accuracy for the out‐of‐field dose is important for predicting the dose to the organs‐at‐risk when they are located outside primary beams. The investigations on evaluating the calculation accuracy of treatment planning systems (TPS) on out‐of‐field dose in existing publications have focused on low energy (6MV) photon. This study evaluates out‐of‐field dose calculation accuracy of AAA algorithm for 15MV high energy photon beams.

Methods:

We used the EGSnrc Monte Carlo (MC) codes to evaluate the AAA algorithm in Varian Eclipse TPS (v.11). The incident beams start with validated Varian phase‐space sources for a TrueBeam linac equipped with Millennium 120 MLC. Dose comparisons between using AAA and MC for CT based realistic patient treatment plans using VMAT techniques for prostate and lung were performed and uncertainties of organ dose predicted by AAA at out‐of‐field location were evaluated.

Results:

The results show that AAA calculations under‐estimate doses at the dose level of 1% (or less) of prescribed dose for CT based patient treatment plans using VMAT techniques. In regions where dose is only 1% of prescribed dose, although AAA under‐estimates the out‐of‐field dose by 30% relative to the local dose, it is only about 0.3% of prescribed dose. For example, the uncertainties of calculated organ dose to liver or kidney that is located out‐of‐field is <0.3% of prescribed dose.

Conclusion:

For 15MV high energy photon beams, very good agreements (<1%) in calculating dose distributions were obtained between AAA and MC. The uncertainty of out‐of‐field dose calculations predicted by the AAA algorithm for realistic patient VMAT plans is <0.3% of prescribed dose in regions where the dose relative to the prescribed dose is <1%, although the uncertainties can be much larger relative to local doses. For organs‐at‐risk located at out‐of‐field, the error of dose predicted by Eclipse using AAA is negligible.

This work was conducted in part using the resources of Varian research grant VUMC40590‐R.
SU‐E‐T‐383: Evaluation of Deep Inspiration Breath‐Hold Technique for Post‐Mastectomy Proton Pencil Beam Scanning Therapy
Authors: Depauw N , Patel S , MacDonald S , Lu H
Published: June 2015

Purpose:

Deep inspiration breath‐hold techniques (DIBH) have been shown to carry significant dosimetric advantages in conventional radiotherapy of left‐sided breast cancer. The purpose of this study is to evaluate the use of DIBH techniques for post‐mastectomy radiation therapy (PMRT) using proton pencil beam scanning (PBS).

Method:

Ten PMRT patients, with or without breast implant, underwent two helical CT scans: one with free breathing and the other with deep inspiration breath‐hold. A prescription of 50.4 Gy(RBE) to the whole chest wall and lymphatics (axillary, supraclavicular, and intramammary nodes) was considered. PBS plans were generated for each patient's CT scan using Astroid, an in‐house treatment planning system, with the institution conventional clinical PMRT parameters; that is, using a single en‐face field with a spot size varying from 8 mm to 14 mm as a function of energy. Similar optimization parameters were used in both plans in order to ensure appropriate comparison.

Results:

Regardless of the technique (free breathing or DIBH), the generated plans were well within clinical acceptability. DIBH allowed for higher target coverage with better sparing of the cardiac structures. The lung doses were also slightly improved. While the use of DIBH techniques might be of interest, it is technically challenging as it would require a fast PBS delivery, as well as the synchronization of the beam delivery with a gating system, both of which are not currently available at the institution.

Conclusion:

DIBH techniques display some dosimetric advantages over free breathing treatment for PBS PMRT patients, which warrants further investigation. Plans will also be generated with smaller spot sizes (2.5 mm to 5.5 mm and 5 mm to 9 mm), corresponding to new generation machines, in order to further quantify the dosimetric advantages of DIBH as a function of spot size.
SU‐E‐T‐387: Evaluation of Effective Treatment Depth in Skin Cancer Treatments with Xoft Electronic Brachytherapy
Authors: Dragojevic I , Hoisak J
Published: June 2015

Purpose:

To evaluate changes in the percent depth dose (PDD) and effective depth of treatment based on exerted force by applicator on the skin during treatments of skin cancer with Xoft Electronic Brachytherapy.

Methods:

To simulate compressible tissue, 5mm tissue‐equivalent bolus was used. An ion chamber (Soft X‐ray Chamber, PTW) and electrometer (Max 4000, Standard Imaging) were used for output measurements. Measurements were done for all available Xoft surface applicators (10, 20, 35, and 50mm cones) with plastic endcap. Fig1 shows the experimental setup. The PDD was measured first with no or minimal pressure of the applicator on the bolus, followed by increasing uniform pressure on the applicator applied with custom cerrobend weights. The measurements were used to calculate the effective PDD and effective depth.

Results:

Force applied with the applicator was plotted against the change in PDD relative to the PDD when no force is applied. For the 10mm cone, moderate force of 5N can change the PDD by more than 20%, (Fig2). The effect is also pronounced for the 20mm cone, while it is minimal for the 35 and 50mm cones. Even when only moderate force is applied, the effective prescription depth can be changed by a several millimeters, which is on the order of the typical prescription depth (Fig3).

Conclusion:

Based on the results of this simulation, excessive pressure applied on the patient's skin by the applicator cone can drastically alter the PDD and effective treatment depth. The effect is most pronounced for the 10mm cone, and to a lesser extent, 20mm, which is significant as these cones tend to be used most frequently in the clinic. Applicator placement therefore may Result in significant consequences such as excessive dose to target, severe skin reaction, permanent discoloration, skin indentation, and poor overall cosmesis upon completion of treatment.
SU‐E‐T‐399: Evaluation of Selection Criteria for Computational Human Phantoms for Use in Out‐Of‐Field Organ Dosimetry for Radiotherapy Patients
Authors: Pelletier C , Jung J , Lee C , Pyakuryal A , Kim J , Lee C
Published: June 2015

Purpose:

To quantify the dosimetric uncertainty due to organ position errors when using height and weight as phantom selection criteria in the UF/NCI Hybrid Phantom Library for the purpose of out‐of‐field organ dose reconstruction.

Methods:

Four diagnostic patient CT images were used to create 7‐field IMRT plans. For each patient, dose to the liver, right lung, and left lung were calculated using the XVMC Monte Carlo code. These doses were taken to be the ground truth. For each patient, the phantom with the most closely matching height and weight was selected from the body size dependent phantom library. The patient plans were then transferred to the computational phantoms and organ doses were recalculated. Each plan was also run on 4 additional phantoms with reference heights and or weights. Maximum and mean doses for the three organs were computed, and the DVHs were extracted and compared. One sample t‐tests were performed to compare the accuracy of the height and weight matched phantoms against the additional phantoms in regards to both maximum and mean dose.

Results:

For one of the patients, the height and weight matched phantom yielded the most accurate results across all three organs for both maximum and mean doses. For two additional patients, the matched phantom yielded the best match for one organ only. In 13 of the 24 cases, the matched phantom yielded better results than the average of the other four phantoms, though the results were only statistically significant at the .05 level for three cases.

Conclusion:

Using height and weight matched phantoms does yield better results in regards to out‐of‐field dosimetry than using average phantoms. Height and weight appear to be moderately good selection criteria, though this selection criteria failed to yield any better results for one patient.
SU‐E‐T‐400: Evaluation of Shielding and Activation at Two Pencil Beam Scanning Proton Facilities
Authors: Remmes N , Mundy D , Stoker J , Classic K , Nelson K , Beltran C , Bues M , Kruse J , Herman M
Published: June 2015

Purpose:

To verify acceptably low dose levels around two newly constructed identical pencil beam scanning proton therapy facilities and to evaluate accuracy of pre‐construction shielding calculations.

Methods:

Dose measurements were taken at select points of interest using a WENDI‐2 style wide‐energy neutron detector. Measurements were compared to pre‐construction shielding calculations. Radiation badges with neutron dose measurement capabilities were worn by personnel and also placed at points throughout the facilities. Seven neutron and gamma detectors were permanently installed throughout the facility, continuously logging data. Potential activation hazards have also been investigated. Dose rates near water tanks immediately after prolonged irradiation have been measured. Equipment inside the treatment room and accelerator vault has been surveyed and/or wipe tested. Air filters from air handling units, sticky mats placed outside of the accelerator vault, and water samples from the magnet cooling water loops have also been tested.

Results:

All radiation badges have been returned with readings below the reporting minimum. Measurements of mats, air filters, cooling water, wipe tests and surveys of equipment that has not been placed in the beam have all come back at background levels. All survey measurements show the analytical shielding calculations to be conservative by at least a factor of 2. No anomalous events have been identified by the building radiation monitoring system. Measurements of dose rates close to scanning water tanks have shown dose rates of approximately 10 mrem/hr with a half‐life less than 5 minutes. Measurements around the accelerator show some areas with dose rates slightly higher than 10 mrem/hr.

Conclusion:

The shielding design is shown to be adequate. Measured dose rates are below those predicted by shielding calculations. Activation hazards are minimal except in certain very well defined areas within the accelerator vault and for objects placed directly in the path of the beam.