bluuebunny/crossref_metadata_2025_split huggingface dataset:
SU‐E‐T‐439: An Improved Formula of Scatter‐To‐Primary Ratio for Photon Dose Calculation
Authors: Zhu T 
Published: July 2014

Purpose:Scatter‐to‐primary ratio (SPR) is an important dosimetric quantity that describes the contribution from the scatter photons in an external photon beam. The purpose of this study is to develop an improved analytical formula to describe SPR as a function of circular field size (r) and depth (d) using Monte Carlo (MC) simulation.Methods:MC simulation was performed for Mohan photon spectra (Co‐60, 4, 6, 10, 15, 23 MV) using EGSNRC code. Point‐spread scatter dose kernels in water are generated. The scatter‐to‐primary ratio (SPR) is also calculated using MC simulation as a function of field size for circular field size with radius r and depth d. The doses from forward scatter and backscatter photons are calculated using a convolution of the point‐spread scatter dose kernel and by accounting for scatter photons contributing to dose before (z' <d) and after (z' >d) reaching the depth of interest, d, where z' is the location of scatter photons, respectively. The depth dependence of the ratio of the forward scatter and backscatter doses is determined as a function of depth and field size.Results:We are able to improve the existing 3‐parameter (a, w, d0) empirical formula for SPR by introducing depth dependence for one of the parameter d0, which becomes 0 for deeper depths. The depth dependence of d0 can be directly calculated as a ratio of backscatter‐to‐forward scatter doses for otherwise the same field and depth. With the improved empirical formula, we can fit SPR for all megavoltage photon beams to within 2%. Existing 3‐parameter formula cannot fit SPR data for Co‐60 to better than 3.1%.Conclusion:An improved empirical formula is developed to fit SPR for all megavoltage photon energies to within 2%.
SU‐E‐T‐440: Elliptical Source Model for IMRT and VMAT with Complex Modulation
Authors: Tang G , Lim S , LoSasso T 
Published: July 2014

Purpose:To improve dose calculation accuracy for highly modulated fields in IMRT and VMAT using an elliptical source model.Methods:For 2 TrueBeam linacs equipped with Millennium and HD MLCs, the source model of 6 and 15 MV beams was optimized with adjustments of the focal spot dimensions in the Varian Eclipse TPS. Starting from the measured focal spot size of 1.75 × 1.75 mm, the y‐dimension of the source was varied while the x‐dimension was kept constant. In addition, all other modeling parameters such as the extrafocal source and dosimetric leaf gap remained unchanged. The different models were compared with measured penumbra for jaws and MLC using ion chamber and diode. For 2 IMRT and 2 VMAT cases, the calculated doses were compared with radiochromic film and EPID measurements.Results:A source size of 1.75 × 0.75 mm was found to be optimal for beam modeling. The calculated penumbra (80–20) of a MLCdefined open field was within 0.8 mm compare to measurement for both the inline and crossline profile for various field sizes and depths. The dose gradients in the direction perpendicular to the MLC travel direction was better modeled compared to the source model with a circular focal spot of 1.75 mm for the fields with rigorous MLC modulation. Local differences up to 8% were found in those regions.Conclusion:The conventional source model using a symmetric focal spot tends to overestimate the penumbra perpendicular to the MLC travel direction, resulting in an inaccurate dose calculation especially for fields with complex MLC modulation. This can be improved with an elliptical source model and it can be considered as a standard as IMRT and VMAT treatments are becoming a norm for the clinic.
SU‐E‐T‐441: Comparison of Dose Distributions for Spot‐Scanned Pencil‐Beam and Scattered‐Beam Proton Treatments of Ocular Tumors
Authors: Deisher A , Whitaker T , Kooy H , Trofimov A , Kruse J 
Published: July 2014

Purpose:To study the cross‐field and depth dose profiles of spot‐scanned pencil beam configurations for the treatment of ocular tumors and to compare their performance to a simulated scattered beam.Methods:Dose distributions in a cubic water phantom were compared for beams that passed through a final 24mm diameter aperture to deposit maximum dose at 2.4cm depth. The pencil‐beam spots formed a hexagonally‐packed ring with a center‐to‐center spacing of 4mm. The protons exited the nozzle with energy 95.5MeV, traversed a 4.5cm water‐equivalent range shifter, and travelled either 42.5cm or 100cm to the phantom surface. The aperture‐to‐phantom distance (APD) was 5.7cm to allow room for eye‐tracking hardware. A configuration with APD=0 was also tested. The scattered beam was generated with energy 159MeV, passed through 127mm of Lexan, exited the final aperture, and travelled 5.7cm to the phantom surface. This latter configuration is comparable to the MGH single scattered beamline. All beams were modelled with TOPAS1.0‐beta6 compiled with GEANT4.9.6p2.Results:The modeled scattered beam produced a distal fall‐off along the central axis of zd90%‐zd10%=3.6mm. For the pencil beam, the zd90%‐zd10% was 1.6mm in all configurations. The scattered beam's cross‐field penumbra at depth of maximum dose was r90%‐ r10%=1.9mm. For the spot‐scanned configuration with the range‐shifter‐tophantom distance (RsPD) of 100cm, similar cross‐field profiles were achieved with r90%‐r10%=2.0mm. At shorter RsPD of 42.5cm, the crossfield penumbras were 5.6mm and 7.7mm for APD=0cm and APD=5.7cm, respectively.Conclusion:For proton treatments employing a range shifter, the cross‐field and central axis dose profiles depend on the quality of the original beam, the size of the range shifter, the distance from the range shifter exit to the patient, and the distance from the final aperture to the patient. A spot‐scanned pencil beam configuration can achieve cross‐field penumbras equal to a scattered beam and superior distal gradients.
SU‐E‐T‐442: Sensitivity of Quality Assurance Tools to Delivery Errors On a Magnetic Resonance‐Imaging Guided Radiation Therapy (MR‐IGRT) System
Authors: Rodriguez V , Li H , Yang D , Kashani R , Wooten H , Dempsey J , Mutic S , Green O 
Published: July 2014

Purpose:To test the sensitivity of the quality assurance (QA) tools actively used on a clinical MR‐IGRT system for potential delivery errors.Methods:Patient‐specific QA procedures have been implemented for a commercially available Cobalt‐60 MR‐IGRT system. The QA tools utilized were a MR‐compatible cylindrical diode‐array detector (ArcCHECK) with a custom insert which positions an ionization chamber (Exradin A18) in the middle of the device, as well as an in‐house treatment delivery verification program. These tools were tested to investigate their sensitivity to delivery errors. For the ArcCHECK and ion chamber, a baseline was established with a static field irradiation to a known dose. Variations of the baseline were investigated which included rotated gantry, altered field size, directional shifts, and different delivery time. In addition, similar variations were tested with the automated delivery verification program that compared the treatment parameters in the machine delivery logs to the ones in the plan. To test the software, a 3‐field conformal plan was generated as the baseline.Results:ArcCHECK noted at least a 13% decrease in passing rate from baseline in the following scenarios: gantry rotation of 1 degree from plan, 5mm change in field size, 2mm lateral shift, and delivery time decrease. Ion chamber measurements remained consistent for these variations except for the 5 second decrease in delivery time scenario which resulted in an 8% difference from baseline. The delivery verification software was able to detect and report the simulated errors such as when the gantry was rotated by 0.6 degrees, the beam weighting was changed by a percent, a single multileaf collimator was moved by 1cm, and the dose was changed from 2 to 1.8Gy.Conclusion:The results show that the current tools used for patient specific QA are capable of detecting small errors in RT delivery with presence of magnetic field.
SU‐E‐T‐443: Developmental Technique for Proton Pencil Beam Measurements: Depth Dose
Authors: Arjomandy B , Lee T , Schultz T , Hsi W , Park S 
Published: July 2014

Purpose:Measurements of depth dose distribution (DDD) of pencil beam in proton therapy can be challenging and time consuming. We have developed a technique that uses two Bragg peak chambers to expedite these measurements with a high accuracy.Methods and Material:We used a PTW water tank and two PTW 10.5 cm3 Bragg peak chambers; one as a field chamber and the other as a reference chamber to measure DDDs for 100–250 MeV proton pencil beams. The reference chamber was positioned outside of the water tank upstream with respect to field chamber. We used Geant4 Monte Carlo Simulation (MCS) to model the ProTom proton beam to generate DDDs. The MCS generated DDDs were used to account for halo effects of proton pencil beam that are not measureable with Bragg peak chambers. We also used PTW PEAKFINDER to measure DDDs for comparison purpose.Results:We compared measured and MCS DDDs with Continuous Slowing Down Approximation (CSDA) ranges to verify the range of proton beams that were supplied by the manufacturer. The agreements between all DDD with respect to CSDA were within ±0.5 mm. The WET for Bragg peak chamber for energies between 100–250 MeV was 12.7 ± 0.5 mm. The correction for halo effect was negligible below 150 MeV and was in order of ∼5‐10% for 150–250 MeV.Conclusion:Use of Bragg Peak chamber as a reference chamber can facilitate DDD measurements in proton pencil beam with a high accuracy. Some corrections will be required to account for halo effect in case of high energy proton beams due to physical size of chamber.
SU‐E‐T‐444: Quantitatively Comparison of Low Dose Spillage Outside of PTV Edge in Arc Therapy Modalities
Authors: Gong J , Sarwan R , Pavord D 
Published: July 2014

Purpose:To quantitatively compare low dose spillage outside of PTV edge in arc therapy modalitiesMethods:The machines used in the study are Tomotherapy Hi‐Arc and Varian 21EX with millennium120 MLC. TPS are TomoPlaning and RayStation for VMAT, respectively. The phantom is a 30cm diameter cylindrical solid water (TOMOTHERAPY, TOMOPHANTOM ASSY). The PTV is 4cm length with ellipsoidal sectional shape with major axis=5cm, minor axis=3cm in the axial plane and reversed in the coronal plane. The PTV volume is created with interpolation. It is located at the center of the phantom. The prescribed dose is 1000×5 cGy to 95% the PTV. The isocenter is set co‐centered with the PTV. EBT‐3 film was used to measure iso‐dose lines at the center plane. Film dosimetry is performed with the RIT, v6.2.Results:the study shows: (1) dose falloff gradient is usually uneven, depending on the PTV shape in the gantry rotation plane. For an elliptical shape, the low dose spillage is wider in the minor axis direction than that in the major axis direction. The more a shape is closer to circular, the more even gradient is all directions; (2)for a circular shape (CAX plane in this study), the maximum dose in % of Rx dose at 2cm from PTV is 55% for Tomo, vs. 70% for VMAT (3) the most rapid dose falloff rang is between 95%–80% IDL for both modalities.Conclusion:Tomo has more rapid dose falloff outside of PTV. In some areas, the gradient is double for Tomo helical than that for LINAC VMAT at same points. Future work will examine the differences between optimization of doses and inherent delivery limitations.
SU‐E‐T‐445: Lateral Optical Density Variation in Flatbed Scanners in Combination with Gafchromic Film
Authors: van Battum LJ , Heukelom S 
Published: July 2014

PurposeThis study investigates the origin of lateral optical density (OD) variation for Gafchromic film (EBT and EBT2) scanned in transmission mode with Epson flatbed scanners (1680 Expression Pro and 10000XL). Effects investigated are: cross talk, optical path length and polarization.MethodsCross talk has been examined with triangular shaped light‐transmission sheets with OD ranging from 0 to opaque. Optical path length has been studied with absorptive and reflective OD‐filters (OD range 0.2 to 2.0). Dependency on light‐polarization on the scanner read out has been investigated using linear polarizer sheets. All experiments have been performed at centre scanner position (norm point) and at several lateral scan positions, without and with (un)irradiated EBT‐film. Dose values used ranged between 0.2 to 9 Gy, yielding an OD‐range between 0.25 to 1.1.ResultsThe lateral OD variation is dose dependent and increases up to 14% at most lateral position for dose up to 9 Gy. Cross talk effect contributes to 0.5% in clinical used OD ranges but equals 2% for extreme high dose gradients. Film induced optical path length will effect the lateral OD variation up to 3% at most lateral points. Light polarization is inherent present in these scanners due to multiple reflection on mirrors. In addition film induced polarization is the most important effect generating the observed lateral OD variation. Both Gafchromic film base and sensitive layer have polarizing capabilities; for the sensitive layer its influence is dose dependent.ConclusionsLateral OD variation origins from optical physics (i.e. polarization and reflection) related to scanner and film construction. Cross talk can be ignored in film dosimetry for clinical used dose values and gradients. Therefore it is recommended to determine the lateral OD variation per film type and scanner.
SU‐E‐T‐446: Evaluation of the Dosimetric Properties of a Diode Detector to Proton Radiosurgery
Authors: Teran A , McAuley G , Slater J D , Slater J M , Wroe A 
Published: July 2014

Purpose:To test the PTW PR60020 proton dosimetry diode in radiation fields relevant to proton radiosurgery applications and evaluate its suitability as a high resolution, real time dosimetry device.Methods:Data was collected using our standard nominal radiosurgery energies of 126 MeV and 155 MeV through a single stage scattering system, corresponding to a range of 9.7 and 15 cm in water respectively. Various beam modulations were tested as part of this study. Depth dose and beam profile measurements were completed with the PTW PR60020 dosimetry diode with comparative measurements using a PTW Markus ionization chamber and EBT2 Gafchromic film. Monte Carlo simulations were also completed for comparison.Results:The single 1 mm2 by 20 μm thick sensitive volume allowed for high spatial resolution measurements while maintaining sufficient sensitive volume to ensure that measurements could be completed without excessive beam delivery. Depth dose profiles exhibited negligible LET dependence which typically impacts film and other solid state dosimetry devices, while beam ranges measured with the PTW diode were within 1 mm of ion chamber data. In an edge on arrangement beam profiles were also measured within 0.5 mm full‐width at half‐maximum at various depths as compared to film and simulation data.Conclusion:The PTW PR60020 proved to be a very useful radiation metrology apparatus for proton radiosurgery applications. Its waterproof and rugged construction allowed for easy deployment in phantoms or water tanks that are commonly used in proton radiosurgery QA. Dosimetrically, the diode exhibited negligible LET dependence as a function of depth, while in edge on arrangement to the incident proton beam it facilitated the measurement of beam profiles with a spatial resolution comparable to both Monte Carlo and film measurements.This project was sponsored in part by funding from the Department of Defense (DOD# W81XWH‐BAA‐10‐1).
SU‐E‐T‐447: Electronic Brachytherapy (EBT) Treatment of Cervical Cancer – First Clinical Experience
Authors: Johnson D , Johnson M , Chan L , Hausen H , Thompson J , Ahmad S 
Published: July 2014

Purpose:To study the first trial patient in which an electronic brachytherapy (EBT) x‐ray source is utilized for treatment of cervical cancer.Methods:During patient treatment, a miniaturized x‐ray source was used in combination with a customized titanium tandem and ovoid applicator set. The semi‐specialized source was modeled with formalisms outlined by AAMP Task Group 43. Multiple models were used to compensate for variable attenuation conditions as a function of source positions. Varian Brachyvision treatment planning software was utilized on CT data sets for dose calculations prior to treatment delivery. The dose was prescribed to “point A” as defined by American Brachytherapy society. Additional treatments plans were created from those clinically utilized in patient care and were recalculated for an existing Ir‐192 source model. Dose volume histograms (DVH) and point dose calculations were compared between the modalities for the clinical condition present in patients treated with EBT.Results:Clinical treatment times, though longer than those typically experienced by Ir‐192 users, were manageable. Instantaneous dose rates at personal positions within the treatment vault were lower than those measured during intra operative radiation therapy and breast EBT treatments. Due to lower average photon energy in EBT, dose gradients within the treatment plans were as expected steeper than those observed in Ir‐192 based brachytherapy. DVH comparisons between Ir‐192 and EBT treatments showed an expected decrease in the integral dose to normal tissues of interest for EBT. In comparing plans created for EBT delivery with those calculated for Ir‐192, average dose values for EBT were more than 4%, 11%, and 9% lower at predefined bladder, rectum and “point B” positions, respectively.Conclusion:For the first time, we have demonstrated that the utilizing electronic brachytherapy system for tandem and ovoid based treatment of cancer of the cervix is feasible, and potentially beneficial, in clinical environment.
SU‐E‐T‐448: On the Perturbation Factor P_cav of the Markus Parallel Plate Ion Chambers in Clinical Electron Beams, Monte Carlo Based Reintegration of An Historical Experiment
Authors: von Voigts‐Rhetz P , Zink K 
Published: July 2014

Purpose:All present dosimetry protocols recommend well‐guarded parallel‐plate ion chambers for electron dosimetry. For the guard‐less Markus chamber an energy dependent fluence perturbation correction pcav is given. This perturbation correction was experimentally determined by van der Plaetsen by comparison of the read‐out of a Markus and a NACP chamber, which was assumed to be “perturbation‐free”. Aim of the present study is a Monte Carlo based reiteration of this experiment.Methods:Detailed models of four parallel‐plate chambers (Roos, Markus, NACP and Advanced Markus) were designed using the Monte Carlo code EGSnrc and placed in a water phantom. For all chambers the dose to the active volume filled with low density water was calculated for 13 clinical electron spectra (E0=6‐21 MeV) at the depth of maximum and at the reference depth under reference conditions. In all cases the chamber's reference point was positioned at the depth of measurement. Moreover, the dose to water DW was calculated in a small water voxel positioned at the same depth.Results:The calculated dose ratio DNACP/DMarkus, which according to van der Plaetsen reflects the fluence perturbation correction of the Markus chamber, deviates less from unity than the values given by van der Plaetsen's but exhibits a similar energy dependence. The same holds for the dose ratios of the other well guarded chambers. But, in comparison to water, the Markus chamber reveals the smallest overall perturbation correction which is nearly energy independent at both investigated depths.Conclusion:The simulations principally confirm the energy dependence of the dose ratio DNACP/DMarkus as published by van der Plaetsen. But, as shown by our simulations of the ratio DW/DMarkus, the conclusion drawn in all dosimetry protocols is questionable: in contrast to all well‐guarded chambers the guard‐less Markus chamber reveals the smallest overall perturbation correction and also the smallest energy dependence.
SU‐E‐T‐449: Optimization and Evaluation: IMRT of Sinoâ,<nasal Carcinoma in Reference to ICRU 83
Authors: Maria Das Joseph K , Aziz Sait A Abdul , Senthil Kumar S , Verma M , Lal P , Kumar S 
Published: July 2014

Purpose:Estimating the potential radiobiological advantage using the sub‐volume concept (ICRU 83)in plan optimization and evaluation of IMRT for sino‐nasal carcinoma.Methods:Ten patients planned with IMRT were selected retrospectively. Two plans were generated for each patient with 7 coplanar field arrangements. The PTV was prescribed 60Gy/30# with an unedited expansion from CTV (CC=5mm, ML=4mm and AP=4mm) and the in‐air PTV filled with a virtual bolus 1cm thick. Proximity structures were defined as per the guidelines. Plan1 was optimized and evaluated based on ICRU 50/62, whereas plan2 was based incorporating the ICRU 83 approach. The dosimetric parameters, including CI, HI, MU were compared and radiobiological metrics estimated using the Biosuite software (NTCP‐LKB model & EUD). The paired T‐test was used to assess the statistical significance of differences of similar parameters between the plans.Results:The mean±sd of PTV was 230.21±88.13cc, where as the sub‐volume PTV was 3.09±1.47cc. PTV dosimetric parameters i.e., D2%, D98%, D95%, D50%, etc., CI and HI were similar for both plans, but OAR and PRV showed reduction in D2%, Dmax, Dmean for plan2 compared with plan1. D2% of optic chiasm 53.66±0.90 vs. 52.47±0.89, p = 0.004; left optic nerve 54.91±1.54 vs. 53.67±1.56, p = 0.001; left retina 54.00±1.34 vs. 51.82±2.52, p = 0.005 for plan1 and plan2 respectively while Dmax, Dmean for other OARs and PRVs demonstrated a similar trend. NTCP for optic chiasm was 7.71±2.86 vs. 5.62±2.24, p = 0.005 without fractionation effect, whereas it was 4.42±2.11 vs. 2.80±1.46, p = 0.006 with fractionation effect for plan1 and plan2 respectively. Similar trend were observed for other OARs and EUDs.7% increase in MU for plan2 was observed.Conclusion:Incorporating the sub volume concept of ICRU 83 in IMRT optimization and evaluation may Resultin reduction of doses to OAR and lower / safer NTCP without compromising TCP.The authors are acknowledging Dr. Julien Uzan and Professor Alan Nahum, Clatterbridge Cancer Centre, Wirral, UK for guidance and sharing the Biosuite software for radiobiological evaluation.