bluuebunny/crossref_metadata_2025 huggingface dataset:
Target localization accuracy in a respiratory phantom using BrainLab ExacTrac and 4DCT imaging
Authors: Matney Jason E. , Parker Brent C. , Neck Daniel W. , Henkelmann Greg , Rosen Isaac I.
Published: February 2017

This study evaluated the accuracy of measuring the motion of an internal target using four‐dimensional computed tomography (4DCT) scanning and the BrainLAB ExacTrac X‐ray imaging system. Displacements of a metal coil implanted in a commercial respiratory phantom were measured in each system and compared to the known motion. A commercial respiratory motion phantom containing a metal coil as a surrogate target was used. Phantom longitudinal motions were sinusoidal with a 4.0 second period and amplitudes ranging from 5–25 mm. We acquired 4DCT and ExacTrac images of the coil at specified respiratory phases and recorded the coordinates of the coil ends. Coil displacement relative to the 0% phase (full‐inhale) position were computed for the ExacTrac and 4DCT imaging systems. Coil displacements were compared to known displacements based on the phantom's sinusoidal motion. Coil length distortion due to 4DCT phase binning was compared to the known physical length of the coil (31 mm). The maximum localization error for both coil endpoints for all motion settings was 3.5 mm for the 4DCT and 0.8 mm for the ExacTrac gating system. Coil length errors measured on the 4DCT were less than 0.8 mm at end inhale/exhale phases, but up to 8.3 mm at mid‐inhalation phases at the largest motion amplitude (25 mm). Due to the fast image acquisition time (100 ms), no coil distortion was observable in the ExacTrac system. 4DCT showed problems imaging the coil during mid‐respiratory phases of higher velocity (phases 20%–30% and 70%–80%) due to distortion caused by residual motion within the 4DCT phase bin. The ExacTrac imaging system was able to accurately localize the coil in the respiratory phantom over all phases of respiration. For our clinic, where end‐respiration phases from 4DCT may be used for treatment planning calculations, the ExacTrac system is used to measure internal target motion. With the ExacTrac system, planning target size and motion uncertainties are minimized, potentially reducing internal target volume margins in gated radiotherapy.

PACS number: 87.56.‐v
An image‐guided technique for planning and verification of supine craniospinal irradiation
Authors: McMahon Ryan L. , Larrier Nicole A. , Wu Q Jackie
Published: February 2017

We present a technique for planning and verification of craniospinal treatment with the patient in the supine position. Treatment delivery and verification is streamlined through the use of modern imaging techniques. Treatments use two lateral brain fields abutted to a single or pair of posterior spine fields. Treatment delivery is simplified by aligning all isocenters in the anterior‐posterior and lateral directions. Patient positioning is accomplished via on‐board kV imaging. Verification of field shape and junctions is accomplished with BB placement and MV portal imaging. Daily treatment is simplified by using only longitundinal couch shifts, which are recorded in the patient chart and RV database. The technique is simple to implement in a clinic that is already using a similar beam arrangement with the patient prone. It requires no additional devices to be fabricated (for immobilization or QA), and it takes advantage of all the existing elements of a modern linac.

PACS number: 87.55.‐x
Evaluation of a new VMAT QA device, or the “X” and “O” array geometries
Authors: Feygelman Vladimir , Zhang Geoffrey , Stevens Craig , Nelms Benjamin E.
Published: February 2017

We introduce a logical process of three distinct phases to begin the evaluation of a new 3D dosimetry array. The array under investigation is a hollow cylinder phantom with diode detectors fixed in a helical shell forming an “O” axial detector cross section (ArcCHECK), with comparisons drawn to a previously studied 3D array with diodes fixed in two crossing planes forming an “X” axial cross section (Delta4). Phase I testing of the ArcCHECK establishes: robust relative calibration (response equalization) of the individual detectors, minor field size dependency of response not present in a 2D predecessor, and uncorrected angular response dependence in the axial plane. Phase II testing reveals vast differences between the two devices when studying fixed‐width full circle arcs. These differences are primarily due to arc discretization by the TPS that produces low passing rates for the peripheral detectors of the ArcCHECK, but high passing rates for the Delta4. Similar, although less pronounced, effects are seen for the test VMAT plans modeled after the AAPM TG119 report. The very different 3D detector locations of the two devices, along with the knock‐on effect of different percent normalization strategies, prove that the analysis results from the devices are distinct and noninterchangeable; they are truly measuring different things. The value of what each device measures, namely their correlation with – or ability to predict – clinically relevant errors in calculation and/or delivery of dose is the subject of future Phase III work.

PACS number: 87.55Qr
Quality assurance using a photodiode array
Authors: Balderson M.J. , Spencer D.P. , Nygren I. , Brown D.W.
Published: February 2017

Improved treatment techniques in radiation therapy provide incentive to reduce treatment margins, thereby increasing the necessity for more accurate geometrical setup of the linear accelerator and accompanying components. In the present paper, we describe the development of a novel device that enables precise and automated measurement of geometric parameters for the purpose of improving initial setup accuracy, and for standardizing repeated quality control activities. The device consists of a silicon photodiode array, an evaluation board, a data acquisition card, and a laptop. Measurements that demonstrate the utility of the device are also presented. Using the device, we show that the radiation light field congruence for both 6 and 15 MV beams is within 1.3 mm. The maximum measured disagreement between radiation field edges and light field edges was , while the smallest disagreement between the light field and radiation field edge was . Because measurements are automated, ambiguities resulting from interobserver variability are removed, greatly improving the reproducibility of measurements across observers. We expect the device to find use in consistency measurements on linear accelerators used for stereotactic radiosurgery, during the commissioning of new linear accelerators, or as an alternative to film or other commercially available devices for performing monthly or annual quality control checks.

PACS numbers: 87.55.Qr, 87.56.Fc, 87.57.N‐, 87.15mn, 87.15mq
Dose tolerance limits and dose volume histogram evaluation for stereotactic body radiotherapy
Authors: Grimm Jimm , LaCouture Tamara , Croce Raymond , Yeo Inhwan , Zhu Yunping , Xue Jinyu
Published: February 2017

Almost 20 years ago, Emami et al. presented a comprehensive set of dose tolerance limits for normal tissue organs to therapeutic radiation, which has proven essential to the field of radiation oncology. The paradigm of stereotactic body radiotherapy (SBRT) has dramatically different dosing schemes but, to date, there has still been no comprehensive set of SBRT normal organ dose tolerance limits. As an initial step toward that goal, we performed an extensive review of the literature to compare dose limits utilized and reported in existing publications. The impact on dose tolerance limits of some key aspects of the methods and materials of the various authors is discussed. We have organized a table of 500 dose tolerance limits of normal structures for SBRT. We still observed several dose limits that are unknown or not validated. Data for SBRT dose tolerance limits are still preliminary and further clinical trials and validation are required. This manuscript presents an extensive collection of normal organ dose tolerance limits to facilitate both clinical application and further research.

PACS numbers: 87.53.Ly, 87.55.dk
Computed tomography dose index and dose length product for cone‐beam CT: Monte Carlo simulations of a commercial system
Authors: Kim Sangroh , Song Haijun , Samei Ehsan , Yin Fang‐Fang , Yoshizumi Terry T.
Published: February 2017

Dosimetry in kilovoltage cone beam computed tomography (CBCT) is a challenge due to the limitation of physical measurements. To address this, we used a Monte Carlo (MC) method to estimate the CT dose index (CTDI) and the dose length product (DLP) for a commercial CBCT system. As Dixon and Boone(1)showed that CTDI concept can be applicable to both CBCT and conventional CT, we evaluated weighted CT dose indexand DLP for a commercial CBCT system. Two extended CT phantoms were created in our BEAMnrc/EGSnrc MC system. Before the simulations, the beam collimation of a Varian On‐Board Imager (OBI) system was measured with radiochromic films (model: XR‐QA). The MC model of the OBI X‐ray tube, validated in a previous study, was used to acquire the phase space files of the full‐fan and half‐fan cone beams. Then, DOSXYZnrc user code simulated a total of 20 CBCT scans for the nominal beam widths from 1 cm to 10 cm. After the simulations, CBCT dose profiles at center and peripheral locations were extracted and integrated (dose profile integral, DPI) to calculate the CTDI per each beam width. The weighted cone‐beam CTDIwas calculated from DPI values and meanand DLP were derived. We also evaluated the differences ofvalues between MC simulations and point dose measurements using standard CT phantoms. In results, it was found thatwascGy for head andwascGy for body scan. The DLP was found to be proportional to the beam collimation. We also found that the point dose measurements with standard CT phantoms can estimate the CTDI within 3% difference compared to the full integrated CTDI from the MC method. This study showed the usability of CTDI as a dose index and DLP as a total dose descriptor in CBCT scans.

PACS number: 87.57.uq
Forward planned intensity modulated radiotherapy (IMRT) for whole breast postoperative radiotherapy. Is it useful? When?
Authors: Morganti Alessio G. , Cilla Savino , de Gaetano Andrea , Panunzi Simona , Digesù Cinzia , Macchia Gabriella , Massaccesi Mariangela , Deodato Francesco , Ferrandina Gabriella , Cellini Numa , Scambia Giovanni , Piermattei Angelo , Valentini Vincenzo
Published: February 2017

The purpose was to compare the dosimetric results observed in 201 breast cancer patients submitted to tangential forward intensity‐modulated radiation therapy (IMRT) with those observed in 131 patients treated with a standard wedged 3D technique for postoperative treatment of whole breast, according to breast size and supraclavicular node irradiation. Following dosimetric parameters were used for the comparison: and for the irradiated volume; and for the ipsilateral lung; and for the heart. Stratification was made according to breast size and supraclavicular (SCV) nodal irradiation. As respect to irradiated volume, a significant reduction of (mean values: versus ) and (mean % values: versus ), and an increase of (mean % values: versus ) were observed with forward IMRT. The homogeneity of dose distribution to target volume significantly improved with forward IMRT in all patient groups, irrespective of breast size or supraclavicular nodal irradiation. When patients treated with supraclavicular nodal irradiation were excluded from the analysis, forward IMRT slightly reduced (mean values: versus ) and (mean values versus ) of the ipsilateral lung. The dose to the heart tended to be lower with IMRT but this difference was not statistically significant. Tangential forward IMRT in postoperative treatment of whole breast improved dosimetric parameters in terms of homogeneity of dose distribution to the target in a large sample of patients, independent of breast size or supraclavicular nodal irradiation. Lung irradiation was slightly reduced in patients not undergoing to supraclavicular irradiation.

PACS numbers: 87.53.Kn; 87.55.de
A new method to deliver supraclavicular radiation in breast radiotherapy for lung sparing
Authors: Yang Bo , Dong Zheng , Lin Mu‐Han , Ma C‐M
Published: February 2017

Due to the angulation of the breast board used for tangential breast irradiation, additional normal lung tissues are included in the supraclavicular field. This work investigates a method to reduce the lung volume and dose delivered during supraclavicular irradiation for breast cancer. Ten patients included for this retrospective study received chest wall and supraclavicular irradiation following radical surgery or breast‐conserving surgery. Three‐dimensional conformal radiation therapy plans were generated using the CMS XiO treatment planning system. The clinical target volume (CTV) of the supraclavicular irradiation is defined as the subcutaneous tissues from 0.5 cm under the anterior skin surface to a 3 cm depth. Only the ipsilateral lung is defined as the organ at risk. In the new method, the couch is rotated 90° and the supraclavicular field is tilted to maintain a normal incident angle to the breast board rather than the couch surface to spare more normal lung tissues. The absolute volume of the ipsilateral lung irradiated, and the volumes of lung tissues receiving 5 Gy and 20 Gy and are analyzed. The new method can reduce the lung volume irradiated by the supraclavicular field significantly. For the ten patients investigated, only 5.3% of the ipsilateral lung is irradiated with the new method, while 14.9% of the ipsilateral lung is irradiated using the conventional method. Compared with the conventional method, the new method reduces by 53.6% and by 59.0%. Our new method does not alter the patient positioning for breast treatment but rotates the couch to deliver a tilted supraclavicular field to maintain adequate CTV coverage and spare more normal lung tissues. The results of this study demonstrated that our new method is effective, and that the reduction of normal lung tissue volume in the field is significant.

PACS number: 87.55.D‐
Understanding the impact of RapidArc therapy delivery errors for prostate cancer
Authors: Oliver Mike , Bush Karl , Zavgorodni Sergei , Ansbacher Will , Beckham Wayne A.
Published: February 2017

The purpose of this study is to simulate random and systematic RapidArc delivery errors for external beam prostate radiotherapy plans in order to determine the dose sensitivity for each error type. Ten prostate plans were created with a single 360° arc. The DICOM files for these treatment plans were then imported into an in‐house computer program that introduced delivery errors. Random and systematic gantry position (0.25°, 0.5°, 1°), monitor unit (MU) (1.25%, 2.5%, 5%), and multileaf collimator (MLC) position (0.5, 1, 2 mm) errors were introduced. The MLC errors were either random or one of three types of systematic errors, where the MLC banks moved in the same (MLC gaps remain unchanged) or opposing directions (increasing or decreasing the MLC gaps). The generalized equivalent uniform dose (gEUD) was calculated for the original plan and all treatment plans with errors introduced. The dose sensitivity for the cohort was calculated using linear regression for the gantry position, MU, and MLC position errors. Because there was a large amount of variability for systematic MLC position errors, the dose sensitivity of each plan was calculated and correlated with plan MU, mean MLC gap, and the percentage of MLC leaf gaps less than 1 and 2 cm for each individual plan. We found that random and systematic gantry position errors were relatively insignificant ( gEUD change) for gantry errors up to 1°. Random MU errors were also insignificant, and systematic MU increases caused a systematic increase in gEUD. For MLC position errors, random MLC errors were relatively insignificant up to 2 mm as had been determined in previous IMRT studies. Systematic MLC shift errors caused a decrease of approximately in the gEUD per mm. For systematic MLC gap open errors, the dose sensitivity was 8.2%/mm and for MLC gap close errors the dose sensitivity was . There was a large variability for MLC gap open/close errors for the ten RapidArc plans which correlated strongly with MU, mean gap width, and percentage of MLC gaps less than 1 or 2 cm. This study evaluates the magnitude of various simulated RapidArc delivery errors by calculating gEUED on various prostate plans.

PACS numbers: 87.55.x, 87.55.D, 87.55.de, 87.55.dk
Initial experience and clinical comparison of two image guidance methods for SBRT treatment: 4DCT versus respiratory‐triggered imaging
Authors: Wang Brian , Rassiah‐Szegedi Prema , Huang Hui Zhao. Jessica , Sarkar Vikren , Szegedi Martin , Kokeny Kristine E. , Anker Christopher J. , Shrieve Dennis C. , Salter Bill J.
Published: February 2017

For Stereotactic Body Radiation Therapy (SBRT) treatment of lung and liver, we quantified the differences between two image guidance methods: 4DCT and ExacTrac respiratory‐triggered imaging. Five different patients with five liver lesions and one lung lesion for a total of 19 SBRT delivered fractions were studied. For the 4DCT method, a manual registration process was used between the 4DCT image sets from initial simulation and treatment day to determine the required daily image‐guided corrections. We also used the ExacTrac respiratory‐triggered imaging capability to verify the target positioning, and calculated the differences in image guidance shifts between these two methods. The mean (standard deviation) of the observed differences in image‐guided shifts between 4DCT and ExacTrac respiratory‐triggered image guidance was left/right , anterior/posterior , superior/inferior , with no difference larger than 5.0 mm in any given direction for any individual case. The largest error occurred in the S/I direction, with a mean of 2.2 mm for the six lesions. This seems reasonable, because respiratory motion and the resulting imaging uncertainties are most pronounced in this S/I direction. Image guidance shifts derived from ExacTrac triggered imaging at two extreme breathing phases (i.e., full exhale vs. full inhale), agreed well (less than 2.0 mm) with each other. In summary, two very promising image guidance methods of 4DCT and ExacTrac respiratory‐triggered imaging were presented and the image guidance shifts were comparable for the patients evaluated in this study.

PACS number: 87.55.ne
Dosimetric effects of manual cone‐beam CT (CBCT) matching for spinal radiosurgery: Our experience
Authors: Schreibmann Eduard , Fox Tim , Crocker Ian
Published: February 2017

Radiosurgical treatment of cranial or extracranial targets demands accurate positioning of the isocenter at the beam and table isocenter, and immobilization of the target during treatment. For spinal radiosurgery, the standard approach involves matching of cone‐beam CT (CBCT) in‐room images with the planning CT (pCT) to determine translation and yaw corrections. The purpose of this study was to assess the accuracy of these techniques compared to advanced automatching using mutual information metrics, with consideration given to volume of interest (VOI) and optimizing translations and rotations in all axes. The dosimetric consequences of our current standard matching techniques were also evaluated. Ten consecutive spinal radiosurgery patients treated in the last year were subjected to analysis. For purposes of this analysis, the automatch using mutual information and a VOI was considered to create “the true isocenter” for positioning the patients. Review of the imaging from this automatch confirmed perfect superimposition of the two datasets within the VOI. Matching the CBCT to the pCT using the automatch allowed assessment of the rotations which had been previously ignored. Recalculation of the dose volume histogram was undertaken for each patient, assuming displacement of the true isocenter to the treated isocenter. Comparisons between the delivered doses and the intended doses were made. The mean absolute lateral/vertical/longitudinal translations and vector displacement between the manual CBCT‐pCT matching isocenter and the true isocenter were 0.13, , and , with a minimum and maximum individual pixel vector shift of 3.2 and 8.94 mm. The mean pitch, yaw, and roll correction for automatch was , 0.25°, and 0.97° with a maximum of 1.65°, 2.92°, and 1.43°. Four of ten patients had a significant change in the coverage of the tumor due to lack of correction of translational and rotational errors. The largest errors were observed in patients with small and irregular target volumes. Our initial results show that precise positioning for spinal radiosurgery cannot be accomplished with manual pCT‐CBCT matching without a clinical strategy to compensate for rotations. In the absence of this, significant underdosing of the tumor may occur.

PACS number: 87.55.Qr, 87.57.uq, 87.55.km
Physical and dosimetric characteristic of high‐definition multileaf collimator (HDMLC) for SRS and IMRT
Authors: Sharma Dayananda Shamurailatpam , Dongre Prabhakar M , Mhatre Vaibav , Heigrujam Malhotra
Published: February 2017

Physical and dosimetric characteristics of HDMLC were studied for SRS6, 6, and 10 MV X‐rays from Novalis Tx. This in‐built tertiary collimator consists of 60 pairs (; and ) of leaves. Properties of HDMLC studied included alignment, readout and radiation field congruence, radiation penumbra, accuracy and reproducibility of leaf position and gap width, static and dynamic leaf shift, tongue‐and‐groove effect, leaf transmission and leakage, leaf travel speed, and delivery of dynamic conformal arc and IMRT. All tests were performed using a calibrated ionization chamber, film dosimetry and DynaLog file analysis. Alignment of leaves with isocenter plane was better than 0.03 cm at all gantry and collimator positions. The congruence of HDMLC readout and radiation field agreed to within for filed sizes ranging from to . Mean 80% to 20% penumbra width parallel (perpendicular) to leaf motion was ) cm, ) cm, and ) cm for SRS6, 6, and 10 MV X‐rays, respectively. Circular field penumbra was comparable to corresponding square field. Average penumbra of field was effectively constant over off‐axis positions of up to 12 cm with mean value of 0.16 at 1.5 cm depth and 0.38 at 10 cm depth. Minimum and maximum effective penumbra along the straight diagonal edge of irregular fields increased from 0.3 and 0.32 cm at 70° steep angle to 0.35 and 0.56 cm at 20° steep angle. Modified Picket Fence test showed average FWHM of 0.18 cm and peak‐to‐peak distance of 1.99 cm for 0.1 cm band and 2 cm interband separation. Dynamic multileaf collimation (DMLC) output factor remained within for 6 MV and for 10 MV X‐rays at all gantry positions, and was reproducible within over a period of 14 months. The static leaf shift was 0.03 cm for all energies, while dynamic leaf shift was 0.044 cm for 10 MV and 0.039 cm for both SRS6 and 6 MV X‐rays. The dose depression and corresponding tongue‐and‐groove size were 24% and 0.17 cm for 6 MV and 19% and 0.20 cm for 10 MV X‐rays. Average transmission through HDMLC was 1.09%, 1.14% and 1.34% for SRS6, 6 and 10 MV X‐rays. Analysis of DynaLog files for leaf speed test in arc dynamic mode, delivery test of dynamic conformal arc, and step‐and‐shoot and sliding window IMRT showed at least 95% or more of the error counts had misplacements , with maximum root mean square (RMS) error value calculated at 0.13 cm. Accurate and reproducible leaf position and gap width, and less leakage and small consistent penumbra over the fields demonstrate HDMLC suitable for high‐dose resolution SRS and IMRT.

PACS number: 87.56.N‐, 87.55.Qr, 87.50.cm, 87.55.de, 87.53.Ly
Evaluation of integrated respiratory gating systems on a Novalis Tx system
Authors: Chang Zheng , Liu TongHai , Cai Jing , Chen Qing , Wang Zhiheng , Yin Fang‐Fang
Published: February 2017

The purpose of this study was to investigate the accuracy of motion tracking and radiation delivery control of integrated gating systems on a Novalis Tx system. The study was performed on a Novalis Tx system, which is equipped with Varian Real‐time Position Management (RPM) system, and BrainLAB ExacTrac gating systems. In this study, the two systems were assessed on accuracy of both motion tracking and radiation delivery control. To evaluate motion tracking, two artificial motion profiles and five patients' respiratory profiles were used. The motion trajectories acquired by the two gating systems were compared against the references. To assess radiation delivery control, time delays were measured using a single‐exposure method. More specifically, radiation is delivered with a 4 mm diameter cone within the phase range of 10%–45% for the BrainLAB ExacTrac system, and within the phase range of 0%–25% for the Varian RPM system during expiration, each for three times. Radiochromic films were used to record the radiation exposures and to calculate the time delays. In the work, the discrepancies were quantified using the parameters of mean and standard deviation (SD). Pearson's product‐moment correlational analysis was used to test correlation of the data, which is quantified using a parameter of r. The trajectory profiles acquired by the gating systems show good agreement with those reference profiles. A quantitative analysis shows that the average mean discrepancies between BrainLAB ExacTrac system and known references are 1.5 mm and 1.9 mm for artificial and patient profiles, with the maximum motion amplitude of 28.0 mm. As for the Varian RPM system, the corresponding average mean discrepancies are 1.1 mm and 1.7 mm for artificial and patient profiles. With the proposed single‐exposure method, the time delays are found to be seconds and seconds for BrainLAB ExacTrac and Varian RPM systems, respectively. The results indicate the systems can track motion and control radiation delivery with reasonable accuracy. The proposed single‐exposure method has been demonstrated to be feasible in measuring time delay efficiently.

PACS numbers: 87.56.bd, 87.56.‐v, 87.55.‐x
Individualized margins for prostate patients using a wireless localization and tracking system
Authors: Rassiah‐Szegedi Prema , Wang Brian , Szegedi Martin , Tward Jonathan , Zhao Hui , Huang Y. Jessica , Sarkar Vikren , Shrieve Dennis , Salter Bill
Published: February 2017

This study investigates the dosimetric benefits of designing patient‐specific margins for prostate cancer patients based on 4D localization and tracking. Ten prostate patients, each implanted with three radiofrequency transponders, were localized and tracked for 40 fractions. “Conventional margin” (CM) planning target volumes (PTV) and PTVs resulting from uniform margins of 5 mm (5M) and 7 mm (7M) were explored. Through retrospective review of each patient's tracking data, an individualized margin (IM) design for each patient was determined. IMRT treatment plans with identical constraints were generated for all four margin strategies and compared. The IM plans generally created the smallest PTV volumes. For similar PTV coverage, the IM plans had a lower mean bladder (rectal) dose by an average of 3.9% (2.5%), 8.5% (5.7%) and 16.2 % (9.8%) compared to 5M, 7M and CM plans, respectively. The IM plan had the lowest gEUD value of 23.8 Gy for bladder, compared to 35.1, 28.4 and 25.7, for CM, 7M and 5M, respectively. Likewise, the IM plan had the lowest NTCP value for rectum of 0.04, compared to 0.07, 0.06 and 0.05 for CM, 7M and 5M, respectively. Individualized margins can lead to significantly reduced PTV volumes and critical structure doses, while still ensuring a minimum delivered CTV dose equal to 95% of the prescribed dose.

PACS numbers: 87.53.Kn, 87.55.D
Commissioning of Varian ring & tandem HDR applicators: reproducibility and interobserver variability of dwell position offsets
Authors: McMahon Ryan , Zhuang Tingliang , Steffey Beverly A. , Song Haijun , Craciunescu Oana I.
Published: February 2017

Studies have shown that source dwells within Varian's HDR CT/MR compatible ring applicators can deviate from intended positions by several millimeters. Quantifying this offset is an important part of commissioning. The aims of this study were to: 1) determine the reproducibility of the offset, 2) study the interobserver variation in the offset's measurement, and 3) quantify the dosimetric impact of the offset. Offsets were measured for four ring applicators: two 30°, one 45°, and one 60°. Dwell positions were measured five times for each ring to determine the reproducibility of source positioning. Experiments were done to compare two separate source wires, as well as different time points within a single source wire's lifecycle. Data were analyzed by three independent observers. To quantify the dosimetric impact of the offset, a treatment plan was generated using BrachyVision. The dose to point A, and the metric for rectum and bladder were calculated with and without the offset. For the 45° and 60° rings, measured offsets were 3.0 mm and 3.6 mm, respectively. The 30° ring showed substantial variation in distal dwell positions (maximum difference between the five experiments of 2.9 mm). Subsequent testing of a replacement ring showed an offset of 2.4 mm that was more reproducible. Offsets varied less than 1 mm between different source wires, and changed less than 1 mm over the course of a source wire's lifecycle. When comparing observers, the average range in a measurement of a dwell position was 0. 5mm (σ = 0.2 mm, max 1.3 mm). The offset resulted in dose variations to point A, bladder, and rectum of less than 1%, 2%, and 5%, respectively. Results indicate that Varian rings can show systematic and random offsets of more than 3 mm. Some can be considered defective and should be replaced. Each applicator should be individually commissioned and reproducibility should be confirmed with multiple tests.

PACS number: 87.56.‐v, 87.53.Jw
Real‐time dose reconstruction for wedged photon beams: a generalized procedure
Authors: Piermattei A. , Greco F. , Fidanzio A. , Azario L. , Porcelli A. , Cilla S. , Sabatino D. , Russo A. , D'Onofrio G. , Russo M.
Published: February 2017

A practical and accurate generalized procedure to reconstruct the isocenter dose for 3D conformal radiotherapy (3DCRT) has been developed for X‐ray open beams supplied by linacs of different manufacturers and equipped with aSi electronic portal imaging devices (aSi EPIDs). This paper reports an extension of the method, to be applied at the wedged X‐ray beams characterized by the wedge attenuation factor .

Using water‐equivalent solid phantoms (SPs) of different thicknesses, w, and photon square fields of sizes, L, the generalized midplane doses and generalized transit signals by 38 beams of six different linacs were determined. The generalized data were fitted by surface equations and used together with the information of the ‘record & verify’ network of the centers. In this manner, for every beam, the reconstruction was obtained in about 25 seconds after the treatment.

To test the in vivo dosimetric procedure, six pelvic treatments that used conformed wedged beams were carried out with three linacs of different manufacturers. For every beam, the comparison between the reconstructed and the computed by the TPS, resulted in an acceptable tolerance level of , estimated for this kind of treatment.

Generally the in vivo dosimetry methods that use EPIDs require: (i) a special effort for the dosimetric commissioning with SPs of different thicknesses, and (ii) extra time for the analysis of the EPID signals. The proposed procedure simplifies the commissioning step and supplies for Varian, Elekta, and Siemens linacs equipped with the aSi EPIDs a quasi‐real time in vivo dosimetry for open and wedged 3DCRT fields.

PACS number: 87.53Xd
Increased beam attenuation and surface dose by different couch inserts of treatment tables used in megavoltage radiotherapy
Authors: Seppälä Jan K.H. , Kulmala Jarmo A.J.
Published: February 2017

The use of solid carbon fiber table materials in radiotherapy has become more common with the implementation of image‐guided radiotherapy (IGRT), since the solid materials give less imaging artifacts than the so‐called tennis racket couchtops. The downside of the solid carbon fiber couch inserts is that they increase the beam attenuation, resulting in increased surface doses and inaccuracies in determine the dose in the patient. The purpose of this study was to evaluate the interaction of 6 and 15 MV photons with eight different couch inserts. The presented results enable direct comparison of the attenuation properties of the studied couchtops. With a direct posterior beam the maximum attenuations reach 3.6% and 2.4% with 6 and 15 M V, respectively. The measured maximum attenuation by a couchtop with an oblique gantry angle was 10.8% and 7.4% at 6 and 15 MV energies, respectively. The skin‐sparing effect was decreased substantially with every couchtop. The highest increases in surface doses were recorded to be four‐ and threefold, as compared to the direct posterior open field surface doses of 6 and 15 MV, respectively. In conclusion, the carbon fiber tabletops decrease the skin‐sparing effect of megavoltage photon energies. The increased beam attenuation and skin doses should be taken into account in the process of treatment planning.

PACS number: 07.90.+c
Patient‐specific CT dosimetry calculation: a feasibility study
Authors: Fearon Thomas , Xie Huchen , Cheng Jason Y. , Ning Holly , Zhuge Ying , Miller Robert W.
Published: February 2017

Current estimation of radiation dose from computed tomography (CT) scans on patients has relied on the measurement of Computed Tomography Dose Index (CTDI) in standard cylindrical phantoms, and calculations based on mathematical representations of “standard man”. Radiation dose to both adult and pediatric patients from a CT scan has been a concern, as noted in recent reports. The purpose of this study was to investigate the feasibility of adapting a radiation treatment planning system (RTPS) to provide patient‐specific CT dosimetry. A radiation treatment planning system was modified to calculate patient‐specific CT dose distributions, which can be represented by dose at specific points within an organ of interest, as well as organ dose‐volumes (after image segmentation) for a GE Light Speed Ultra Plus CT scanner. The RTPS calculation algorithm is based on a semi‐empirical, measured correction‐based algorithm, which has been well established in the radiotherapy community. Digital representations of the physical phantoms (virtual phantom) were acquired with the GE CT scanner in axial mode. Thermoluminescent dosimeter (TLDs) measurements in pediatric anthropomorphic phantoms were utilized to validate the dose at specific points within organs of interest relative to RTPS calculations and Monte Carlo simulations of the same virtual phantoms (digital representation). Congruence of the calculated and measured point doses for the same physical anthropomorphic phantom geometry was used to verify the feasibility of the method. The RTPS algorithm can be extended to calculate the organ dose by calculating a dose distribution point‐by‐point for a designated volume. Electron Gamma Shower (EGSnrc) codes for radiation transport calculations developed by National Research Council of Canada (NRCC) were utilized to perform the Monte Carlo (MC) simulation. In general, the RTPS and MC dose calculations are within 10% of the TLD measurements for the infant and child chest scans. With respect to the dose comparisons for the head, the RTPS dose calculations are slightly higher (10%–20%) than the TLD measurements, while the MC results were within 10% of the TLD measurements. The advantage of the algebraic dose calculation engine of the RTPS is a substantially reduced computation time (minutes vs. days) relative to Monte Carlo calculations, as well as providing patient‐specific dose estimation. It also provides the basis for a more elaborate reporting of dosimetric results, such as patient specific organ dose volumes after image segmentation.

PACS numbers: 87.55.D‐, 87.57.Q‐, 87.53.Bn, 87.55.K‐
Measurements to predict the time of target replacement of a helical tomotherapy
Authors: Kampfer Severin , Schell Stefan , Duma Marciana N. , Wilkens Jan J. , Kneschaurek Peter
Published: February 2017

Intensity‐modulated radiation therapy (IMRT) requires more beam‐on time than normal open field treatment. Consequently, the machines wear out and need more spare parts. A helical tomotherapy treatment unit needs a periodical tungsten target replacement, which is a time consuming event. To be able to predict the next replacement would be quite valuable. We observed unexpected variations towards the end of the target lifetime in the performed pretreatment measurements for patient plan verification. Thus, we retrospectively analyze the measurements of our quality assurance program. The time dependence of the quotient of two simultaneous dose measurements at different depths within a phantom for a fixed open field irradiation is evaluated. We also assess the time‐dependent changes of an IMRT plan measurement and of a relative depth dose curve measurement. Additionally, we performed a Monte Carlo simulation with Geant4 to understand the physical reasons for the measured values. Our measurements show that the dose at a specified depth compared to the dose in shallower regions of the phantom declines towards the end of the target lifetime. This reproducible effect can be due to the lowering of the mean energy of the X‐ray spectrum. These results are supported by the measurements of the IMRT plan, as well as the study of the relative depth dose curve. Furthermore, the simulation is consistent with these findings since it provides a possible explanation for the reduction of the mean energy for thinner targets. It could be due to the lowering of low energy photon self‐absorption in a worn out and therefore thinner target. We state a threshold value for our measurement at which a target replacement should be initiated. Measurements to observe a change in the energy are good predictors of the need for a target replacement. However, since all results support the softening of the spectrum hypothesis, all depth‐dependent setups are viable for analyzing the deterioration of the tungsten target. The suggested measurements and criteria to replace the target can be very helpful for every user of a TomoTherapy machine.

PACS numbers: 87.55.N‐, 87.55.Qr, 87.55.T‐, 87.56.bd
The effect of gantry spacing resolution on plan quality in a single modulated arc optimization
Authors: Mihaylov Ivaylo B. , Curran Bruce , Sternick Edward
Published: February 2017

Volumetric‐modulated arc technique (VMAT) is an efficient form of IMRT delivery. It is advantageous over conventional IMRT in terms of treatment delivery time. This study investigates the relation between the number of segments and plan quality in VMAT optimization for a single modulated arc. Five prostate, five lung, and five head‐and‐neck (HN) patient plans were studied retrospectively. For each case, four VMAT plans were generated. The plans differed only in the number of control points used in the optimization process. The control points were spaced 2°, 3°, 4°, and 6° apart, respectively. All of the optimization parameters were the same among the four schemes. The 2° spacing plan was used as a reference to which the other three plans were compared. The plan quality was assessed by comparison of dose indices (DIs) and generalized equivalent uniform doses (gEUDs) for targets and critical structures. All optimization schemes generated clinically acceptable plans. The differences between the majority of reference and compared DIs and gEUDs were within 3%. DIs and gEUDs which differed in excess of 3% corresponded to dose levels well below the organ tolerances. The DI and the gEUD differences increased with an increase in plan complexity from prostates to HNs. Optimization with gantry spacing resolution of 4° seems to be a very balanced alternative between plan quality and plan complexity.

PACS number: 87.55.de
Antiscatter grid use in pediatric digital tomosynthesis imaging
Authors: King Jenna M. , Elbakri Idris A. , Reed Martin
Published: February 2017

The objective of this study was to assess the effect of antiscatter grid use on tomosynthesis image quality. We performed an observer study that rated the image quality of digital tomosynthesis scout radiographs and slice images of a Leeds TO.20 contrast‐detail test object embedded in acrylic with and without a grid. We considered 10, 15, 20 and 25 cm of acrylic to represent the wide range of patient thicknesses encountered in pediatric imaging. We also acquired and rated images without a grid at an increased patient dose. The readers counted the total number of visible details in each image as a measure of relative image quality. We observed that the antiscatter grid improves tomosynthesis image quality compared to the grid‐out case, which received image quality scores similar to grid‐in radiography. Our results suggest that, in order to achieve the best image quality in exchange for the increase in patient dose, it may often be appropriate to include an antiscatter grid for pediatric tomosynthesis imaging, particularly if the patient thickness is greater than 10 cm.

PACS number: 87.57.‐s
Isocenter verification for linac‐based stereotactic radiation therapy: review of principles and techniques
Authors: Rowshanfarzad Pejman , Sabet Mahsheed , O'Connor Daryl J. , Greer Peter B.
Published: February 2017

There have been several manual, semi‐automatic and fully‐automatic methods proposed for verification of the position of mechanical isocenter as part of comprehensive quality assurance programs required for linear accelerator‐based stereotactic radiosurgery/radiotherapy (SRS/SRT) treatments. In this paper, a systematic review has been carried out to discuss the present methods for isocenter verification and compare their characteristics, to help physicists in making a decision on selection of their quality assurance routine.

PACS numbers: 87.53.Ly, 87.56.Fc, 87.56.‐v
Calculating the peak skin dose resulting from fluoroscopically guided interventions. Part I: Methods
Authors: Jones A. Kyle , Pasciak Alexander S.
Published: February 2017

While direct measurement of the peak skin dose resulting from a fluoroscopically‐guided procedure is possible, the decision must be made a priori at additional cost and time. It is most often the case that the need for accurate knowledge of the peak skin dose is realized only after a procedure has been completed, or after a suspected reaction has been discovered. Part I of this review article discusses methods for calculating the peak skin dose across a range of clinical scenarios. In some cases, a wealth of data are available, while in other cases few data are available and additional data must be measured in order to estimate the peak skin dose. Data may be gathered from a dose report, the DICOM headers of images, or from staff and physician interviews. After data are gathered, specific steps must be followed to convert dose metrics, such as the reference point air kerma or the kerma area product (KAP), into peak skin dose. These steps require knowledge of other related factors, such as the f‐factor and the backscatter factor, tables of which are provided in this manuscript. Sources of error and the impact of these errors on the accuracy of the final estimate of the peak skin dose are discussed.

PACS numbers: 87.59.Dj, 87.53.Bn
An opposed matched field IMRT technique for prostate cancer patients with bilateral prosthetic hips
Authors: Fattahi Shahin , Ostapiak Orest Z.
Published: February 2017

Intensity‐modulated radiation therapy (IMRT) has gained wide‐spread use for treating patients with prostate cancer, yet developing a plan for patients with bi‐lateral metal hip prostheses implants may be challenging. The high atomic number of the metallic hips not only gives rise to streak artifacts that obscure anatomy but also attenuates laterally directed fields by a significant amount that cannot be reliably ascertained from the CT dataset. A common approach to planning directs five IMRT fields such that incidence through the metal hips is avoided. While this technique generally gives adequate PTV coverage, it may escalate the rectal dose if beams, which would otherwise be incident from a lateral direction, are angled toward a posterior direction in order to avoid the prosthesis. In this work, we propose and investigate a new technique which alleviates this problem by introducing asymmetric opposed fields that are edge‐matched along a plane that is tangent to the metal prostheses. With this approach, a posterior oblique field is oriented closer to the lateral direction but does not irradiate the ipsilateral prosthesis. The portion of the target eclipsed by the prosthesis is irradiated by the opposed matched anterior oblique field which, again, avoids the corresponding ipsilateral prosthesis. While the proposed technique may improve rectal sparing and PTV coverage, the dose along the match plane is sensitive to intrafraction motion. In the worse case of intrafraction motion perpendicular to the plane occurring in the time interval between the deliveries of successive fields of the opposed matched pair, the induced error is typically about 5 cGy per mm of target motion for a 200 cGy fraction. To reduce the induced error, several approaches to broadening the penumbra at the match plane were investigated and compared to conventional IMRT plans for three patients. Phantom measurements were performed to evaluate the effectiveness of these approaches. Match‐plane shifts of 4 mm in a single step, in two 2 mm steps, and in four 1 mm steps, were effective in reducing the worse case induced error to 2.8 cGy per mm. Imposing match‐plane shifts precludes the use of intensity modulation for the opposed matched field pairs. Therefore, we favor an approach whereby the opposed matched fields overlap by 4 mm. Since both fields contribute fluence to the overlap region, the worse case induced error was observed to be typically within 2.9 cGy per mm. In conclusion, the use of this technique should be considered for patients with bilateral metal hip implants who do not meet dose‐volume criteria by conventional IMRT techniques.

PACS number: 87.55.‐x