bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐36: A GPU‐Accelerated Monte‐Carlo Dose Calculation Platform and Its Application Toward Validating a ViewRay Beam Model
Authors: Wang Y , Mazur T , Green O , Hu Y , Wooten H , Yang D , Zhao T , Mutic S , Li H
Published: June 2015

Purpose:

To build a fast, accurate and easily‐deployable research platform for Monte‐Carlo dose calculations. We port the dose calculation engine PENELOPE to C++, and accelerate calculations using GPU acceleration. Simulations of a Co‐60 beam model provided by ViewRay demonstrate the capabilities of the platform.

Methods:

We built software that incorporates a beam model interface, CT‐phantom model, GPU‐accelerated PENELOPE engine, and GUI front‐end. We rewrote the PENELOPE kernel in C++ (from Fortran) and accelerated the code on a GPU. We seamlessly integrated a Co‐60 beam model (obtained from ViewRay) into our platform. Simulations of various field sizes and SSDs using a homogeneous water phantom generated PDDs, dose profiles, and output factors that were compared to experiment data.

Results:

With GPU acceleration using a dated graphics card (Nvidia Tesla C2050), a highly accurate simulation – including 100*100*100 grid, 3×3×3 mm3 voxels, <1% uncertainty, and 4.2×4.2 cm2 field size – runs 24 times faster (20 minutes versus 8 hours) than when parallelizing on 8 threads across a new CPU (Intel i7‐4770). Simulated PDDs, profiles and output ratios for the commercial system agree well with experiment data measured using radiographic film or ionization chamber. Based on our analysis, this beam model is precise enough for general applications.

Conclusions:

Using a beam model for a Co‐60 system provided by ViewRay, we evaluate a dose calculation platform that we developed. Comparison to measurements demonstrates the promise of our software for use as a research platform for dose calculations, with applications including quality assurance and treatment plan verification.
SU‐E‐T‐37: A GPU‐Based Pencil Beam Algorithm for Dose Calculations in Proton Radiation Therapy
Authors: Kalantzis G , Leventouri T , Tachibana H , Shang C
Published: June 2015

Purpose:

Recent developments in radiation therapy have been focused on applications of charged particles, especially protons. Over the years several dose calculation methods have been proposed in proton therapy. A common characteristic of all these methods is their extensive computational burden. In the current study we present for the first time, to our best knowledge, a GPU‐based PBA for proton dose calculations in Matlab.

Methods:

In the current study we employed an analytical expression for the protons depth dose distribution. The central‐axis term is taken from the broad‐beam central‐axis depth dose in water modified by an inverse square correction while the distribution of the off‐axis term was considered Gaussian. The serial code was implemented in MATLAB and was launched on a desktop with a quad core Intel Xeon X5550 at 2.67GHz with 8 GB of RAM. For the parallelization on the GPU, the parallel computing toolbox was employed and the code was launched on a GTX 770 with Kepler architecture. The performance comparison was established on the speedup factors.

Results:

The performance of the GPU code was evaluated for three different energies: low (50 MeV), medium (100 MeV) and high (150 MeV). Four square fields were selected for each energy, and the dose calculations were performed with both the serial and parallel codes for a homogeneous water phantom with size 300×300×300 mm3. The resolution of the PBs was set to 1.0 mm. The maximum speedup of ∼127 was achieved for the highest energy and the largest field size.

Conclusion:

A GPU‐based PB algorithm for proton dose calculations in Matlab was presented. A maximum speedup of ∼127 was achieved. Future directions of the current work include extension of our method for dose calculation in heterogeneous phantoms.
SU‐E‐T‐39: A Logistic Function‐Based Model to Predict Organ‐At‐Risk (OAR) DVH in IMRT Treatment Planning
Authors: Chen S , Zhang H , Zhang B , D'souza W
Published: June 2015

Purpose:

To investigate the feasibility of a logistic function‐based model to predict organ‐at‐risk (OAR) DVH for IMRT planning. The predicted DVHs are compared to achieved DVHs by expert treatment planners.

Methods:

A logistic function is used to model the OAR dose‐gradient function. This function describes the percentage of the prescription dose as a function of the normal distance to PTV surface. The slope of dose‐gradient function is function of relative spatial orientation of the PTV and OARs. The OAR DVH is calculated using the OAR dose‐gradient function assuming that the dose is same for voxels with same normal distance to PTV. Ten previously planned prostate IMRT plans were selected to build the model, and the following plan parameters were calculated as possible features to the model: the PTV maximum/minimum dose, PTV volume, bladder/rectum volume in the radiation field, percentage of bladder/rectum overlapping with PTV, and the distance between the bladder/rectum centroid and PTV. The bladder/rectum dose‐gradient function was modeled and applied on 10 additional test cases, and the predicted and achieved clinical bladder/rectum DVHs were compared: V70 (percentage of volume receiving 70Gy and above), V65, V60, V55, V50, V45, V40.

Results:

The following parameters were selected as model features: PTV volume, and distance of centroid of rectum/bladder to PTV. The model was tested with 10 additional patients. For bladder, the absolute difference (mean±standard deviation) between predicted and clinical DVHs is V70=−0.3±3.2, V65=−0.8±3.9, V60=1.5±4.3, V55=1.7±5.3, V50=−0.6±6.4, V45=0.6±6.5, and V40=0.9±5.7, the correlation coefficient is 0.96; for rectum, the difference is V70=1.5±3.8, V65=1.2±4.2, V60=−0.1±5.3, V55=1.0±6.6, V50=1.6±8.7, V45=1.9±9.8, and V40=1.5±10.1, and the correlation coefficient is 0.87.

Conclusion:

The OAR DVH can be accurately predicted using the OAR dose‐gradient function in IMRT plans. This approach may be used as a quality control tool and aid less experienced planners determine benchmarks for plan quality.
SU‐E‐T‐44: A Micro‐Raman Spectroscopy Study of the Dose‐Dependence of EBT3 GafChromicTM Films for Quantifying the Degree of Molecular Polymerization
Authors: Callens M , Crijns W , Simons V , De Wolf I , Depuydt T , Maes F , Haustermans K , D'hooge J , D'Agostino E , Pfeiffer H , Van Den Abeele K
Published: June 2015

Purpose:

Radiochromic films, such as the poly‐diacetylene‐based EBT3 GafChromic™ films (Ashland Specialty Ingredients, Wayne, NY, USA), are widely used for dosimetry applications because of their clear energy independence, high spatial resolution, near tissue equivalence and easy handling. The films undergo a slight color change by radiation‐induced polymerization of diacetylene monomers. But more importantly, the film becomes optically less transparent with increasing radiation dose, with a saturation starting between 10 and 20 Gy, i.e. a common SBRT dose level. In contrast to the chromatic properties, less attention has been given to the underlying molecular mechanism that induces this partial color change and strongly reduces the transparency. Therefore, the current work investigates the variation of the molecular composition of the active layer of EBT3 films for an SBRT dose range.

Method:

Uncoated EBT3 films were irradiated with a 6 MV photon beam using dose levels between 0 and 20 Gy. The relative variation of the polymer content as a function of the applied radiation dose was studied using micro‐Raman spectroscopy. Raman spectroscopy with a 633 nm probe laser incident on the active layer allowed to identify the film constituents and to estimate the amount of poly‐diacetylenes from the intensities of the unique molecular vibrations of the molecule.

Results:

The normalized intensity of all polymeric vibrations, and most notably the polymeric triple and double carbon‐carbon bonds at 2058 cm−1 and 1446 cm−1 respectively, increase with increasing dose up to a saturation level starting at about 10 Gy, indicating a corresponding increase and saturation of the amount of polymers. This molecular saturation process is the main cause of the non‐linear dose response (i.e. a transparency reduction) and of the limited dose range of the studied films.

Conclusion:

Raman spectroscopy provides new and more fundamental insights in the mechanism of the dose‐dependence of EBT3 films.

This work was funded by the Research Foundation ‐ Flanders (FWO).
SU‐E‐T‐47: A Monte Carlo Model of a Spot Scanning Proton Beam Based On a Synchrotron Proton Therapy Accelerator
Authors: Xie C , Lin H , Jing J , Chen C , Cao R , Pei X
Published: June 2015

Purpose:

To build the model of a spot scanning proton beam for the dose calculation of a synchrotron proton therapy accelerator, which is capable of accelerating protons from 50 up to 221 MeV.

Methods:

The spot scanning beam nozzle is modeled using TOPAS code, a simulation tool based on Geant4.9.6. The model contained a beam pipe vacuum window, a beam profile monitor, a drift chamber, two plane‐parallel ionization chambers, and a spot‐position monitor consisted of a multiwire ionization chamber. A water phantom is located with its upstream surface at the isocenter plane. The initial proton beam energy and anglar deflection are modeled using a Gaussian distribution with FWHM (Full Widths at Half Maximum) deponding on its beam energy. The phase space file (PSF) on a virtual surface located at the center between the two magnets is recorded. PSF is used to analyze the pencil beam features and offset the pencil beam position. The source model parameters are verificated by fitting the simulated Result to the measurement.

Results:

The simulated percentage depth dose (PDD) and lateral profiles of scanning pencil beams of various incident proton energies are verificated to the measurement. Generally the distance to agreement (DTA) of Bragg peaks is less than 0.2cm. The FWHM of Gaussian anglar distribution was adjusted to fit the lateral profile difference between the simulation and the measurement to less than 2∼3cm.

Conclusion:

A Monte Carlo model of a spot scanning proton beam was bullt based on a synchrotron proton therapy accelerator. This scanning pencil beam model will be as a block to build the broad proton beam as a proton TPS dose verification tool.
SU‐E‐T‐49: A Multi‐Institutional Study of Independent Dose Verification for IMRT
Authors: Baba H , Kamima T , Takahashi R , Kawai D , Sugawara Y , Yamamoto T , Sato A , Yamashita M , Tachibana H
Published: June 2015

Purpose:

AAPM TG114 does not cover the independent verification for IMRT. We conducted a study of independent dose verification for IMRT in seven institutes to show the feasibility.

Methods:

384 IMRT plans in the sites of prostate and head and neck (HN) were collected from the institutes, where the planning was performed using Eclipse and Pinnacle3 with the two techniques of step and shoot (S&S) and sliding window (SW). All of the institutes used a same independent dose verification software program (Simple MU Analysis: SMU, Triangle Product, Ishikawa, JP), which is Clarkson‐based and CT images were used to compute radiological path length. An ion‐chamber measurement in a water‐equivalent slab phantom was performed to compare the doses computed using the TPS and an independent dose verification program. Additionally, the agreement in dose computed in patient CT images between using the TPS and using the SMU was assessed. The dose of the composite beams in the plan was evaluated.

Results:

The agreement between the measurement and the SMU were −2.3±1.9 % and −5.6±3.6 % for prostate and HN sites, respectively. The agreement between the TPSs and the SMU were −2.1±1.9 % and −3.0±3.7 for prostate and HN sites, respectively. There was a negative systematic difference with similar standard deviation and the difference was larger in the HN site. The S&S technique showed a statistically significant difference between the SW. Because the Clarkson‐based method in the independent program underestimated (cannot consider) the dose under the MLC.

Conclusion:

The accuracy would be improved when the Clarkson‐based algorithm should be modified for IMRT and the tolerance level would be within 5%.
SU‐E‐T‐50: A Multi‐Institutional Study of Independent Dose Verification Software Program for Lung SBRT
Authors: Kawai D , Takahashi R , Kamima T , Baba H , Yamamoto T , Kubo Y , Ishibashi S , Higuchi Y , Takahashi H , Tachibana H
Published: June 2015

Purpose:

The accuracy of dose distribution depends on treatment planning system especially in heterogeneity‐region. The tolerance level (TL) of the secondary check using the independent dose verification may be variable in lung SBRT plans. We conducted a multi‐institutional study to evaluate the tolerance level of lung SBRT plans shown in the AAPM TG114.

Methods:

Five institutes in Japan participated in this study. All of the institutes used a same independent dose verification software program (Simple MU Analysis: SMU, Triangle Product, Ishikawa, JP), which is Clarkson‐based and CT images were used to compute radiological path length. Analytical Anisotropic Algorithm (AAA), Pencil Beam Convolution with modified Batho‐method (PBC‐B) and Adaptive Convolve (AC) were used for lung SBRT planning. A measurement using an ion‐chamber was performed in a heterogeneous phantom to compare doses from the three different algorithms and the SMU to the measured dose. In addition to it, a retrospective analysis using clinical lung SBRT plans (547 beams from 77 patients) was conducted to evaluate the confidence limit (CL, Average±2SD) in dose between the three algorithms and the SMU.

Results:

Compared to the measurement, the AAA showed the larger systematic dose error of 2.9±3.2% than PBC‐B and AC. The Clarkson‐based SMU showed larger error of 5.8±3.8%. The CLs for clinical plans were 7.7±6.0 % (AAA), 5.3±3.3 % (AC), 5.7±3.4 % (PBC ‐B), respectively.

Conclusion:

The TLs from the CLs were evaluated. A Clarkson‐based system shows a large systematic variation because of inhomogeneous correction. The AAA showed a significant variation. Thus, we must consider the difference of inhomogeneous correction as well as the dependence of dose calculation engine.
SU‐E‐T‐51: A National QA Audit of QA Systems Used for IMRT and VMAT Patient QA
Authors: Seravalli E , Houweling A , Van Gellekom M , Kaas J , Kuik M , Loeff E , Raaben T , De Pooter J , de Vries W , Van de Kamer J
Published: June 2015

Purpose:

To independently validate patient‐specific quality assurance for IMRT and VMAT plans using the same set of treatment plans for all institutes.

Methods:

In February 2014 we devised a set of treatment plans: simple IMRT/VMAT plans; more complex IMRT/VMAT plans and a stereotactic VMAT plan, all 6MV for both Varian and Elekta linacs. In total we used 5 Varian and 8 Elekta plans. The plans were imported in the institute's treatment planning system for dose computation on the phantom of the audit team and the institute's phantom. Additionally, 10x10 cm2 fields were made and computed on both phantoms. Next, the audit team performed measurements using the audit equipment. So far, 18 audits have been performed and we expect to have concluded the audits by June 2015. The measurements were performed using an ionization chamber (PinPoint, PTW), Gafchromic film and a 2D ionization chamber array, all in an octagonal phantom (Octavius, PTW, see Figure). Differences between the measured and computed 2D dose distributions were investigated using a gamma analysis with a 5%/1mm criterion for the stereotactic treatment plan and 3%/3mm for the other plans. Additionally, the participating centres performed QA measurements of the same treatment plans according to their local protocol and equipment.

Results:

For the 10x10 field on the phantom, the first 18 audits showed differences with respect to the planning of −0.21 (range: −1.8; 2.2)%. See Table 1 for more results. The findings compared well with the QA measurement results reported by the institutions according to their local protocols.

Conclusion:

These preliminary results demonstrate that such a national QA audit is feasible. Importing and computing the prepared treatment plans in the planning systems in use in the country is achievable. The local QA systems provided similar results as found with the audit.
SU‐E‐T‐61: A Practical Process for Fabricating Passive Scatter Proton Beam Modulation Compensation Filters Using 3D Printing
Authors: Zhao T , Drzymala R
Published: June 2015

Purpose:

The purpose of this project was to devise a practical fabrication process for passive scatter proton beam compensation filters (CF) that is competitive in time, cost and effort using 3D printing.

Methods:

DICOM compensator filter files for a proton beam were generated by our Eclipse (Varian, Inc.) treatment planning system. The compensator thickness specifications were extracted with in‐house software written in Matlab (MathWorks, Inc.) code and written to a text file which could be read by the Rhinoceros 5, computer‐aided design (CAD) package (Robert McNeel and Associates), which subsequently generated a smoothed model in a STereoLithographic also known as a Standard Tesselation Language file (STL). The model in the STL file was subsequently refined using Netfabb software and then converted to printing instructions using Cura. version 15.02.1. for our 3D printer. The Airwolf3D, model HD2x, fused filament fabrication (FFF) 3D printer (Airwolf3D.com) was used for our fabrication system with a print speed of 150mm per second. It can print in over 22 different plastic filament materials in a build volume of 11” × 8” × 12”. We choose ABS plastic to print the 3D model of the imprint for our CFs.

Results:

Prints of the CF could be performed at a print speed of 70mm per second. The time to print the 3D topology for the CF for the 14 cm diameter snout of our Mevion 250 proton accelerator was less than 3 hours. The printed model is intended to subsequently be used as a mold to imprint a molten wax cylindrical to form the compensation after cooling. The whole process should be performed for a typical 3 beam treatment plan within a day.

Conclusion:

Use of 3D printing is practical and can be used to print a 3D model of a CF within a few hours.
SU‐E‐T‐62: A Preliminary Experience of Using EPID Transit Dosimetry for Monitoring Daily Dose Variations in Radiation Treatment Delivery
Authors: Yao R , Chisela W
Published: June 2015

Purpose:

To investigate the use of EPID transit dosimetry for monitoring daily dose variations in radiation treatment delivery.

Methods:

A patient with head and neck cancer treated using nine field IMRT beams was used in this study. The prescription was 45 Gy in 25 fractions. A KV CBCT was acquired before each treatment on a Varian NTX linear accelerator. Integrated images using MV EPID were acquired for each treatment beam. Planning CT images, treatment plan, and daily integrated images were imported into a commercial QA software Dosimetry Check (v4r4 Math Resolutions, LLC, Columbia, MD) to calculate 3D dose of the day assuming 25 fractions treatment. Planning CT images were deformed and registered to each daily CBCT using Varian SmartAdapt (v11.MR2). ROIs were then propagated from planning CT to daily CBCT. The correlation between maximum, average dose of ROIs and ROI volume, center of mass shift, Dice Similarity Coefficient (DSC) were investigated.

Results:

Not all parameters investigated showed strong correlations. For PTV and CTV, the average dose has inverse correlation with their volume change (correlation coefficient −0.52, −0.50, respectively) and DSC (−0.59, −0.59, respectively). The average dose of right parotid has correlation with its volume change (0.56). The maximum dose of spinal cord has correlation with the center of mass superior‐inferior shift (0.52) and inverse correlation with the center of mass anterior‐posterior shift (−0.73).

Conclusion:

Transit dosimetry using EPID images collected during treatment delivery offers great potential to monitor daily dose variations due to patient anatomy change, motion, and setup errors in radiation treatment delivery. It can provide a patient‐specific QA tool valuable for adaptive radiation therapy. Further work is needed to validate the technique.
SU‐E‐T‐64: A Programmable Moving Insert for the ArcCHECK Phantom for Dose Verification of Respiratory‐Gated VMAT
Authors: Gaede S , Young H , Mulligan M , Jordan K
Published: June 2015

Purpose:

To present a customized programmable moving insert for the ArcCHECK™ phantom that can, in a single delivery, check both entrance dosimetry, while simultaneously verifying the delivery of respiratory‐gated VMAT.

Methods:

The cylindrical motion phantom uses a computer‐controlled stepping motor to move an insert inside a stationery sleeve. Insert motion is programmable and can include rotational motion in addition to linear motion along the axis of the cylinder. The sleeve fits securely in the bore of the ArcCHECK™. Interchangeable inserts, including an A1SL chamber, optically‐stimulated luminescence dosimeters, radiochromic film, or 3D gels, allow this combination to be used for commissioning, routine quality assurance, and patient‐specific dosimetric verification of respiratory‐gated VMAT. Before clinical implementation, the effect of a moving insert on the ArcCHECK™ measurements was considered. First, the measured dose to the ArcCHECK™ containing multiple inserts in the static position was compared to the calculated dose during multiple VMAT treatment deliveries. Then, dose was measured under both sinusoidal and real‐patient motion conditions to determine any effect of the moving inserts on the ArcCHECK™ measurements. Finally, dose was measured during gated VMAT delivery to the same inserts under the same motion conditions to examine any effect of various beam “on‐and‐off” and dose rate ramp “up‐and‐down”. Multiple comparisons between measured and calculated dose to different inserts were also considered.

Results:

The pass rate for the static delivery exceeded 98% for all measurements (3%/3mm), suggesting a valid setup for entrance dosimetry. The pass rate was not altered for any measurement delivered under motion conditions. A similar Result was observed under gated VMAT conditions, including agreement of measured and calculated dose to the various inserts.

Conclusion:

Incorporating a programmable moving insert within the ArcCHECK™ phantom provides an efficient verification of respiratory‐gated VMAT delivery that is useful during commissioning, routine quality assurance, and patient‐specific dose verification.

Prototype phantom development and testing was performed in collaboration with Modus Medical Devices Inc. (London, ON). No financial support was granted.
SU‐E‐T‐66: A Prototype for Couch Based Real‐Time Dosimetry in External Beam Radiotherapy
Authors: Ramachandran P
Published: June 2015

Purpose:

The main purpose of this study is to design a prototype for couch‐based based real time dosimetry system in external beam radiotherapy

Methods:

A prototype of 100 ionization chambers was designed on a printed circuit board by etching the copper layer and each ionization chamber was wired to a 50 pin connector. The signals from the two 50 pin connectors collected from the ionization chambers were then transferred to a PXI module from National Instruments. The PXI module houses a current amplifier that amplifies the charge collected from the ionization chamber. The amplified signal is then sent to a digital multimeter module for converting the analog signal to digital signal. A software was designed in labview to read and display the signals obtained from the PXI module. A couch attachment frame was designed to house the 100 ionization chamber module. The frame was fixed underneath the treatment couch for measuring the dose during treatment.

Resutls:

The ionization chamber based prototype dosimetry was tested for simple radiotherapy treatment fields and found to be a useful device for measuring real time dosimetry at the treatment couch plane. This information could be used to assess the delivered dose to a patient during radiotherapy. It could be used as an invivo dosimeter during radiotherapy.

Conclusion:

In this study, a prototype for couch based real time dosimetry system was designed and tested. The prototype forms a basis for the development of large scale couch based real time dosimetry system that could be used to perform morning QA prior to treatment, assess real time doses delivered to patient and as a device to monitor the output of the treatment beam.

Peter MacCallum Cancer Foundation
SU‐E‐T‐68: A Quality Assurance System with a Web Camera for High Dose Rate Brachytherapy
Authors: Ueda Y , Hirose A , Oohira S , Isono M , Tsujii K , Miyazaki M , Kawaguchi Y , Konishi K , Teshima T
Published: June 2015

Purpose:

The purpose of this work was to develop a quality assurance (QA) system for high dose rate (HDR) brachytherapy to verify the absolute position of an 192Ir source in real time and to measure dwell time and position of the source simultaneously with a movie recorded by a web camera.

Methods:

A web camera was fixed 15 cm above a source position check ruler to monitor and record 30 samples of the source position per second over a range of 8.0 cm, from 1425 mm to 1505 mm. Each frame had a matrix size of 480×640 in the movie. The source position was automatically quantified from the movie using in‐house software (built with LabVIEW) that applied a template‐matching technique. The source edge detected by the software on each frame was corrected to reduce position errors induced by incident light from an oblique direction. The dwell time was calculated by differential processing to displacement of the source. The performance of this QA system was illustrated by recording simple plans and comparing the measured dwell positions and time with the planned parameters.

Results:

This QA system allowed verification of the absolute position of the source in real time. The mean difference between automatic and manual detection of the source edge was 0.04 ± 0.04 mm. Absolute position error can be determined within an accuracy of 1.0 mm at dwell points of 1430, 1440, 1450, 1460, 1470, 1480, 1490, and 1500 mm, in three step sizes and dwell time errors, with an accuracy of 0.1% in more than 10.0 sec of planned time. The mean step size error was 0.1 ± 0.1 mm for a step size of 10.0 mm.

Conclusion:

This QA system provides quick verifications of the dwell position and time, with high accuracy, for HDR brachytherapy.

This work was supported by the Japan Society for the Promotion of Science Core‐to‐Core program (No. 23003).
SU‐E‐T‐82: A Study On Enhanced Dynamic Wedge (EDW) Dosimetry Using 2D Seven29 Ion Chamber Array Detector
Authors: Kumar Syam , Aparna
Published: June 2015

Purpose:

To study the dosimetric properties of Enhanced Dynamic Wedge (EDW) using PTW Seven29 ion chamber array

Methods:

PTW Seven29 ion chamber array and Solid Water phantoms for different depths were used for the study. The study was carried out in Varian Clinac ix with photon energies, 6MV & 15MV. Primarily the solid water phantoms with the 2D array were scanned using a CT scanner (GE Optima 580) at different depths. These scanned images were used for EDW planning in an Eclipse treatment planning system (version 10). Planning was done for different wedge angles and for different depths for 6MV & 15MV. A dose of 100 CGy was delivered in each cases. For each delivery, calculated the Monitoring Unit (MU) required. Same set‐up was created before delivering the plans in Varian Clinac‐ix. For each clinically relevant depth and for different wedge angles, the same MU was delivered as calculated. Different wedged dose distributions where reconstructed from the measured 2D array data using the in‐house developed excel program.

Results:

It is observed that the shoulder like region in the profile which reduces as depth increases. For the same depth and energy, the percentage difference between planned and measured dose is lesser than 3%. For smaller wedge angles, the percentage difference is found to be greater than 3% for the largest wedge angle. Standard deviation between measured doses at shoulder region for planned and measured profiles is 0.08 and 0.02 respectively. Standard deviations between planned and measured wedge factors for different depths (2.5cm, 5cm, 10cm, and 15cm) are (0.0021, 0.0007, 0.0050, 0.0001) for 6MV and (0.0024, 0.0191, 0.0013, 0.0005) for 15MV respectively.

Conclusion:

The 2D Seven29 ion chamber array is a good tool for the Enhanced Dynamic Wedge (EDW) dosimetry.
SU‐E‐T‐84: A Survey On Radiotherapy Table Tolerances and Couch Overrides of Over 350 Institutions
Authors: Chinsky B , Patel R , Panfil J , Surucu M , Roeske J
Published: June 2015

Purpose:

Table overrides are relatively common in radiotherapy, yet are a potential safety concern. The goal of this study was to survey current departmental policies on treatment couch overrides and table tolerance values used clinically.

Methods:

A 25 question electronic survey on couch overrides and tolerances was sent to full members of the AAPM. In the first part of the survey, participants were asked: if table overrides were allowed at their institution, who was allowed to perform these overrides, and if imaging was required with overrides. In the second part of the survey, individuals were asked to provide table tolerance data for the following sites: brain/head & neck, lung, breast, abdomen/pelvis and prostate. Each site was further divided into IMRT/VMAT and 3D conformal techniques. Free‐text spaces were provided, allowing respondents to enter any table tolerance data they were unable to specify under the treatment sites listed.

Results:

A total of 361 individuals responded, of which approximately half participated in the couch tolerances portion of the survey. Overall, 86% of respondents’ institutions allow couch tolerance overrides at treatment. Therapists were the most common staff members permitted to perform overrides, followed by physicists, dosimetrists, and physicians, respectively. Of the institutions allowing overrides, 34% reported overriding daily. More than half of the centers require documentation of the override and/or a setup image (acquired after override) to radiographically verify the treatment site. With respect to table tolerances, groups resulting from the free‐text responses were at the two extremes; SRS/SBRT were the tightest, while clinical setup/mets/extremities were the most generous. There was no qualitative difference between IMRT/VMAT and 3D conformal table tolerances.

Conclusion:

This work is intended to stimulate a discussion within the radiotherapy community. This discussion, supplemented by the survey results provides an opportunity for institutions to enhance patient safety during daily treatment delivery.
SU‐E‐T‐95: An Alternative Option for Reducing Lung Dose for Electron Scar Boost Irradiation in Post‐Mastectomy Breast Cancer Patients with a Thin Chest Wall
Authors: Lee Y , Kumar P , Mitchell M
Published: June 2015

Purpose:

Breast cancer patients who undergo a mastectomy often require post‐mastectomy radiation therapy (PMRT) due to high risk disease characteristics. PMRT usually accompanies scar boost irradiation (10–16Gy in 5–8 fractions) using en face electrons, which often results in increased dose to the underlying lungs, thereby potentially increasing the risk of radiation pneumonitis. Hence, this study evaluated water‐equivalent phantoms as energy degraders and as an alternative to a bolus to reduce radiation dose to the underlying lungs for electron scar boost irradiation.

Methods:

Percent depth dose (PDD) profiles of 6 MeV (the lowest electron energy available in most clinics) were obtained without and with commercial solid water phantoms (1 to 5mm by 1mm increments) placed on top of electron cones. Phantom attenuation was measured by taking a ratio of outputs with to without the phantoms in 10×10cm2 cone size for monitor unit (MU) calculation. In addition, scatter dose to contralateral breast was measured on a human‐like phantom using two selected scar (short and long) boost patient setups.

Results:

The PDD plots showed that the solid water phantoms and the bolus had similar dosimetric effects for the same thickness. Lower skin dose (up to 3%) to ipsilateral breast was observed with a 5mm phantom compared with a 5mm bolus (up to 10%) for all electron cones. Phantom attenuation was increased by 50% with about a 4.5mm phantom. Also, the energy degraders caused scatter dose to contralateral breast by a factor of 3 with a 5mm phantom.

Conclusion:

Our results demonstrate the feasibility of using water‐equivalent phantoms to reduce lung dose using en face electrons in patients with a thin chest wall undergoing PMRT. The disadvantages of this treatment approach (i.e., the increase in MUs and treatment time, and clinically insignificant scatter dose to the contralateral breast given usually 10Gy) are outweighed by its above clinical benefits.
SU‐E‐T‐101: An Arc Based Treatment Technique for Proton PBS Beam Utilizing Overlaid Bragg Peak Spots
Authors: Wang Z
Published: June 2015

Purpose:

When Bragg peaks from proton PBS beams are overlaid from a large number of directions, it creates a localized dose distribution with rapid falloff. By positioning these overlaid Bragg peak (OBP) spots strategically throughout the targeted volume, a uniform and conformal dose distribution can be achieved. When PBS spots from all the directions are delivered in an arc fashion, it becomes the Volume Modulated Proton Therapy (VMPT) technique which corresponds to the VMAT technique in photon therapy.

Methods:

PBS beam from an IBA machine and a 20cm radius cylindrical phantom were simulated with GATE/GEANT4. To position the OBP spot at the center of the phantom, single energy PBS beam from 10/36/360 equally‐spaced directions around the phantom was used. To position the OBP spot at an off‐center position, PBS beams with various energies were used. To form a uniform dose distribution in 1D/2D, OBP spots are positioned in arrays with spot intervals and weight optimized.

Results:

Dose profile for single energy OBP spot improved when the number of beam angles increased from 10 to 36, but remained same from 36 to 360 angles. Multi‐energy OBP spot had a slightly non‐symmetrical profile due to the different energy proton beams used. By optimizing each OBP spot's weight, 11 OBP spots with 10mm interval size created a uniform dose distribution covering a length of 10cm. Similarly, 11×11 OBP spots in an array of 10mm interval size created a uniform dose distribution covering an area of 10×10cm2.

Conclusion:

We demonstrated the technique of using OBP spots to deliver uniform dose in 1D/2D. This technique can be easily extended through the optimization of position and weight of OBP spots in 3D and becomes the arc based VMPT technique. Further studies are needed to explore the optimization methods and full potential of the VMPT technique.
SU‐E‐T‐102: An Efficient Method to Quantitatively Detect Sub‐Millimeter MLC Bank Alignment Errors Using a Well‐Known QA Device
Authors: Nguyen A , Patton K , Paul T , Duggar W , Yang C
Published: June 2015

Purpose:

To introduce an ultra‐sensitive procedure that can detect and quantify systematic MLC bank alignment error down to 0.2 mm or less. The procedure takes only 5 minutes to run and employs the familiar MapCheck2 device.

Methods:

Elekta iCOM v13.0 was used to create and deliver a modified 4‐Quadrant field on the Elekta Infinity Linac. The 2D dose distribution was collected on the MapCheck2, at 100 Source‐Detector‐Distance with no external buildup. Using SNC Patient v6.2.3 this planar dose was compared against a reference planar dose established preferably right after commissioning or the most recent Annual QA. To obtain baseline results and tolerance levels leaf bank alignment shifts ranging from −1.0 mm (in‐field) to +1 mm (out‐of‐field) in 0.1 mm increment were intentionally induced. Each shifted field is matched against a reference field with zero shift. A Y‐profile through the CAX, perpendicular to MLC traveling direction, was selected in the software. Percentage dose differences at selected points on this profile were recorded. The mean dose differences were then plotted against the corresponding shift distances.

Results:

The test is extremely sensitive in picking up MLC alignment error. Shift down to 0.2 mm can be easily detected. Furthermore, the relationship between dose deviations and MLC bank alignment errors is shown to be quite linear. For our setup, each 0.1 mm shift in MLC bank would Result in approximately 2.7% dose deviation from baseline. Based on this Result, monthly QA tolerance level at was set at +/−7%, which translates roughly to 0.25 mm in alignment error.

Conclusion:

A novel method to quantify MLC alignment error was introduced. The test is short, simple yet very effective. It's been implemented clinically as part of our institution monthly machine QA.
SU‐E‐T‐120: Analytic Dose Verification for Patient‐Specific Proton Pencil Beam Scanning Plans
Authors: Chang C , Mah D
Published: June 2015

Purpose:

To independently verify the QA dose of proton pencil beam scanning (PBS) plans using an analytic dose calculation model.

Methods:

An independent proton dose calculation engine is created using the same commissioning measurements as those employed to build our commercially available treatment planning system (TPS). Each proton PBS plan is exported from the TPS in DICOM format and calculated by this independent dose engine in a standard 40 × 40 × 40 cm water tank. This three‐dimensional dose grid is then compared with the QA dose calculated by the commercial TPS, using standard Gamma criterion. A total of 18 measured pristine Bragg peaks, ranging from 100 to 226 MeV, are used in the model. Intermediate proton energies are interpolated. Similarly, optical properties of the spots are measured in air over 15 cm upstream and downstream, and fitted to a second‐order polynomial. Multiple Coulomb scattering in water is approximated analytically using Preston and Kohler formula for faster calculation. The effect of range shifters on spot size is modeled with generalized Highland formula. Note that the above formulation approximates multiple Coulomb scattering in water and we therefore chose not use the full Moliere/Hanson form.

Results:

Initial examination of 3 patient‐specific prostate PBS plans shows that agreement exists between 3D dose distributions calculated by the TPS and the independent proton PBS dose calculation engine. Both calculated dose distributions are compared with actual measurements at three different depths per beam and good agreements are again observed.

Conclusion:

Results here showed that 3D dose distributions calculated by this independent proton PBS dose engine are in good agreement with both TPS calculations and actual measurements. This tool can potentially be used to reduce the amount of different measurement depths required for patient‐specific proton PBS QA.
SU‐E‐T‐122: Anisotropic Analytical Algorithm (AAA) Vs. Acuros XB (AXB) in Stereotactic Treatment Planning
Authors: Mynampati D , Kuo H , Yaparpalvi R , Scripes P Godoy , Tome W
Published: June 2015

Purpose:

To evaluate dosimetric differences between superposition beam model (AAA) and determinant photon transport solver (AXB) in lung SBRT and Cranial SRS dose computations.

Methods:

Ten Cranial SRS and ten Lung SBRT plans using Varian, AAA _11.0 were re‐planned using Acuros _XB_11.0 with fixed MU. 6MV photon Beam model with HD120_MLC used for dose calculations. Four non‐coplanar conformal arcs used to deliver 21Gy or 18Gy to SRS targets (0.4 to 6.2cc). 54Gy (3Fractions) or 50Gy (5Fractions) was planned for SBRT targets (7.3 to 13.9cc) using two VAMT non‐coplanar arcs. Plan comparison parameters were dose to 1% PTV volume (D1), dose to 99% PTV volume( D99), Target mean (Dmean), Conformity index (ratio of prescription isodose volume to PTV), Homogeneity Index [ (D2%‐D98%)/Dmean] and R50 (ratio of 50% of prescription isodose volume to PTV). OAR parameters were Brain volume receiving 12Gy dose (V12Gy) and maximum dose (D0.03) to Brainstem for SRS. For lung SBRT, maximum dose to Heart and Cord, Mean lung dose (MLD) and volume of lung receiving 20Gy (V20Gy) were computed. PTV parameters compared by percentage difference between AXB and AAA parameters. OAR parameters and HI compared by absolute difference between two calculations. For analysis, paired t‐test performed over the parameters.

Results:

Compared to AAA, AXB SRS plans have on average 3.2% lower D99, 6.5% lower CI and 3cc less Brain_V12. However, AXB SBRT plans have higher D1, R50 and Dmean by 3.15%, 1.63% and 2.5%. For SRS and SBRT, AXB plans have average HI 2 % and 4.4% higher than AAA plans. In both techniques, all other parameters vary within 1% or 1Gy. In both sets only two parameters have P>0.05.

Conclusion:

Even though t‐test results signify difference between AXB and AAA plans, dose differences in dose estimations by both algorithms are clinically insignificant.
SU‐E‐T‐134: Assessing the Capabilities of An MU Model for Fields as Small as 2cm in a Passively Scattered Proton Beam
Authors: Simpson R , Ghebremedhin A , Gordon I , Patyal B
Published: June 2015

Purpose:

To assess and expand the capabilities of the current MU model for a passively scattered proton beam. The expanded MU model can potentially be used to predict the dose/MU for fields smaller than 2cm in diameter and reduce time needed for physical calibrations.

Methods:

The current MU model accurately predicted the dose/MU for more than 800 fields when compared to physical patient calibrations. Three different ion chambers were used in a Plastic Water phantom for physical measurements: T1, PIN, and A‐16. The original MU model predicted output for fields that were affected by the bolus gap factor (BGF) and nozzle extension factor (NEF). As the system was tested for smaller treatment fields, the mod wheel dependent field size factor (MWDFSF) had to be included to describe the changes observed in treatment fields smaller than 3cm. The expanded model used Clarkson integration to determine the appropriate value for each factor (field size factor (FSF), BGF, NEF, and MWDFSF), to accurately predict the dose/MU for fields smaller than 2.5cm in effective diameter.

Results:

The expanded MU model demonstrated agreement better than 2% for more than 800 physical calibrations that were tested. The minimum tested fields were 1.7cm effective diameter for 149MeV and 2.4cm effective diameter for 186MeV. The inclusion of Clarkson integration into the MU model enabled accurate prediction of the dose/MU for very small and irregularly shaped treatment fields.

Conclusion:

The MU model accurately predicted the dose/MU for a wide range of treatment fields used in the clinic. The original MU model has been refined using factors that were problematic to accurately predict the dose/MU: the BGF, NEF, and MWDFSF. The MU model has minimized the time for determining dose/MU and reduced the time needed for physical calibrations, improving the efficiency of the patient treatment process.
SU‐E‐T‐135: Assessing the Clinical Impact of Approximations in Analytical Dose Calculations for Proton Therapy
Authors: Schuemann J , Giantsoudi D , Grassberger C , Paganetti H
Published: June 2015

Purpose:

To estimate the clinical relevance of approximations made in analytical dose calculation methods (ADCs) used for treatment planning on tumor coverage and tumor control probability (TCP) in proton therapy.

Methods:

We compared dose distributions planned with ADC to delivered dose distributions (as determined by TOPAS Monte Carlo (MC) simulations). We investigated 10 patients per site for 5 treatment sites (head‐and‐neck, lung, breast, prostate, liver). We evaluated differences between the two dose distributions analyzing dosimetric indices based on the dose‐volume‐histograms, the γ‐index and the TCP. The normal tissue complication probability (NTCP) was estimated for the bladder and anterior rectum for the prostate patients.

Results:

We find that the target doses are overestimated by the ADC by 1–2% on average for all patients considered. All dosimetric indices (the mean dose, D95, D50 and D02, the dose values covering 95%, 50% and 2% of the target volume, respectively) are predicted within 5% of the delivered dose. A γ‐index with a 3%/3mm criteria had a passing rate for target volumes above 96% for all patients. The TCP predicted by the two algorithms was up to 2%, 2.5%, 6%, 6.5%, and 11% for liver and breast, prostate, head‐and‐neck and lung patients, respectively. Differences in NTCP for anterior‐rectum and bladder for prostate patients were less than 3%.

Conclusion:

We show that ADC provide adequate dose distributions for most patients, however, they can Result in underdosage of the target by as much as 5%. The TCP was found to be up to 11% lower than predicted. Advanced dose‐calculation methods like MC simulations may be necessary in proton therapy to ensure target coverage for heterogeneous patient geometries, in clinical trials comparing proton therapy to conventional radiotherapy to avoid biases due to systematic discrepancies in calculated dose distributions, and, if tighter range margins are considered.

Fully funded by NIH grants.
SU‐E‐T‐137: Attenuation of Carbon Fiber IGRT Couch Top for SBRT By Using a MapCheck 2 and Ion Chamber
Authors: Dou K , Li B , Jacobs M , Laser B , Lerma F , Sarfaraz M
Published: June 2015

Purpose:

It is common to use posterior oblique beams in intensity‐modulated radiation therapy (IMRT) and volumetric‐modulated arc therapy (VMAT) for regular and stereotactic body radiotherapy (SBRT). Beam attenuation by the treatment couch is not negligible when the couch is in the beam portal. In this study, we have correlated relative dose versus beam angle through a MapCheck 2TM diode array and an ionization chamber measurement.

Methods:

A Varian TrueBeam linear accelerator equipped with the image‐guided radiation therapy (IGRT) carbon fiber couch was used for delivery of radiation with different photon energies of 6MV, 15MV and 6MV FFF. A MapCHECK 2TM diode array with a MapPhan was employed for the measurement at three field sizes (3 by 3, 5 by 5, and 10 by 10 cm). The independent measurement was performed with an ionization chamber placed at the center of an acrylic cylindrical phantom at the isocenter. The measured data with the MapCHECK 2TM diode array were averaged over the central 2×2 cm diodes for each measurement. The couch attenuation was deduced from the angular dependence with and without the couch involvement.

Results:

For the MapCHECK 2TM with MapPhan used only the obvious attenuation was observed at the beam angle between 5 and 10 degree from the angular dependence using a 6MV photon beam. The similar Result was obtained for the MapCHECK 2TM /MapPhan with the couch involvement. The couch attenuation was then deduced from the difference of these two sets of data measurements. Maximum couch attenuation was found up to 4.2% which was verified with the ion chamber measurement.

Conclusion:

A maximum attenuation of 4.2% was found for the carbon fiber couch top using the MapCHECK 2TM/MapPhan, which is consistent with the ion chamber measurement.