Measured Dose Profiles Research Articles

SU-E-T-177: Deconvolution of Line Dose Profiles with Ionization Chambers - Experiences with the First Software Implementation in Mephisto 3.0.

To evaluate the first implementation of a deconvolution algorithm in a commercial water phantom scanning software. Line dose profile measurements in a water phantom are an essential part of a quality assurance system and in base data measurements in radiotherapy. Usually these measurements are performed with waterproof ionization chambers of various sizes. These dose profile measurements are broadened by the Gaussian response functions of the detectors. In recent studies we showed that the undisturbed line dose profiles can be reconstruced by iterative deconvolution of the measured signal profiles with the Gaussian detector response functions. Recently, the proposed method was implemented in Mephisto 3.0. In this work we analyze the applicability and the limits of the deconvolution algorithm for several chambers by comparing the result with diode measurements. As long as the dose gradient becomes not too steep the deconvolution algorithm is able to reconstruct the undisturbed dose profiles with sufficient accuracy. Deviations occur for smallest field sizes in which the width of the detector's lateral response functions reaches the dimensions of the field. A simple chart for those limits is derived. The implemented deconvolution algorithm allows a fast and simple correction of measured dose profiles broadened by the volume effect of the ionization chambers. It offers therefore for the first time a clinical deconvolution of the profiles on a regular base and by this the implementation of undisturbed base data in the treatment planning systems as well as in the quality assurance process in modern radiotherapy.

WE‐C‐BRB‐07: Benchmarking of the TOPAS Monte Carlo System against Phantom Dose Measurements in Proton Therapy

Purpose: To validate the accuracy of Monte Carlo (MC) simulations performed with TOPAS (TOol for PArticle Simulation; to be released to the proton therapy community) against phantom measurements in proton therapy. Methods: We used TOPAS, a new MC simulation tool that provides a user‐friendly interface to the Geant4 package, to simulate proton depth‐dose curves from the MGH gantry treatment heads in double‐ scattering mode. Simulations were compared to QA measurements of SOBPs. Field flatness and field symmetry from all clinical options (i.e. combinations of double‐scattering system, range modulator wheel and beam current modulation settings) have been simulated and benchmarked against measurements performed with 2D dose detectors. A step‐compensator consisting of two sections of different thickness was placed upstream a water phantom in which we measured lateral and longitudinal dose profiles. Results: For all treatment options TOPAS depth‐dose curves are within clinical required accuracy, the range being within +1/−1.5 mm and the modulation within ±3 mm of the requested specifications. SOBP simulations also validate time‐dependent geometry and beam current in TOPAS, as the modulator wheel is rotating with the beam current modulation changing at the same time. The field flatness of simulated protons is within ±2% the average dose measured in a plane transverse to the beam axis. Field symmetries show slight discrepancies compared to measured data. In the case of the step‐compensator, TOPAS correctly predicts the lateral width of the beam and longitudinal profiles at different transverse positions. In the transverse profiles, the edge scattering effects due to multiple Coulomb scattering were correctly reproduced by TOPAS. Conclusions: We demonstrated TOPAS accuracy to reproduce measured dose profiles in a variety of homogeneous phantom setups relevant for proton therapy. The accuracy in the benchmarking achieved with TOPAS simulations provides the basis for routine clinical dose calculations. NIH/NCI P01 CA021239

SU-E-T-289: On the Use of a Diode Array for the Commissioning of Dynamically-Wedged Asymmetric Fields Generated by Varian EDW's in the Pinnacle Treatment Planning System.

To present a validation study of enhanced dynamic wedges (EDWs) implemented into Pinnacle3 treatment planning system (TPS) using a diode array, Monte Carlo (MC) simulation, and measurements. Modeling of EDW dose distribution in the Pinnacle3 TPS is based on a combination of open-field beam data and the Varian 'Golden Segmented Treatment Table' (GSTT) unique to each photon beam. To validate the EDW models, dose profiles of 6- and 10-MV photon beams from a Clinac 2100C/D were measured in virtual water at depths from near- surface to 30 cm for a wide range of asymmetric field sizes and wedge angles using the Profiler-2 diode array system. The EDW output factors for asymmetric fields were measured in solid water using a small-volume cylindrical ionization chamber placed at a depth of 10 cm on the central axis. In addition, absolute doses on central axis at depths of 5, 10, 15 and 20 cm were measured in virtual water using a small-volume cylindrical chamber for comparison with the collapsed-cone convolution (CCC) calculations. The 6- and 10-MV photon beams emerging from the treatment head of the Clinac 2100C/D were fully simulated using the Monte Carlo code BEAMnrc and DOSXYZnrc. This code was used to calculate the central-axis percentage depth doses and dose profiles for the open and dynamically-wedged fields in a water phantom. The off-axis dose profiles of various EDWs computed with the CCC dose model and MC simulations agreed with measured EDW dose distributions to an accuracy of better than 2%/2 mm. Measured EDW output factors used for MU calculations in Pinnacle3 TPS agreed with MC predictions within 1-2%. The utility of Profiler-2 diode array system to measure dose profiles for commissioning of EDWs into the Pinnacle3 TPS has been demonstrated to have acceptable accuracy for clinical implementation.

SU-E-I-42: Measurement of X-Ray Beam Width and Geometric Efficiency in MDCT Using Radiochromic Films.

The purpose of this work was to measure the x-ray beam width and geometric efficiency (GE) of a multi detector computed tomography scanner (MDCT) for different beam collimations using radiochromic films. In MDCT, the primary beam width extends the nominal beam collimation to irradiate the active detector elements uniformly (called 'over-beaming') which contributes to increased radiation dose to the patient compared to single detector CT. Therefore, the precise determination of the primary beam width and GE is of value for any CT dose calculation using Monte Carlo or analytical methods. Single axial dose profiles free in air were measured for 6 different beam collimations nT for a Siemens SOMATOM Sensation 64 Scanner with Gafchromic XR-QA2 films. The films were calibrated relative to the measured charge of a PTW semiflex ionization chamber (type: 31010) for a single rotation in the CT scanner at the largest available beam collimation of 28.8 mm. The beam energy for all measurements in this work was set to 120 kVp. For every measured dose profile and beam collimation the GEin-air and the full-width-at-half- maximum value (FWHM) as a value for the x-ray beam width was determined. Over-beaming factors FWHM / nT were calculated accordingly. For MDCT beam collimations from 7.2 (12×0.6 mm) to 28.8 (24×1.2 mm) the geometric efficiency was between 58 and 85 %. The over- beaming factor ranged from 1.43 to 1.11. For beam collimations of 1×5 mm and 1×10 mm the GE was 77 % and 84 % respectively. The over-beaming factors were close to 1, as expected. This work has shown that radiochromic films can be used for accurate x-ray beam width and geometric efficiency measurements due to their high spatial resolution. The measured free-in-air geometric efficiency and the over-beaming factor depend strongly on beam collimation.

SU-E-J-56: Localization Error in Gamma Radiosurgery with 3T MR Due to Fiducial Box Chemical Shift from Incorrect Liquid Filling Materials.

Use of a 3D MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) polymer gel dosimeter to evaluate potential localization and positioning errors due to the use of incorrect liquid filling material (baby oil instead of copper sulfate) for a gamma radiosurgery (GSR) MRI fiducial box. Two phantoms were used to evaluate geometric errors due to chemical shift for 'baby oil' fiducial markers: 1) a cylindrical grid phantom with Teflon rod insert; 2) a human skull phantom with a MAGIC (Gelatin 9%; Methacrylic acid 4%; CuSO4 0.1mM; Ascorbic acid 2mM; Glucose 10%) gel capsule to simulate an intracranial target. Both phantoms were stereotactically MR and CT scanned for GSR treatment planning using a 'baby oil' GSR MRI fiducial box and a standard CT fiducial box. The gel capsule was given 15Gy @ 50% isodose line using the 4mmcollimator based on 'baby oil' fiducial box MR images. Then, the skull phantom with capsule was scanned in 3T GE Signa® EXCITETM clinical scanner (Spin Echo pulse sequence) for 3D dose distribution read-out. Dose distribution errors were analyzed based on gel results for the combined MR and CT plans. For comparison, deviation of grid phantom teflon rods were directly measured from MR and CT stereotactic images. Gel dose profile measurements indicate a 3.8mm shift in the phase encoding direction for the 'baby oil' fiducial marker for 3T MRI based GSR treatment planning. A 3.7mm shift in Teflon rod position was also confirmed through direct comparison of CT and MR images. Both results showsignificant target localization and positioning errors can be caused if inappropriate filling materials are used for GSR MRI fiducial boxes. Thisstudy also indicates that MAGIC polymer gel can be used as a versatile toolfor GSR 3D quality assurance.

Comparison of methods for assessing geometric efficiency on multi-detector CT scanners

The aim of the current study was to compare the film method against the method based on a new CT slice detector in assessing geometric efficiency (GE) of x-ray beams utilized by a multi-detector CT (MDCT) scanner. Measurements of GE were performed using radiographic films and a solid state CT slice detector for all beam qualities, collimations and focal spot sizes available on an MDCT scanner. Repeatability of GE measurements was assessed. The radiographic film and the solid state detector methods were compared to each other in regard to efficacy in measuring free-in-air GE. The values of GE determined using the radiographic film method were found to range between 48.5% and 90.6%. Differences between values obtained with the radiographic film method and corresponding values obtained with the solid state detector were less than 10% exceeding 5% for only one case. Both methods show that wide beams have higher GE values compared to thin ones. The use of large instead of small focal spot was found to deteriorate GE values by up to 23.1%. Beam quality did not seem to influence GE of the various collimations. When thin beam collimations are employed, a considerable amount of the radiation is wasted for non-imaging purposes. Both film and solid state probe methods are capable of measuring GE of thin as well as wide collimations. The solid state detector is the easiest to use, however its usefulness is reduced by the fact that it cannot measure dose profiles of beam collimations available for step-and-shoot mode of operation.

Evaluation of the Effects of Inhomogeneities on Dose Profiles Using Polymer Gel Dosimeter and Monte Carlo Simulation in Gamma Knife

Introduction Polymer gel dosimeters offer a practical solution to 3D dose verification for conventional radiotherapy as well as intensity-modulated and stereotactic radiotherapy. In this study, EGSnrc calculated and PAGAT polymer gel dosimeter measured dose profiles from single shot irradiation with 18 mm collimator of Gamma Knife in homogeneous and inhomogeneous phantoms were compared with each other. Materials and Methods The head phantom was a custom-built 16 cm diameter plexiglas sphere. Inside the phantom, there were two cubic cutouts for inserting the gel vials and inhomogeneities. Following irradiation with the Gamma Knife unit, the polymer gel dosimeters were scanned with a 1.5 T MRI scanner. For the purpose of simulation the simplified channel of 60Co source of Gamma Knife BEAMnrc and for extracting the 3D dose distribution in the phantom, DOSXYZnrc codes were used. Results Within high isodose levels (>80%), there are dose differences higher than 7%, especially between air inserted and PTFE inserted phantoms, which were obtained using both simulation and experiment. This means that these values exceed the acceptance criterion of conformal radiotherapy and stereotactic radiosurgery (i.e., within some isodose levels, less than 93% of prescription dose are delivered to the target). Conclusion The discrepancies observed between the results obtained from heterogeneous and homogeneous phantoms suggest that Leksell Gamma Knife planning system (LGP) predictions which assume the target as a homogeneous material must be corrected in order to take care of the air- and bone-tissue inhomogeneities.

The Verification of iPlan Commissioning by Radiochromic EBT2 Films

Purpose: To evaluate the measured dose distributions using radiochromic EBT2 films for small fields in iPlan (BrainLab) commissioning. Methods: Radiochromic EBT2 films were irradiated with 6 MV photons on a Varian Trilogy linac using polystyrene phantoms. The measurements included dose profiles and depth doses for field sizes of 1 × 1, 2 × 2, 3 × 3, 4 × 4, and 10 × 10 cm2 etc. The dose profile measurements were taken at the depth of 5 cm. The calibration films were irradiated at dmax(1.4 cm) for doses up to 6 Gy. Films were scanned using an Epson 10,000 XL flatbed scanner with 72 dpi resolution. Pixel values were converted to doses using the established calibration-curve. The 2D dose distributions were generated from the film data analysis. In-house software was utilized to compare the measured doses from films with the treatment planning data. In addition, selected patients’ SRS fields were also measured with the EBT2 films for comparison with iPlan. An EDGETM detector was also used to check the centralaxis doses for the SRS patients’ measurements. Results and Discussion: The measured planar dose distributions achieved more than 98% and 95% passing rates with a set of 2%/2 mm dose and DTA criteria for all square fields and all patient treatment fields (2), respectively. Agreement with measurement data with the EDGETM detector at the central axis (±1%) was found with the plan data. This is the first report for SRS small photon-field measurement using the latest radiochromic film, EBT2. The results shown in this work indicate that the use of EBT2 film provides accurate dosimetry measure-ments for small photon beams. The measurements show excellent agreement with the iPlan TP commissioning data. Conclusions: The patient-specific EBT2 film QA for iPlan SRS patients showed good results. The EBT2 films could potentially be a useful dosimeter in verification of commissioning as well as patient-specific QA for SRS cases.

Dosimetric performance and array assessment of plastic scintillation detectors for stereotactic radiosurgery quality assurance

To compare the performance of plastic scintillation detectors (PSD) for quality assurance (QA) in stereotactic radiosurgery conditions to a microion-chamber (IC), Gafchromic EBT2 films, 60 008 shielded photon diode (SD) and unshielded diodes (UD), and assess a new 2D crosshair array prototype adapted to small field dosimetry. The PSD consists of a 1 mm diameter by 1 mm long scintillating fiber (BCF-60, Saint-Gobain, Inc.) coupled to a polymethyl-methacrylate optical fiber (Eska premier, Mitsubishi Rayon Co., Ltd., Tokyo, Japan). Output factors (S(c,p)) for apertures used in radiosurgery ranging from 4 to 40 mm in diameter have been measured. The PSD crosshair array (PSDCA) is a water equivalent device made up of 49 PSDs contained in a 1.63 cm radius area. Dose profiles measurements were taken for radiosurgery fields using the PSDCA and were compared to other dosimeters. Moreover, a typical stereotactic radiosurgery treatment using four noncoplanar arcs was delivered on a spherical phantom in which UD, IC, or PSD was placed. Using the Xknife planning system (Integra Radionics Burlington, MA), 15 Gy was prescribed at the isocenter, where each detector was positioned. Output Factors measured by the PSD have a mean difference of 1.3% with Gafchromic EBT2 when normalized to a 10 × 10 cm(2) field, and 1.0% when compared with UD measurements normalized to the 35 mm diameter cone. Dose profiles taken with the PSD crosshair array agreed with other single detectors dose profiles in spite of the presence of the 49 PSDs. Gamma values comparing 1D dose profiles obtained with PSD crosshair array with Gafchromic EBT2 and UD measured profiles shows 98.3% and 100.0%, respectively, of detector passing the gamma acceptance criteria of 0.3 mm and 2%. The dose measured by the PSD for a complete stereotactic radiosurgery treatment is comparable to the planned dose corrected for its SD-based S(c,p) within 1.4% and 0.7% for 5 and 35 mm diameter cone, respectively. Furthermore, volume averaging of the IC can be observed for the 5 mm aperture where it differs by as much as 9.1% compared to the PSD measurement. The angular dependency of the UD is also observed, unveiled by an under-response around 2.5% of both 5 and 35 mm apertures. Output Factors and dose profiles measurements performed, respectively, with the PSD and the PSDCA were in agreement with those obtained with the UD and EBT2 films. For stereotactic radiosurgery treatment verification, the PSD gives accurate results compared to the planning system and the IC once the latter is corrected to compensate for the averaging effect of the IC. The PSD provides precise results when used as a single detector or in a dense array, resulting in a great potential for stereotactic radiosurgery QA measurements.

Dosimetry of a new P‐32 ophthalmic applicator

The potential of P-32 ophthalmic applicator irradiation after pterygium excision has been demonstrated as an alternative to Sr∕Y-90 irradiation. This study aimed to provide the clinical dosimetry for this new applicator. The prototype of a cylindrical P-32 applicator was fabricated according to the Monte Carlo (MC)-based design study. At a nominal activity of 6 mCi (0.22 GBq), the absorbed dose rate at the front surface (i.e., reference dose rate) was measured by using an extrapolation ionization chamber (EC). The radiochromic film (RCF) was also used to measure the reference dose, axial depth dose distributions and transaxial dose profiles at various depths in water. The reference dose rate was 3.89 ± 0.14 cGy∕s for EC and 3.84 ± 0.25 cGy∕s for RCF. The depth dose rate was reduced approximately by an order of magnitude for every 2 mm depth in water. Measured depth doses in depths of 0.5-2.5 mm agreed with Monte Carlo data within ±3%. Due to nonuniform absorption of P-32 into an absorbent disk, the dose profiles were not symmetric and decreased more rapidly toward the periphery than those predicted by the MC. The authors confirmed no leakage of P-32 activities and negligible exposure rate around the hand grip of the applicator. The P-32 applicator can deliver therapeutic doses to the surface of the conjunctiva, while sparing the lens better than Sr∕Y-90 applicators. The doses at any points from the P-32 applicator could be calculated by using the measured dosimetry data. They also confirmed no leakage of the source, reliable integrity of the applicator, and negligible exposure level around the hand grip of the applicator. However, due to a possibility of nonuniform distributions of P-32 in an absorbent disk, measuring dose profiles as well as the reference dose rate for every new applicator would be recommended.

A novel dosimetry system for computed tomography using phototransistor

Computed tomography (CT) dosimetry normally uses an ionization chamber 100 mm long to estimate the computed tomography dose index (CTDI), however some reports have already indicated that small devices could replace the long ion chamber to improve quality assurance procedures in CT dosimetry. This paper presents a novel dosimetry system based in a commercial phototransistor evaluated for CT dosimetry. Three detector configurations were developed for this system: with a single, two and four devices. Dose profile measurements were obtained with them and their angular responses were evaluated. The results showed that the novel dosimetry system with the phototransistor could be an alternative for CT dosimetry. It allows to obtain the CT dose profile in details and also to estimate the CTDI in longer length than the 100 mm pencil chamber. The angular response showed that the one device detector configuration is the most adequate among the three configurations analyzed in this study.

Dosimetric Evaluation of Linac Photon Small Fields using MAGIC Polymer Gels

Introduction: In radiotherapy, methods of treatment planning are becoming increasingly more complicated. This requires verification of the doses delivered to increasingly smaller and more precise regions. Radiotherapy techniques are continuously employing smaller and smaller field sizes to deliver tighter radiation doses with higher therapeutic ratios, generating interest among researchers to provide reliable dosimetry for beams and treatment plans collimated to small field sizes. In this study, the dosimetry of these fields was evaluated in clinical applications by using polymer gel dosimetry. Material and Methods: The MAGIC polymer gel was used in this study. The gel samples were manufactured and poured into phantoms and calibration vials and were irradiated with a 6 MV x-ray beam. The R2 maps of the dose distributions were obtained from the gel MR images. The depth dose distributions and dose profile measurements were measured in different fields at a depth of 5 cm in gel, and were compared against another technique using a pinpoint chamber. Results: Comparison of the results of gel and pinpoint chamber measurements showed largest differences between the dose profile measurements in the low dose regions (near the edges). In these regions, the pinpoint-chamber measured penumbra width was at most 3.2 mm wider than those given by gel dosimeters. For a 30 × 30 mm2 field, maximum difference between gel and pinpoint chamber was 2 mm within the depth of maximum dose region, and were 2 mm, 3 mm and 2 mm for 20 × 20 mm2, 10 × 10 mm2 and 5 × 5 mm2 fields, respectively. The maximum differences within the buildup region for 30 × 30 mm2, 20 × 20 mm2, 10 × 10 mm2 and 5 × 5 mm2 fields were 9%, 4.5%, 2% and 9%, respectively. Discussion and Conclusion: The differences in the depth dose distributions and dose profile measurements between MAGIC polymer gel and pinpoint chamber are attributable to different factors. The dosimetry of these fields using ionization chambers has limitations regarding uncertainty, due to the relatively large sizes of ionization chambers with respect to small field sizes and the lack of charged-particle equilibrium in such small fields. Pinpoint chambers are too large for very small photon fields and smaller dosimeters should be used. Fewer limitations in the dosimetry of small fields exist when using polymer gel dosimeters. Hence polymer gel dosimeters are suitable for dosimetry of photon small fields.

Towards MRI-guided linear accelerator control: gating on an MRI accelerator

To boost the possibilities of image guidance in radiotherapy by providing images with superior soft-tissue contrast during treatment, we pursue diagnostic quality MRI functionality integrated with a linear accelerator. Large respiration-induced semi-periodic target excursions hamper treatment of cancer of the abdominal organs. Methods to compensate in real time for such motion are gating and tracking. These strategies are most effective in cases where anatomic motion can be visualized directly, which supports the use of an integrated MRI accelerator. We establish here an infrastructure needed to realize gated radiation delivery based on MR feedback and demonstrate its potential as a first step towards more advanced image guidance techniques. The position of a phantom subjected to one-dimensional periodic translation is tracked with the MR scanner. Real-time communication with the MR scanner and control of the radiation beam are established. Based on the time-resolved position of the phantom, gated radiation delivery to the phantom is realized. Dose distributions for dynamic delivery conditions with varying gating windows are recorded on gafchromic film. The similarity between dynamically and statically obtained dose profiles gradually increases as the gating window is decreased. With gating windows of 5 mm, we obtain sharp dose profiles. We validate our gating implementation by comparing measured dose profiles to theoretical profiles calculated using the knowledge of the imposed motion pattern. Excellent correspondence is observed. At the same time, we show that real-time on-line reconstruction of the accumulated dose can be performed using time-resolved target position information. This facilitates plan adaptation not only on a fraction-to-fraction scale but also during one fraction, which is especially valuable in highly accelerated treatment strategies. With the currently established framework and upcoming improvements to our prototype-integrated MRI accelerator, we will realize more intricate MRI-guided linear accelerator control in the near future.

MO-F-BRB-04: Multi-Source Modeling of Flattening Filter Free (FFF) Beam and the Optimization of Model Parameters

Purpose: To suggest a novel analytic beam source model for a flattening filter free(FFF) beam, a multi‐source model and its optimization method applicable to a treatment planning system were developed. Methods: Previous three source model was improved by introducing off axis ratio (OAR) of primary photon fluence to generate cone shape profiles. The parameters of the model and the OAR were determined from measured head scatter factors and a measured dose profile of a 40 × 40 cm2 field size using a line search optimization technique. A new method to acquire gradient terms for OAR's was developed to enhance the speed of the optimization process. The improved model was validated with measured dose profiles from 3 × 3 cm2 to 40 × 40 cm2 field sizes at 6 and 10 MV from a TrueBeamTM STx linear accelerator. Furthermore, planar dose distributions for clinically used radiation fields were also calculated and compared with measurements using a 2D array detector using the gamma index method. Results: All dose values for the calculated profiles agreed with the measured dose profiles within 0.5% at 6 and 10 MV beams, except for some low dose regions for larger field sizes. A slight overestimation was seen in the lower penumbra region near the field edge for the large field sizes by 1 % ∼ 4%. The planar dose calculations showed comparable passing rates (> 98%) when the criterion of the gamma index method was selected to be 3%/3 mm. Conclusions: Developed source model showed good agreements between measured and calculated dose distributions. The model is easily applicable to any other linear accelerator using FFF beams as the data required include only the measured PDD, dose profiles and output factors for various field sizes, which are easily acquired during conventional beam commissioning process.

SU-E-T-144: Study of the Effect of the Optical Density to Dose Conversion Calibration Curves on the Measurement of Transverse Lateral Dose Profiles of Proton Beams Using Kodak XV Films

Purpose: To investigate the effect of the optical density (OD) to dose conversion calibration curves on the measurement of transverse lateral dose profiles of proton beams using Kodak XV films. Methods: A number of XV films were exposed to different doses in a 10 cm × 10 cm field of passively scattered range modulated beams of different incident energies. The films were placed at the center of spread out Bragg peak of the beam inside plastic water phantoms. The films were scanned by a Vidar Scanner using Scanditronix/ Wellhofer OmniPro‐Accept6.4 software. Different OD‐dose calibration curves were generated and compared. Transverse lateral dose profiles were obtained from an irradiated film using different OD‐dose calibration curves and were compared to evaluate the differences. The lateral dose profiles of proton pencil beam spots (PPBS) were also obtained using the XV films and one of these OD‐dose calibration curves and were compared with ion chamber measurement. Results: OD‐dose calibration curves obtained by giving same doses, but with beams of different incident energies were found to have differences in all dose regions. But, the relative lateral dose profiles (LDP) were not affected by the differences in the calibration curves. Thus, the incident energy of the range modulated beam is not an important consideration for obtaining the film OD‐dose calibration curves for relative LDP measurements. The XV film measured relative LDP of the PPBS agreed well with that from ion chamber measurement in the high dose region, but was seen to be noisier in the long tail region. Conclusions: OD‐dose calibration curves created with given doses from any range modulated passively scattered beam, irrespective of the energy of the incident beam, are found to be suitable for measuring the relative LDP of broad fields of passively scattered proton beams and PPBS with XV films.

SU‐E‐I‐72: Evaluation of CTDIw under the New Scan Modes of GE HD 750 Scanner

Purpose: To evaluate the weighted computed tomography dose index (CTDIw) under the high resolution mode of the state‐of‐the‐art GE Discovery HD750 scanner. The pencil ionization chamber measurements were cross‐validated with thermoluminescent dosimeters (TLDs).Methods: CT scans were performed on a GE Discovery CT 750 HD scanner under high resolution axial mode, using the scan parameters of 120 kVp, and 200 mA, 1 sec/rotation. Two beam collimations were used in this study: 40mm and 10mm. A Standard Imaging Exradin A101 CT ionization chamber (effective length 12.08 cm) was used to determine CTDIw. To calculate CTDIw using TLDs, the CT dose profiles at both central and peripheral positions in a CTDI head phantom were measured with arrays of TLD‐100 chips. With the measured dose profiles, integrations along the z‐direction within ± 50 mm were performed to calculate CTDI100 values. CTDIw with TLD was determined and compared with pencil chamber measurements.Results: In the studied scan modes, for 40 mm scan thickness, CTDIw was determined to be 34.37 mGy and 35.67 mGy (in terms of dose to air) from charge readings with pencil chamber and TLD‐measured CT dose profile, respectively. For 10 mm scan thickness, CTDIw was determined to be 41.97 mGy and 44.60 mGy with the pencil chamber and TLDs, respectively. Conclusions: The CTDIw determined by the TLD‐measured dose profile agreed well with pencil chamber measurements for both scan thicknesses studied. Future work will include measurements using additional scan thicknesses and under another new scan mode provided by the scanner: Gemstone Spectral Imaging (GSI) mode. Comparison with regular scan modes will be performed as well.

SU‐E‐I‐67: Monte Carlo Evaluation of CT‐Scanner Scatter Dose to Operator and Comparison with Measurements

Purpose: To investigate the scatter dose received by a CT‐scanner operator from walls and ceiling as a function of the height of the shielding barrier between the operator and the scanner. To compare Monte Carlo (MC) simulated data with measurement. Methods: MC simulations were performed, using the Penelope algorithm. The CT scanner and a typical, lead‐shielded control console were modeled in cylindrical coordinates. An x‐ray photon spectrum of 120 kVp impinging on a water‐phantom was modeled and the resulting scatter dose profile obtained in CT suite and operator room. This simulation was repeated with varying heights of the lead shield: between 6 ft. and 15ft (height of the concrete ceiling). Finally measurements were obtained of a cross section of the scatter dose profile in the operator room, using a 64 slice CT scanner (Toshiba Aquilion), a water‐equivalent phantom and an NaI‐based survey meter; the measured dose profiles are compared with the simulated ones. Dose reduction at the operator console resulting from increased wall height was measured and compared to simulation. Results: Simulating the extension of the lead shield from a height of 6 ft. all the way to the ceiling resulted in dose reduction at the operator console of two orders of magnitude. Measured dose profiles show good qualitative agreement with the simulated profiles. Measured dose reduction for increased barrier height was larger than simulated, likely owing to the simplified geometry used in simulating the CT scanner and room geometry. Conclusions: Additional contribution of dose scattered at the ceiling and back wall of an operator console can be significant; its varying contribution is given as a function of barrier height. MC simulation provides good qualitative agreement with the measurement. Improved modeling of the CT‐suite is likely needed to enhance quantitative agreement.

SU-E-T-560: The Use of EBT2 Films for IPlan Commissioning Verification

Purpose: To evaluate the measured dose distributions using radiochromic EBT2 films for small fields in iPlan (BrainLab) commissioning. Methods: Radiochromic EBT2 films were irradiated with 6MV photons on a Varian Trilogy linac using polystyrene phantoms. The measurements included dose profiles and depth doses for field sizes of 1×1, 2×2, 3×3, 4×4, and 10×10cm2 etc. The dose profile measurements were taken at the depth of 5cm. The calibration films were irradiated at dmax (1.4cm) for doses up to 6 Gy. Films were scanned using an Epson 10000XL flatbed scanner with 72 dpi resolution. Pixel values were converted to doses using the established calibration‐curve. The 2D dose distributions were generated from the film data analysis. In‐house software was utilized to compare the measured doses from films with the associated treatment planing data. In addition, sample patients' SRS plans were also measured with the EBT2 films for comparison with iPlan. MapCHECK was also used to check the central‐axis doses for the SRS patients' measurements. Results: The measured planar dose distributions achieved more than 95% passing rates with a set of 2%/2mm dose and DTA criteria for all patient treatment fields (< 5×5cm2). Agreement with measurement data with MapCHECK at the central axis (+1%) was found with the plan data. This is the first report for SRS small photon‐field measurements using the latest radiochromic film, EBT2. The results shown in this work indicate that the use EBT2 film provides accurate dosimetry measurements for small photon beams. The measurements show excellent agreement with validated small ion chamber measurements data that were entered into iPlan as commissioning data. Conclusions: The patient‐specific EBT2 film QA for iPlan SRS patients showed good results. The EBT2 films could potentially be a useful dosimeter in verification of commissioning as well as patient‐specific QA for SRS cases.

SU-E-T-214: Volumetric Particle Beam Quality Assurance

Purpose: Quality assurance (QA) and dose output measurements for particle therapy, where the range varies across the field is a challenge. Our goal was to devise a system that was both accurate and efficient. Methods: We have utilized a MatriXX(IBA) device in a protective sleeve within water to measure absolute dose and profiles. The MatriXXcan be positioned remotely to various depths in water. For high energy electrons (using a custom compensator‐CC) and protons (range compensator) depths may be chosen according to the therapeutic ranges across the field. Measured data was compared to phantom calculated profiles using the I'mRTsoftware (IBA). Two CC were constructed, CT scanned, and calculated, on top of a test phantom. The CC in front of a water phantom was irradiated with 20 MeV electrons. One CC was intentionally altered (added plastic narrow ring) to ensure a deviation could be detected. Setup and measurement times were recorded. Results: We found good agreement between measured absolute dose/profiles and calculations for two CCs at three depths (6.17, 8.17 and 10.17 cm). The Gamma index (3 mm, 3%) was calculated to be met over the majority of irradiated field. The MatriXX accurately detected the intentionally altered CC by for the measurement vs. the calculated plan. Setup and measurement times were reasonable for routine patient specific QA (∼5 min. per CC tested) excluding setup. Conclusions: The results indicate our measurement system using the MatriXX device in water was sufficient for measuring dose and profiles for electrons. We plan on performing dose output and profile measurements for proton patients when we go clinical in 2012. We anticipate performing QA for as many as 6 proton fields in <1hour. We have devised a measurement system that is accurate, efficient and sensitive to deviations.

SU‐E‐T‐435: Comparison in Dose Distributions Between 2D‐Arrays for Pre‐Treatment IMRT QA

Purpose: In recent years, many 2D‐arrays have been developed as dose verification. Therefore, the comparison in the dose accuracy between the 2D‐arrays is important especially for complex intensity modulated radiation therapy (IMRT). In this study, we compared dosimetric properties between various 2D‐arrays and also evaluated the accuracy of absolute dose and dose profile measurements for clinical IMRT fields. Methods: In this study, the following detectors were used to check the dose accuracy: MapCHECK (Sun Nuclear, Melbourne, FL), EPID (Varian Medical Systems, Palo Alto, CA), EPID‐based dosimetry (EPIDose, Sun Nuclear, Melbourne, FL), COMPASS (IBA Dosimetry, Germany) and EDR2 film (Eastman Kodak, Rochester, NY), Exradin A‐14SL ion chamber (0.016 cc, Standard Imaging, Middleton, WI). First the dosimetric properties of the 2D‐arrays were compared with dose linearity, dose rate, and output factor for 6 and 15 MV photon beams. Next the accuracy of the absolute dose and dose profile was evaluated for 21 clinical IMRT fields. Results: All detector responses for the dose linear were within 1%, and for the dose rates agreed within a standard deviation of ±1.2%. Except for EPID, the output factors were insignificant deviation. This is because EPID is the fluence measurement. In almost 2D‐arrays, the point dose agreed with treatment planning system (TPS) in less than 3% in the clinical fields. As for EPID, the accuracy that is equal to other detectors is obtained by some correction factors. Pass rates of each detector for TPS were more than 95% in the gamma analysis (criteria: 3 mm/3%). EPIDose is in a good agreement with TPS. This is because EPIDose has high resolution. Conclusions: All 2D arrays used in this study showed almost the same accuracy for clinical IMRT fields. EPIDose has better resolution than other 2D arrays and thus is expected for dose distributions with a small field.