EDR2 Film Research Articles

SU-E-T-128: Characterizing the Uniformity of Gafchromic EBT2 Film Response in a Cobalt-60 Field

Purpose: The goal of this work is to evaluate the uniformity of International Specialty Products Gafchromic EBT2 film with a high-resolution metric. The effect on uniformity of the marker dye (blue-channel) correction and time post-exposure are investigated. Methods: Film uniformity was quantified by using EBT2 and Kodak EDR2 film to measure the flatness of a cobalt-60 field. EBT2 films were digitized 24, 48, and 72 hours post-exposure and analyzed using three Methods: the manufacturer-recommended blue-channel correction, an in-house blue-channel correction, and a net optical density (OD) measurement in the red color channel. The field flatness was calculated from the horizontal and vertical dose profiles taken through the center of the cobalt-60 field for each analysis method and compared to the EDR2 results. Results: The blue-channel correction methods improved the flatness measured with EBT2 film by up to 1% over the net OD method. Additionally, the percent standard deviations of the flatness measurements with the blue-channel correction methods were up to 0.7% (1-sigma) lower than the net OD method. The flatness measured with EDR2 film was 1.4% to 1.8% better than the flatness measured with EBT2 film using the net OD method. The flatness did not exhibit any unidirectional change with time and showed a maximum change of 0.4%. Conclusions: Using field flatness as a metric for response uniformity, EBT2 film analyzed with the net OD method demonstrates substantial non-uniformities relative to EDR2 film. The blue-channel correction considerably improves the uniformity of EBT2 response and reduces the film-to-film response variation relative to the net OD method. The uniformity of EBT2 is independent of post-exposure time. The blue-channel correction can be used to reduce uncertainty in relative dosimetric measurements with EBT2 film.

SU-E-T-567: Small Field Dosimetry of Flattening Filter Free Beams

Purpose: To examine small field dosimetry of flattening filter free (FFF) beams. Methods: Output and relative profiles were measured at 10 cm depth in water equivalent plastic for the 6 MV FFF, 10 MV FFF, and the flattened 6 MV beams of a TrueBeam accelerator (Varian Medical Systems) using EDR2 film. Measurements were obtained for square field sizes of 3, 2, 1, and 0.5 cm, and for 1 and 0.5 cm MLC defined fields with collimator jaws set to 10 cm × 10 cm and to the vendor's recommendation (margins of 8 mm in the leaf direction and 2 mm in the perpendicular direction). Dose on a 1 mm grid was calculated using the analytic anisotropic algorithm (AAA), which was commissioned using fields >= 3 cm × 3 cm. Results: For 2 cm and 3 cm, the output agreement was better than 2%. At 1 and 0.5 cm, the differences ranged from 6% to 9% and 31 % to 44%, respectively. For the 1 cm MLC field, differences were 50%) by up to 2 mm. Interestingly, the agreement in the y‐direction was best for 0.5 cm, although the width for the outer (dose = 2 cm without additional commissioning. For smaller fields, additional output factors may improve the absolute dosimetry. Adjusting target spotsize may improve the profile agreement in the y‐direction. Author receives research support from Varian Medical Systems

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.

SU-E-T-201: Issues Encountered in Film-Based IMRT QA

Purpose: To investigate characteristics of EDR2 film-based IMRT QA with an eye towards moving from isodose line comparisons to more objective gamma statistical analysis. Methods: IMRT QA was performed on Siemens Linacs using a traditional ion-chamber-and-film phantom setup for step-and-shoot IMRT plans. All beams were delivered at their actual gantry angles and to the same film. Fiducial beams were used to fix film location for comparison with calculated dose. Dose was calculated on the Pinnacle planning system (8.0m). The EDR2 films were developed using a Kodak PP-XOMAT film processor that has been remanufactured by Picker. Films were scanned with a VIDAR VXR-16 Dosimetry Pro film digitizer using RIT 113 v5.2 software. Calibration films were created using crossed step wedges with doses calculated by Pinnacle. Gamma was calculated using the standard software settings of 3% / 4 mm. Results: For each QA film, the normalization doses (target to measured), mean gamma, and percentage of points failing the gamma limit (gamma < 1) were recorded. These percentages were found to be considerably higher in many cases than the often-used limit of 5%. This problem was reduced when a new calibration film was used every week and developed together with the QA films; however, the mean fraction of points failing the gamma criterion was still 7%, with 45% of films failing. The mean gamma was 0.4. The product of (target dose) * (mean gamma) * (3%/100%) was relatively constant and averaged 2.3 cGy. Conclusions: Various factors were found to be relevant to the deviations observed, especially film calibration and the target dose (which sets the dose scale for gamma). In many cases, the problem was due to a poorly functioning film processor, resulting in films with streaks and artifacts; this problem was solved by service but recurs on occasion.

SU-E-T-108: EBT2 Film Based Dosimetry Incorporating Correction for Spatial-Temporal Dose Response Variation

Purpose: Implementation of radiochromic film, such as GafChromic EBT2 film, as a quantitative 2‐dimensional dosimeter requires an array of equipment calibrations and corrections. We have developed a holistic model‐based calibration/correction mechanism for radiochromic film dosimetry. Methods: One challenge of using radiochromic film is that most of the densitometry scanners in use are designed for qualitative, not quantitative use. We performed careful characterization and quantification of the systematic and random components of all temporal and spatial optical density variations. In the proposed workflow, the spatial‐dependent dose response is estimated in the calibration stage under a novel model‐based optimization framework. A benchmark calibration process has been developed for the full (optical density, space) calibration map. For improved robustness, a simplified model that assumes separability of spatial variation and dose dependence was developed. The corresponding QA tool utilizes a consistent correction principle and maps the observed pixel intensity value based on the estimated spatially varying calibration map. The developed method has been tested on seven plans (both IMRT and VMAT) and compared with commercial software correction results. Dosimetry accuracy was validated against EDR2 film by comparison with treatment planning system calculated dose distribution using 3%‐3mm gamma criterion.Results: Under equivalent test conditions, the proposed method achieves gamma passing rates with EBT2 that are comparable to or better than a correction method of a commercial counterpart. Such corroboration is consistent for both IMRT and VMAT plans under gamma tests Conclusions: We have developed a systematic approach to comprehensively calibrate and use EBT2 film, accounting for temporal and spatial dose response variations. Specifically, a formal generative model and an estimation methodology was developed to achieve good balance between measurement sensitivity and robustness. This process has been implemented with in‐house software. Research supported in part by AACR career development award.

SU‐E‐T‐781: Reduction of Thread Effect for Superficial Tomotherapy Treatments

Purpose: Helical tomotherapy can be useful in the treatment of superficial treatment volumes. As a consequence of the helical delivery, a thread pattern is evident in measured dose distributions. This goal of this work is to reduce this effect.Methods: Radiation treatment plans were created utilizing pitches of 0.287, 0.360, 0.430 and 0.500 with a 5 cm field width with the helical tomotherapy treatment planning system (TPS). The plans were delivered to a cylindrical solid water phantom with 1.0 cm bolus. EDR2 film was placed between the solid water and the bolus in the target volume region. Back‐to‐back delivery of procedures with different pitch to constitute a single treatment fraction was considered. Results: The measured percent difference of minima and maxima of the thread pattern was determined to be 5%, 12%, 10%, and 16% for pitch values of 0.287, 0.360, 0.430 and 0.500, respectively. The calculated thread magnitude was within 2% of the measured. Combining procedures with pitches of 0.287 &amp; 0.360 and 0.287 &amp; 0.430 resulted in interference patterns in the threading. For the pitch combination of 0.287 &amp; 0.430, reduced variation (∼3%) was largely observed; however, limited regions of greater magnitude minima and maxima (± 5% of the mean dose) were also observed. An attempt was made to modify the target volume in order to create two plans with equal pitch values that combine to form a destructive interference pattern and, hence, a uniform dose. However, this was not readily achievable with the clinical version of the TPS Conclusions: The thread pattern can be mitigated with the use of a combination of delivery procedures with differing pitch, specifically 0.287 and 0.430. The TPS was able to predict the magnitude of the thread effect within 2% of the measured for the superficial target for all cases considered.

SU-E-T-736: Dosimetric Evaluation with Heterogeneity in Eclipse Treatment Planning Dose Calculation Algorithms

Purpose: Dose calculation accuracy utilizing anisotropic analytical algorithm (AAA) and pencil beam convolution (PBC) with multiple inhomogeneous phantoms was evaluated with measurements. Methods: A five layer virtual phantom was created in the Eclipse planning system. Each 5 cm layer was assigned in terms of water, air, water, bone, and water materials. Plans with a 6MV beam in different field sizes were calculated in AAA_8615 (with heterogeneity correction) and PBC_8615 (with and without heterogeneity correction). Combinations of solid water, PVC, and Styrofoam were then manufactured to acquire doses with parallel plate chamber and EDR2 films. Next, a seven field IMRT plan in PBC without heterogeneity was created within a Rando phantom. A cylindrical target volume was defined and 95% target was covered by prescription doses. Plans were then regenerated by recalculating this optimized base plan with keeping identical beam parameters but switching algorithms. The base plan was delivered to the phantom with TLDs and films. Measurements were compared with all calculations at same locations. Results: AAA shows a better dose prediction in all materials from measurements (Fig. 1–Fig.4). Discrepancies in air and bone regions due to lacks of lateral scatters and beam hardening were observed. In IMRT, both PBC with heterogeneity and AAA mean calculated target doses were <1% compared to TLD measurements; and PBC without heterogeneity values were lower by 11%. AAA produced better matches in profiles (Fig.5) as well. Conclusions: The findings suggest that AAA is more appropriate in dose predictions especially when lung and bone are involved.

SU-E-T-485: Clinical Evaluation of Inverse Planning Models and IMRT Delivery Systems in the Framework of AAPM TG-119 Protocol

Purpose/Objective: To evaluate the overall performance of treatment planning and MLC delivery systems for clinical implementation of IMRT modality. Materials/Methods: A 6-MV beam of photons from Elekta Synergy linac, modeled into Pinnacle treatment planning system (TPS), (Version 9.0) was used to develop plans from a suite of benchmark test cases provided in the TG-119 protocol. Each test case had a planning tumor volume (PTV) and an organ-at-risk (OAR) embedded into the rectangular phantom. These tests were planned, delivered, measured and analyzed using state-of-the-art dosimetry systems. Per-field planar isodose distributions were measured with the MapCHECK-2 diode-array device (Sun Nuclear Corporation, Melbourne, FL). A radiographic EDR-2 film, placed at one coronal plane in the phantom, was exposed to all fields with planned gantry and collimator angles. RIT113 scanning software (V5.3) was used to perform film dosimetry. Measured and planned dose distributions were analyzed using gamma criteria of 3%/3-mm. For absolute dosimetry, each test plan was delivered on a 30-cm×30-cm×20-cm homogenous solid-water phantom to assess agreement between planned and measured dose in high and low dose regions using a calibrated 0.125cc cylindrical ion chamber. Upon achieving quantitative confidence limits on baseline values established in the TG-119 protocol, the H&N phantom of the Radiation Physics Center (RPC) was irradiated with its planned IMRT fields and returned to RPC for dosimetric analysis. Results: The MapCHECK-2 and EDR-2 film measurements reproduced the planned isodose distributions with 99–100% gamma-test passing rates for all IMRT test plans. Ion-chamber dosimetry showed <2% difference between the planned and measured doses in the high and low dose regions. Our institution passed the RPC IMRT credentialing test using the RPCˈs H&N dosimetry phantom. Conclusions: AAPM TG- 119 protocol provides a useful resource for the clinical evaluation of the IMRT modality consisting of Pinnacle TPS and Elekta synergy MLC delivery system.

SU‐E‐T‐425: Plan‐Specific Gamma Analysis Acceptance Criteria for Multi‐Chamber Arrays

Purpose: Gamma analysis acceptance criteria for patient‐specific IMRT QA are subject to many variables including the measurement device, plan complexity and the threshold value used for the dose distribution. In this work we demonstrate plan‐specific gamma analysis using the IBA Matrix Evolution phantom. Methods: A series of previously treated prostate patient plans were selected to represent moderately complex dose distributions. The plans were delivered to the same phantom plane containing the ionchamber array and again with EDR2 film. Gamma criteria of 3%/3mm were evaluated using thresholds of 10% and a meaningful, plan‐specific threshold of 50%. Differences in number of pixels passing were evaluated for both detectors and thresholds using film as the “gold standard”. All plans were delivered in a composite manner using treatment gantry angles. Passing rates for both detectors were compared to determine detector‐specific acceptance criteria. The clinical results from the last 329 prostate patients were compared with these criteria.Results: The passing rates for the 10% threshold between film and the ionchamber array were 99.2% and 92.1%, respectively. Passing rates for the 50% threshold were 99.5% and 96.7%, respectively. These rates represent an approximate 7% and 3% absolute reduction in passing criteria for detector and plan‐specific analysis. The average passing rate for the initial 329 clinical prostate cases measured with the array using the 50% threshold was 94.6% Conclusions: AAPM TG‐119 suggests a passing rate of 88–90% for composite distributions using film. It is important to understand the relationship between film (high resolution) and the detector array and dose threshold used clinically. The 3% drop between film and array in points passing using a plan‐specific threshold of 50% for our test cases indicates an acceptable range of 85–87%. Our clinical passing rates ranged between 82.0% to 99.7% with 97.6% of cases meeting the aforementioned range.

SU-E-T-478: TomoTherapy® with 180 Beam Angles Per Rotation for Small Target Off-Axis Treatment

Purpose: Investigate the effect of increasing the number of beam angles per rotation to treat small off-axis targets with helical tomotherapy. Methods: TomoTherapy® gantry rotates continuously although radiation beam angles are discretized in the optimization. At each projection angle, the leaf open/close pattern is assumed unchanged. However, for small off-axis targets, the beam motion within a projection (7 degree) may require substantial portion of leaves to open/close for better target conformity. An off-axis (17 cm) target of 1 cm diameter in a cheese phantom was used in the planning study. The plans were optimized with 180 and 51 beam angles with source super sampling and the results were compared. The optimization constraints are the same for the two plans. Measurements were taken for a plan with four cylindrical targets of 1 cm diameter located at different radii from the central axis along a line in a cheese phantom. The plan was optimized and delivered with 180 beam angles on a Hi-Art® system. An EDR2 film was placed in the plane lined up with the four targets to measure the transverse dose distribution. Ion chamber point measurements were taken near three targets of the four. Results: Compared to the plan with 51 beam angles, the plan with 180 angles showed a 70% decrease of (D2-D98) for target dose uniformity, a 27% decrease of D100 for OAR avoidance and a 12% decrease of beam-on time. The film measurement showed good agreement between calculation and measurement. The point dose measurements showed agreement within a 1.6% difference. Conclusions: Delivery with 180 beam angles can improve the plan quality for small off- axis targets over 51 beam angles with source super sampling. Both target dose uniformity and OAR avoidance are improved. The plan with 180 beam angles was delivered and the measurements agreed with the calculation.

SU‐D‐BRA‐05: Evaluation of the Accuracy of 3DVH+Mapcheck2 Predictions of Dose to Virtual Ion Chamber and Film in Composite IMRT QA

Purpose: Two common methods of performing patient‐specific IMRT QA are the composite dose (ion chamber and film) and beam‐by‐beam dose measurement (diode array). Neither method by itself gives a complete enough measure of the dose distribution. DVH+Mapcheck2 was evaluated for its ability to provide both types of doses with just the array measurement. Methods: The dosimetric results from 15 patient‐specific IMRT QA tests performed by composite irradiation of ion chamber and EDR film (in coronal plane) in phantom were compared to the predicted doses for the same locations based on Mapcheck 2 measured doses of each beam within each plan and interpretation by 3DVH software. The ratio of the measured to predicted ion chamber doses was calculated and the Gamma metric was used to compare the relationship between the film doses to the Eclipse or 3DVH predicted planar doses. Also, the 3D gamma value was calculated in the 3DVH software which compares the Eclipse to 3DVH predicted dose distributions. For the 2D and 3D Gamma metrics, 2% dose and 2mm DTA were used. In addition, a simple dose difference was performed using either a 2% or 3% dose difference tolerance. Results: The mean ratio(SD) of the chamber to 3DVH predicted dose was 1.013(.015) while the ratio of the measured to Eclipse calculated dose to the chamber was 1.003(.012). The mean percentage of pixels failing the 3D gamma=1 metric was 1.2%(1.4%) while the failure rate for the 2D Gamma metric (3DVH to Eclipse) was 1.1%(1%). The Gamma failure rate for the film vs. Eclipse was 1.6%(2%). Mean dose differences, highly sensitive measures of dosimetric difference when not paired with DTA, were not significantly different between 3DVH+Mapcheck2 or film vs. Eclipse. Conclusions: 3DVH+Mapcheck2 accurately predicts the doses one would have measured had one instead used an ion chamber and coronal film in plastic phantom.Sun Nuclear has provided support for this project

SU‐E‐T‐341: Dosimetric Verification of a Wedge Shaped Midline Block for Teletherapy in Cervical Cancers

Purpose: Designing a wedge shaped midline block (WMLB) for teletherapy treatments in early cervical cancers, its dosimetric verification and clinical implementation. Methods: The method of designing a WMLB is discussed based on the dose distribution in coronal plane obtained from treatment planning system (TPS) for manual afterloading intracavitary brachytherapy (ICBT). Horizontal profiles are taken with Kodak EDR2 film and 0.6cc ionization chamber (IC) under water equivalent slab &amp; water phantom respectively at isocenter of telecobalt machine with WMLB at tray level. Using these measurements, WMLB was configured in the unit modeling of TPS. Clinical plans (AP/PA; Three fields, Box field techniques) related to the treatment were created with central shielding by conventional rectangular midline block (RMLB) and WMLB in TPS for a field size of 15cm × 15cm in AP/PA &amp; 8cm × 15cm for Lateral field portals under simulated phantom geometry. Dose calculations (3D) were done at isocenter and were verified with IC (absolute &amp; relative) dosimetric measurements. Results: Wedge shaped MLB was found to be symmetrical within &amp;#61617;1.5% accuracy which was confirmed through horizontal profiles generated by Film, IC &amp; TPS. The calculated doses from TPS were verified with ionometric measurements (Central and Off Axis) taken at isocenter and were found to be well within +/−3.3%. Conclusions: The use of a RMLB results in in‐homogeneity of dose around the target and should be avoided. A WMLB on the other hand would certainly improve the dose homogeneity around the target thereby minimizing the dose to the critical structures (bladder and rectum).

SU‐C‐BRB‐05: Quality Assurance of Gated RapidArc Stereotactic Body Irradiation

Purpose: Develop a reproducible QA methodology for commissioning gated RapidArc SBRT and patient treatment. Metrics used for patient specific QA, namely distance‐to‐agreement (DTA) and gamma index, are examined to determine if standard values used for IMRT are applicable for displacementgated treatments. Methods: Methods followed that used for IMRT patient‐specific QA. Slabs of solid water with 1.5 mm radio‐opaque markers were placed on a Respiratory Gating Platform (Standard Imaging), and the phantom was aligned to markers placed on its stationary base. Treatment plans were based on 4D and planning CTs acquired while the phantom was moved sinusoidally. Gating parameters were determined by fluoroscopic imaging prior to treatment. 3D square fields, RapidArc plans with spherical targets (1–10 cm diameter), and patient plans were delivered to the moving phantom with a 1 mm gating aperture. Dose distributions were measured with Kodak EDR2 film and compared to distributions planned with Eclipse v8.9 using RIT 113v5.2 software to determine the mean DTA. Results: For square fields the DTA averaged over the different field sizes was 3.4 mm in the direction parallel to the motion and 2.7 mm perpendicular. The average 2D DTA was 3.8 mm. For the gated RapidArc treatments, the average DTAs were 3.4 mm parallel to the motion and 1.9 mm perpendicular. The 2D DTA was 3.0 mm. There was a trend for the DTA for small targets to be larger than for large targets. For the patient plan the 2D DTA for the gated treatment was 2.6 mm, and 0.3 mm with the phantom static. Conclusions: A repeatable QA method for gated RapidArc SBRT has been implemented. With a fluoroscopic gating aperture of 1 mm, use of a gamma index criterion of (3%, 3 mm) and a DTA of 3 mm is valid for small targets with gated delivery.

SU‐D‐BRA‐04: Technical Evaluation of TomoTherapy Roll Correction

Purpose: Evaluate the effectiveness of the roll correction feature of the TomoTherapy Hi‐ART system. Methods: A roll‐sensitive plan was designed and delivered to a cylindrical solid water phantom to test the accuracy and utility of roll corrections. Cylindrical target structures containing coaxial inner avoidance structures were placed adjacent to the plane bisecting the phantom and 7cm laterally off central axis. The phantom was positioned at isocenter with the target‐plane parallel to the couch surface. Varying degrees of phantom roll were induced and dose to the targets and inner avoidance structures was measured using Kodak EDR2 films placed in the target‐plane. Normalized point doses were compared with baseline (no roll) data to determine the sensitivity of the test and the effectiveness of the roll correction feature. Gamma analysis between baseline, roll‐corrected and uncorrected films was performed using RIT software. Results: On average, significant deviations from baseline point dose measurements (±3 standard deviations) are observed beyond 0.4° of uncorrected phantom roll. Conversely, all point dose measurements with roll correction applied are indistinguishable from baseline (&lt;3 standard deviations) up to our maximum tested roll of 5°. For measurements in the rolled phantom without roll correction, gamma pass rates drop below 90% starting at an average of 0.25°. Gamma pass rates for all measurements with roll correction applied were greater than 97%. Conclusion: Using our rigid phantom and roll‐sensitive test plan, rotational misalignment greater than 0.25° can be detected and results in observable differences between planned and delivered dose. Utilizing the roll correction feature accurately corrects for the effects of such misalignment. Further work is required to evaluate the efficacy of roll correction in clinical circumstances where deformation accompanies rotation and to assess the clinical impact of using rotational and translational corrections compared to translational corrections alone.

SU-E-T-481: Dosimetric Verification of Large IMRT Fields at Shorter SSDs Using Two-dimensional Arrays - a Comparison to EDR-2 Films

Purpose: The dosimetric verification of large IMRT fields is crucial, if the target length is not entirely covered by the two‐dimensional detector arrays, due to its limited active area for the measurements. Therefore, our aim is to overcome this limitation by doing the measurements at shorter SSDs, where the treatment field size is smaller compared to the field size at isocenter. Methods: Four cervical carcinoma patient plans with large IMRT fields (PTV Length‐20.50, 21.50, 22.50 and 24.00cm) were chosen, where PTV length was not entirely covered by I'matriXX two‐dimensional array (Active area 24×24 cm2) placed at the isocenter. To overcome this, for each patient, IMRT plans (6MV, Sliding window) were verified at 95cm (array at isocenter), 90cm, 85cm and 80cm SSDs using I'matriXX and EDR‐2 films. The dose planes measured with I'matriXX and EDR‐2 films were compared with their effective dose planes at TPS. For gamma index analysis, 3%/3mm criteria were used. Initially, I'matriXX and films were calibrated at corresponding SSDs. Results: For I'matriXX Vs TPS, as the SSD decreases, significant improvement in pass rate of gamma was observed. At shorter SSDs, gamma was improved up to 4.4%. This is due to the improved coverage of the fields at I'matriXX plane at shorter SSDs and it is consistent for all the cases. For I'matriXX, the average pass rate of gamma was 97.75±2.63%, 98.96±0.86%, 99.36±0.50% and 99.52±0.33% for 95cm, 90cm, 85cm and 80cm SSDs, respectively. I'matriXX showed better results at shorter SSDs than EDR‐2 films. For film, which is field size independent, the average pass rate was 97.66±0.80%, 97.16±1.14%, 96.57±1.62% and 94.43±2.30% for 95, 90, 85 and 80cm SSDs. Both I'matriXX and films are sensitive to calibration and correction factors at different SSDs. Conclusions: By using proper calibration and correction factors, I'matriXX may be used at shorter SSDs for the verification of larger IMRT fields.

SU-E-T-170: Development and Testing of a Single Exposure Film Calibration Procedure

Purpose: To develop and test the sensitivity of a single exposure film calibration procedure using a dynamic wedge. Method and Materials:A 60o dynamic wedge was used to expose EDR2 film and generate optical density (OD) to dose calibration curves in DoseLab Pro. Doses between 50 and 500 cGy were examined. Dosimetric data was generated from a Profiler2 diode array. This calibration curve was compared to one generated by the standard approach of irradiating 8 separate sheets of film to known doses. The sensitivity of the wedged‐film calibration procedure was also tested through the following 4 investigations. First, film measurements were repeated 10 times to determine the variability of the film response. Second, profiler measurements were repeated 10 times to determine the variability of the profiler response. Third, positional offsets of the profiler were investigated (up to 4mm off‐center — the size of the diode spacing). Fourth, different amounts of solid water build‐up were used (between 1 and 10cm) above the profiler and corresponding film. Results: The calibration curve generated with the wedge exposure compared very well to that generated with the standard 8‐film approach, being less than 2.5% locally different (0.8% relative to maximum) at doses above 50 cGy. Irradiation of multiple films introduced a maximum deviation of 1.2%, while multiple profiler measurements introduced a maximum deviation of 0.5%. Shifts in the position of the profiler introduced a maximum error of 3.0%. These deviations represent good robustness of the calibration procedure. Consistent calibration curves were generated regardless of the extent of build‐up material used. Each film exposure and software analysis was completed in 1 minute and 5 seconds, respectively. Conclusions: This work has developed a single exposure film calibration procedure. This offers physicists a rapid and robust film calibration method.

SU‐E‐T‐454: Feasibilty of Image‐Guided Total Marrow Irradiation Using Helical TomoTherapy

Purpose: Investigation of the technical feasibility and dosimetry of image‐ guided total marrow irradiation using helical TomoTherapy in the treatment of relapsed multiple myeloma patients. Irradiation is followed by a hematopietic stemcell transplantation. This study concentrates on planning and dosimetry Methods: Patients are simulated in supine position with H&amp;N mask and pelvic cast. TomoTherapy plans for the upper body (head‐knees) were generated (pitch 0.45, modulation factor 2.0, field width 5cm). Dose prescription was 4x3Gy=12Gy. Dose verification is performed with the IˈmRT MatriXX. Beam‐on‐time is +/−30min. TMI‐stability measurement was done with 2 ICs on different distances in 1.5m solid water equivalent blocks. Lower extremities were planned conventional and with TomoTherapy. Dose in the junction region between upper and lower body plans was verified with EDR2 film for both techniques. Results: It is possible to create clinically acceptable plans for the upper body with TomoTherapy. Verification shows acceptable results in the lung region (gamma 5%, 7mm). Measurement in pelvis region is less good as in lung region. TMI‐stability measurement shows differences in dose till 4% measured/calculated. During TMI a 2.5% dose drift was seen by the detectors. Plans for the extremities were more homogeneous in the junction region with TomoTherapy comparing with the conventional technique. Conclusions: TPS has difficulties to find a good ratio between couch and gantry speed. Differences seen at TMI‐stability measurements can be pointed to drift and stability of one rotation. Differences are depending on system stability (magnetron, target, …). Patient specific QA is necessary before treatment. Differences between the gamma index of the lungs and pelvis can be pointed to the fact that the planning is optimized very well for the lung region. Pitch and modulation factor are optimized for this region. For the junction region tomotherapy plans give a better homogeneity than 1 tomotherapy and 1 conventional plan.

SU-E-T-468: A Novel Complexity Index to Evaluate IMRT QA

Purpose: To establish a quantitative complexity measure for IMRT plans. Methods: We propose a quantitative complexity index (CI) of IMRT plans that considers the dose variations within each IMRT field and compares to dose variations uniform rectangular fields of comparable size. The ratio of area to perimeter of a 1‐D dose profile is a measure of complexity, as the complexity of dose profile increases, this ratio would decrease as the perimeter would increase for a given area. This ratio would be greatest for an open field. By normalizing ratio of an IMRT field to that of uniform field of similar size, the variation of this ratio with field size would be eliminated. The complexity index of a plan would be weighted average of complexity indices of its individual fields. A graph of gamma versus CI would guide the clinical physicist in accepting an IMRT QA in a more rational manner. The calculated dose distributions were used to calculate the CI. We exposed EDR films in solid phantom to uniform and intensity modulated fields using 6 MV photons. The films were normal to CAX of beams. Results: The gamma analyses of these films were performed using 3% dose, 3mm DTA criteria with a 10% dose threshold. Some of the uniform and the modulated fields were measured and analyzed repeatedly to evaluate the random uncertainty. We noticed a gradual decrease in the percent of pixels with gamma values &#8804;1 with increasing complexity — from about 94% for large uniform fields to about 83% for modulated fields. The decrease was significant compared to the random uncertainty. Small open fields (4×4) had lower % pixel with gamma &#8804;1 than larger fields (10×10) (88%). Conclusion: A quantitative complexity analysis was established and would be a useful tool in evaluating gamma analyses of IMRT QA.

SU‐E‐T‐329: Degrading Electron Beam Energy in Rotational Total Skin Electron Therapy

Purpose: In rotational total skin electron therapy (RTSET), degraded electron beam energy may be required for patient treatment. A method is developed by inserting an acrylic plate in TBIe‐tray to deliver a lower energy beam. The change in percentage depth dose (PDD) verses thickness of plate is studied. Methods: A Rando phantom is used on a rotational platform at an extended SSD (=332 cm) with a linac gantry angled at 90°. A special mode with a TBIe‐insert delivers a 6 MeV electron beam on a Varian 21EX. An acrylic plate is inserted in the TBIe‐tray to degrade 6 MeV electron beam energy. Three different thickness of acrylic plates are 4, 6, and 10 mm. For PDD measurements, Kodak EDR films are sandwiched in the Rando phantom slides. The film surface is oriented parallel to the incident electron beam. The films are analyzed using the RIT113 dosimetry software with a Vidar‐VXR 16 Pro Scanner. Results: Several physical parameters dmax, R50, and Rp are determined in the PDD curves. These parameters vary with the inserted plate thickness. The dmax is around 0.4 cm without acrylic plate (t = 0) and decreases as the plate thickness increases. For 10 mm plate thickness, dmax shifts to the surface of the phantom. The mean energy E0 at the phantom surface can be obtained from R50 for each degraded electron beam using the relationship between Eo and R50. The E0 decreases as the plate thickness increases, indicating 6 MeV electron beam degraded to a lower energy beam. Conclusions: The method to degrade electron beam energy in RTSET offers advantages of easy setup and manageable beam energy than spoiler technique. Most of the treatment parameters can be readily obtained from the PDD data for each degraded electron beam.

Monte Carlo electron source model validation for an Elekta Precise linac

Electron radiation therapy is used frequently for the treatment of skin cancers and superficial tumors especially in the absence of kilovoltage treatment units. Head-and-neck treatment sites require accurate dose distribution calculation to minimize dose to critical structures, e.g., the eye, optic chiasm, nerves, and parotid gland. Monte Carlo simulations can be regarded as the dose calculation method of choice because it can simulate electron transport through any tissue and geometry. In order to use this technique, an accurate electron beam model should be used. In this study, a two point-source electron beam model developed for an Elekta Precise linear accelerator was validated. Monte Carlo data were benchmarked against measured water tank data for a set of regular and circular fields and at 95, 100, and 110 cm source-to-skin-distance. EDR2 Film dose distribution data were also obtained for a paranasal sinus treatment case using a Rando phantom and compared with corresponding dose distribution data obtained from Monte Carlo simulations and a CMS XiO treatment planning system. A partially shielded electron field was also evaluated using a solid water phantom and EDR2 film measurements against Monte Carlo simulations using the developed source model. The major findings were that it could accurately replicate percentage depth dose and beam profile data for water measurements at source-to-skin-distances ranging between 95 and 110 cm over beam energies ranging from 4 to 15 MeV. This represents a stand-off between 0 and 15 cm. Most percentage depth dose and beam profile data (better than 95%) agreed within 2%/2 mm and nearly 100% of the data compared within 3%/3 mm. Calculated penumbra data were within 2 mm for the 20 x 20 cm2 field compared to water tank data at 95 cm source-to-skin-distance over the above energy range. Film data for the Rando phantom case showed gamma index map data that is similar in comparison with the treatment planning system and the Monte Carlo source model. The gamma index showed good agreement (2%/2 mm) between the Monte Carlo source model and the film data. Percentage depth dose and beam profile data were in most cases within a tolerance of 2%/2 mm. The biggest discrepancies were in most cases recorded in the first 6 mm of the water phantom. Circular fields showed local dose agreement within 3%/3mm. Good agreement was found between calculated dose distributions for a paranasal sinus case between Monte Carlo, film measurements and a CMS XiO treatment planning system. The electron beam model can be easily implemented in the BEAMnrc or DOSXYZnrc Monte Carlo codes enabling quick calculation of electron dose distributions in complex geometries.