FilmQA Pro Software Research Articles

Evaluation of Patient-Specific Quality Control (QC) for Markerless Dynamic Tumor Tracking (MDTT) Deliveries

To report on the patient-specific QC for MDTT for liver and lung tumors treated using the dome-of-liver/diaphragm as a surrogate for the target position and compare to the QC of conventional fiducial-based tracking. Vero4DRT is a linear accelerator that has a gimbal-mounted waveguide and collimation system allowing the radiation beam to perform dynamic tumor tracking based on a patient's respiratory motion. A correlation model is built between an external IR marker placed on the chest and an internal structure, which may be implanted fiducial surrogates or in the case of MDTT, an anatomic landmark. During treatment an orthogonal kV image pair is taken every second. The auto detected internal structure position versus the predicted position is recorded in treatment log files. Five IMRT tracking plans were delivered to two commercial motion phantoms with in-house additions for patient-specific tracking QC. For point dose measurements, a 0.6cc farmer chamber was placed inside the dynamic tracking phantom which contains fiducials for fiducial-based tracking delivery and an anatomic landmark for MDTT. For 2D dose distribution measurements, a radiographic film insert for the Quasar Respiratory Motion Phantom was constructed. The phantom contains both fiducials and has a liver-dome shape at one end to facilitate MDTT. Both motion platforms were interfaced with software that enabled patient-specific respiratory motion traces acquired during a 4DCT scan. Each plan was delivered three times on each phantom: 1) in static mode, 2) during fiducial-based tracking, and 3) during MDTT. Chamber measurements taken during static and tracked delivery were compared to the average chamber dose from the treatment planning software (TPS). Film distributions measured during tracked deliveries were compared to the static film distribution using gamma analysis with FILMQA PRO software. Finally, dynamic tracking treatment statistics, extracted from log files, were compared between fiducial and markerless tracking deliveries. All chamber measurements resulted in dose differences <2.5% compared with the TPS. Dose differences between fiducial-based and MDTT were 0.26% on average (range 0.03-0.62%). For film dose map analysis, gamma pass rates were >95% for all plans for all tracking methods. The average gamma pass rate for 3%/3mm was 99.4% (fiducial-based) vs 98.4% (markerless). For 2%/2mm, the average gamma pass rate was 96.4% (fiducial-based tracking) vs 95.6% (MDTT). Dynamic treatment statistics from logs files reported an average 3D absolute deviation from predicted position of 0.52mm (±0.24) for fiducial tracking and 1.19mm (±0.50) for MDTT. Both fiducial-based and MDTT plans meet passing criteria for patient-specific QC. 3D absolute deviation of detected versus predicted position are larger for MDTT compared to fiducial tracking, however no significant impact on dose delivery was measured.

Evaluation of a new radiochromic film dosimeter, Gafchomic EBT4, for VMAT, SABR and HDR treatment delivery verification

Objective. To evaluate a new film for radiotherapy dosimetry, Gafchromic EBT4, compared to the current EBT3. To evaluate dose-response and verify test cases in MV external beam and HDR brachytherapy. Approach. Three lots (batches) of EBT4 and three lots of EBT3 films were calibrated at 6 MV over 0–1200 cGy range, using FilmQAPro software. Signal-to-noise of pixel value, reported dose (RD), and factors affecting dosimetry accuracy were evaluated (rotation of the film at scanning, energy response and post-exposure darkening). Both films were exposed to clinical treatment plans (VMAT prostate, SABR lung, single HDR source dwell, and ‘pseudo’ 3-channel HDR cervix brachytherapy). Film-RD was compared to TPS-calculated dose. Main results. EBT4 calibration curves had characteristics more favourable than EBT3 for radiation dosimetry, with improved signal to noise in film-RD of EBT4 compared to EBT3 (increase of average 46% in red and green channels at 500 cGy). Film rotation at scanning and post-exposure darkening was similar for the two films. The energy response of EBT4 is similar to EBT3. For all clinical case studies, EBT4 provided better agreement with the TPS-planned doses than EBT3. VMAT prostate gamma 3%/3 mm passing rate, EBT4 100.0% compared to EBT3 97.9%; SABR lung gamma 2%/2 mm, EBT4 99.6% and EBT3 97.9%; HDR cervix gamma 3%/2 mm, EBT4 97.7% and EBT3 95.0%. Significance. These results show EBT4 is superior to EBT3 for radiotherapy dosimetry validation of TPS plan delivery. Fundamental improvements in noise profile and calibration curve are reported for EBT4. All clinical test cases showed EBT4 provided equivalent or smaller difference in measured dose to TPS calculated dose than EBT3. Baseline data is presented on the achievable accuracy of film dosimetry in radiotherapy using the new Gafchromic EBT4 film.

IMRT national audit in Portugal

PurposeThe IAEA newly developed “end-to-end” audit methodology for on-site verification of IMRT dose delivery has been carried out in Portugal in 2018. The main goal was to evaluate the physical aspects of the head and neck (H&N) cancer IMRT treatments. This paper presents the national results. MethodsAll institutions performing IMRT treatments in Portugal, 20 out of 24, have voluntarily participated in this audit. Following the adopted methodology, a Shoulder, Head and Neck End-to-End phantom (SHANE) – that mimics an H&N region, underwent all steps of an IMRT treatment, according to the local practices. The measurements using an ionization chamber placed inside the SHANE phantom at four reference locations (three in PTVs and one in the spinal cord) and an EBT3 film positioned in a coronal plane were compared with calculated doses. FilmQA Pro software was used for film analysis. ResultsFor ionization chamber measurements, the percent difference was within the specified tolerances of ±5% for PTVs and ±7% for the spinal cord in all participating institutions. Considering film analysis, gamma passing rates were on average 96.9%±2.9% for a criterion of 3%/3 mm, 20% threshold, all above the acceptance limit of 90%. ConclusionsThe national results of the H&N IMRT audit showed a compliance between the planned and the delivered doses within the specified tolerances, confirming no major reasons for concern. At the same time the audit identified factors that contributed to increased uncertainties in the IMRT dose delivery in some institutions resulting in recommendations for quality improvement.

Selection of gamma analysis acceptance criteria in IMRT QA using Gafchromic EBT3 film dosimetry

AbstractBackground and purposeThis study reported the justification and selection of acceptable γ criteria with respect to low (6 MV) and high (15 MV) photon beams for intensity-modulated radiation therapy quality assurance (IMRT QA) using the Gafchromic external beam therapy 3 (EBT3) film.Materials and methodsFive-field step-and-shoot IMRT was used to treat 16 brain IMRT patients using the dual-energy DHX-S linear accelerator (Varian Medical System, Palo Alto, CA, USA). Dose comparisons between computed values of the treatment planning system (TPS) and Gafchromic EBT3 film were evaluated based on γ analysis using the Film QA Pro software. The dose distribution was analysed with gamma area histograms (GAHs) generated using different γ criteria (3%/2 mm, 3%/3 mm and 5%/3 mm) for the 6 and 15 MV photon beams, to optimise the best distance-to-agreement (DTA) criteria with respect to the beam energy.ResultsFrom the comparison between the dose distributions acquired from the TPS and EBT3 film, a DTA criterion of 3%/2 mm showed less dose differences (DDs) with passing rates up to 93% for the 6 MV photon beams, while for the 15 MV a relaxed DTA criterion of 5%/3 mm was consistent with the DD acceptability criteria with a 95% passing rate.ConclusionsOur results suggested that high-energy photon beams required relaxed DTA criteria for the brain IMRT QA, while low-energy photon beams showed better results even with tight DTA criteria.

Commissioning and evaluation of a radiochromic EBT3 film dosimetry system

AbstractPurposeThis work reports our study to commission a radiochromic film dosimetry system using the timely EBT3 film. We carried out dosimetric evaluations on different characteristics of photon beams (e.g., flatness, symmetry and penumbra) in radiation dose delivery.Materials and MethodsA Varian linear accelerator producing 6 and 15 MV photon beams with 120 multi-leaf collimator was used in this study. PTW ionisation chamber was used to measure the beam characteristics such as symmetry, flatness and penumbra and these measurements were used to commission the radiochormic EBT3 film dosimetry system. The results of irradiated films were analysed using the radiochromic film QA Pro software 2016.ResultsThe measured film doses were analysed at two different colour channels (green and red) using two scanning geometries (i.e., upper or lower side of film facing the scanner light source) at two dose levels (10 and 40 Gy). The difference between the ionisation chamber and film results was found insignificant and within the acceptable range as per the World Health Organisation standard.ConclusionResults of the comparison between the ionisation chamber and film measurements show that our radiochormic EBT3 film dosimetry system is reliable and cost-effective in the output measurement of a linear accelerator. Our measurements confirm that our EBT3 film dosimetry agreed well with the ionisation chamber, and can be used as a re-validation tool for linear accelerator quality control.

Dosimetric effects of a repositioning head frame system and treatment planning system dose calculation accuracy.

This work aims to study the effect on surface dose and dose distribution caused by the Elekta Fraxion cranial immobilization system. The effect of Fraxion inclusion in Elekta Monaco treatment planning system and its calculation accuracy is also checked. To study the dose attenuation, a cylindrical phantom was located over the Elekta Fraxion with an IBA CC13 ionization chamber placed in the central insert at the linac isocenter. Dose measurements at multiple gantry angles were performed for three open fields, 10 × 10 cm, 5 × 5 cm and other smaller 2 × 2 cm. Measured doses were compared with the ones calculated by Monaco. Surface dose and dose distribution in the buildup region were measured placing several Gafchromic Films EBT3 at linac CAX between the slabs of a RW3 phantom located over Fraxion and read using FilmQA Pro software. Measures were performed for two open field sizes and results were compared with Monaco calculations. Measurements show a 1% attenuation for 180° gantry angle but it can be as high as 3.4% (5 × 5 open field) for 150°/210° gantry angle, as with these angles the beam goes through the Fraxion's headrest twice. If Fraxion is not included in the calculation Monaco calculation can result in a 3% difference between measured and calculated doses, while with Fraxion in the calculation, the maximum difference is 0.9%. Fraxion increases 3.7 times the surface dose, which can be calculated by Monaco with a difference lower than 2%. Monaco also calculated correctly the PDD for both open fields (2%) when Fraxion is included in the calculation. This work shows that the attenuation varies with gantry angle. The inclusion of Fraxion in Monaco improves the calculation from 3% difference to 1% in the worst case. Furthermore, the surface dose increment and the dose in the buildup region are correctly calculated.

29. End-to-End tests of the new Elekta Gamma Knife ICON

Introduction The objective of this study is to evaluate the overall performances of the new Elekta Gamma Knife Icon via End-to-End tests. Methods This study consists of three different End-to-End tests: misalignment, point dose and dose distribution discrepancies. The first study evaluates the misalignment of the dose delivery by using a phantom made of plexiglass wherein radiochromic films have been inserted. CBCT images of this phantom have been acquired and were using for the dose planning. The dose planning consisted of positioning 4 mm isocenters on specific localizations. Films were analyzed with ImageJ software. The second study evaluates point dose discrepancies. We used the anthropomorphic CIRS STEEV phantom and a PTW PinPoint ionization chamber. A dose planning, consisting of a single 4 mm isocenter centered on the sensitive volume of the ionization chamber, were performed on the CT of the phantom and the whole workflow including mask and pillow personalization and CBCT repositioning were followed step-by-step. Deviation of ten dose measurements was analyzed. The third study evaluates dose distribution discrepancies. We used the anthropomorphic CIRS STEEV phantom and a film stack insert filled with seven radiochromic films. A realistic dose planning (25 isocenters (4 and 8 mm collimators, some plugged), prescription dose of 4.8 Gy at the 50% isodose) was performed on the CT of the phantom using the Convolution algorithm then the whole workflow including mask and pillow personalization and CBCT acquisitions were followed. Film distribution doses were compared with theoretical distribution using 3%/1mm gamma index analysis via FilmQA Pro software (Ashland). Results Misalignment errors found were 0.25 mm and 0.27 mm in x and z directions. Point dose discrepancy found was 0.18% when the phantom has been moved and when the re-alignment procedure with CBCT acquisition was performed. Average number of pixels passing the 3%/1 mm gamma index criteria is 97.5% (min. 93.6% and max. 99.45%). Conclusions The sum of these results demonstrates the overall quality of the new Elekta Gamma Knife ICON and offers the guarantee of high quality procedures for hypo fractionated and frameless treatment. Informations given by the CBCT (ie. Leksell coordinates here) are enough trustable to assure the quality of treating a patient at the right localization and with the right dose.

TU‐D‐209‐06: Head and Neck Tissue Dose From X‐Ray Scatter to Physicians Performing Cardiovascular Procedures

Purpose:The purpose of this work was to characterize the spatial distribution of scatter radiation to the head and neck of a physician performing an x‐ray interventional procedure and assess brain, eye lens, and carotid artery dose.Methods:Radiographic x‐ray beams were tuned to match the peak energy (56 to 106 keV) and HVL (3.5 to 6.5 mm Al) of x‐ray scatter originating from a patient during a fluoroscopic procedure. The radiographic beam was directed upon a Rando phantom from an inferior‐left location to mimic a typical patient‐operator geometric relationship. A lead‐equivalent protective garment was secured to the phantom. Direct exposure Gafchromic film (XRQA2) was placed between the transverse plane layers of the head and neck region of the phantom and exposed with 4 scatter‐equivalent radiographic beams. A 3×3 cm2 film placed at the left collar of the phantom was used to monitor incident dose in the position of a radiation monitoring badge. The films were converted to 2D dose distribution maps using FilmQA Pro software and an Epson 11000‐XL scanner. The 2D dose distributions maps were normalized by the left collar dose and the percent of left collar dose (%LCD) was calculated for select tissues.Results:The dose maps had high dynamic range (104) and spatial detail. Considering all transverse planes and 4 scatter beam qualities, the median %LCD values were: whole brain 8.5%, left brain 13%, right brain 5.4%, left eye lens 67%, right eye lens 25%, left carotid artery 72%, and right carotid artery 28%.Conclusion:Scatter radiation dose to an operator can be simulated using a tuned radiographic beam and used to expose a phantom and Gafchromic film, thereby creating detailed 2D dose distribution maps. This work facilitates individualized estimation of dose to select head and neck tissues based on an operator's radiation monitoring badge value.

SU‐F‐T‐490: Separating Effects Influencing Detector Response in Small MV Photon Fields

Purpose:Different detector properties influence their responses especially in field sizes below the lateral electron range. Due to the finite active volume, the detector density and electron perturbation at other structural parts, the response factor is in general field size dependent. We aimed to visualize and separate the main effects contributing to detector behavior for a variety of detector types. This was achieved in an experimental setup, shielding the field center. Thus, effects caused by scattered radiation could be examined separately.Methods:Signal ratios for field sizes down to 8 mm (SSD 90 cm, water depth 10 cm) of a 6MV beam from a Siemens Primus LINAC were recorded with several detectors: PTW microDiamond and PinPoint ionization chamber, shielded diodes (PTW P‐60008, IBA PFD and SNC Edge) and unshielded diodes (PTW E‐60012 and IBA SFD). Measurements were carried out in open fields and with an aluminum pole of 4 mm diameter as a central block. The geometric volume effect was calculated from profiles obtained with Gafchromic EBT3 film, evaluated using FilmQA Pro software (Ashland, USA).Results:Volume corrections were 1.7% at maximum. After correction, in small open fields, unshielded diodes showed a lower response than the diamond, i.e. diamond detector over‐response seems to be higher than that for unshielded diodes. Beneath the block, this behavior was amplified by a factor of 2. For the shielded diodes, the overresponse for small open fields could be confirmed. However their lateral response behavior was strongly type dependent, e.g. the signal ratio dropped from 1.02 to 0.98 for the P‐60008 diode.Conclusion:The lateral detector response was experimentally examined. Detector volume and density alone do not fully account for the field size dependence of detector response. Detector construction details play a major role, especially for shielded diodes.

SU-G-BRC-09: Experimental Validation of a VMAT Planning Technique for Patients with Hip Prosthesis

Purpose: To validate and evaluate the dosimetric accuracy of two different VMAT planning techniques for patients with hip prosthesis of unknown composition. Methods: A phantom was designed to model a patient with one stainless steel hip prosthesis. PTVs and critical structures contours were copied from a prostate cancer patient. Simultaneous integrated boost VMAT plans covering the prostate and pelvic lymph nodes (50.4Gy and 70Gy in 28 fractions) and sparing the rectum and bladder were generated in Eclipse using AAA dose calculation. Our clinical dose constraints were used for PTV coverage and rectum and bladder sparing. Three VMAT plans were generated. First an unconstrained plan (UN) was created. Two other plans reducing the entrance dose through the prosthesis were developed: a plan using avoidance sector (AV) and a plan with maximum dose constraint on the hip prosthesis (MP). The three plans were measured with gafchromic EBT3 film and analyzed with FilmQAPro software. Results: The three VMAT plans resulted in an average gamma 2%/2mm of 48.8%, 95.4%, 98.4% (UN, AV and MP plan respectively) and an average gamma 3%/3mm of 76.3%, 99.7%, 99.9% (UN, AV and MP plan respectively). The avoidance sector plan exhibited higher measured doses of up to 3% of the prescription dose at the prosthesis interface. The plan with constraints on the prosthesis showed excellent agreement at the prosthesis interface. Conclusion: For the phantom presented in this study, VMAT plans with a maximum dose constraint on the prosthesis resulted in best agreement with measured doses.

SU‐E‐T‐464: Implementation and Validation of 4D Acuros XB Dose Calculations

Purpose:In this abstract we implement and validate a 4D VMAT Acuros XB dose calculation using Gafchromic film. Special attention is paid to the physical material assignment in the CT dataset and to reported dose to water and dose to medium.Methods:A QUASAR phantom with a 3 cm sinusoidal tumor motion and 5 second period was scanned using 4D computed tomography. A CT was also obtained of the static QUASAR phantom with the tumor at the central position. A VMAT plan was created on the average CT dataset and was delivered on a Varian TrueBeam linear accelerator. The trajectory log file from this treatment was acquired and used to create 10 VMAT subplans (one for each portion of the breathing cycle). Motion for each subplan was simulated by moving the beam isocentre in the superior/inferior direction in the Treatment Planning System on the static CT scan. The 10 plans were calculated (both dose to medium and dose to water) and summed for 1) the original HU values from the static CT scan and 2) the correct physical material assignment in the CT dataset. To acquire a breathing phase synchronized film measurements the trajectory log was used to create a VMAT delivery plan which includes dynamic couch motion using the Developer Mode. Three different treatment start phases were investigated (mid inhalation, full inhalation and full exhalation).Results:For each scenario the coronal dose distributions were measured using Gafchromic film and compared to the corresponding calculation with Film QA Pro Software using a Gamma test with a 3%/3mm distance to agreement criteria. Good agreement was found between calculation and measurement. No statistically significant difference in agreement was found between calculations to original HU values vs calculations to over‐written (material‐assigned) HU values.Conclusion:The investigated 4D dose calculation method agrees well with measurement.

SU‐E‐T‐815: In Vivo Dosimetry and Daily Arc Junction Verification Using EBT3 Gafchromic Film for Pediatric Cranio‐Spinal Irradiation

Purpose:Perform dosimetric comparison between planned and delivered dose in the junction area, measure daily dose variation in the arc junction area for pediatric patients treated for medulloblastoma using Craniospinal axis irradiation(CSI)Material and methodsDose comparison in the junction area, daily dose variation in the arc junction area for a Rando Phantom and 5 pediatric patients treated using CSI technique were analyzed. Plans were created using the Eclipse treatment planning system. Two arcs for cranium and 1 arc for spine region were used. Planar dose matrix was created by projecting phantom and patient plan into the ArcCheck phantom. EBT3 film was placed in the middle of ArcCheck plug to measure dose distribution in the junction areaDuring patient treatment, strip of EBT3 film was placed daily at each junction area for verification. EBT3 films were scanned using a flatbed scanner, Epson Expression 10000 XL. Film QA pro software was used to analyze film. Scanning and analysis was performed according to vendor recommendations and AAPM TG‐55 report. Films were scanned and analyzed daily after each treatment and at the end of treatment course. Planar dose distributions from films were compared with planar dose distribution from treatment planning system.Results:Comparison of planned vs. measured dose distributions for patients have passing rates of 90%–100% with 3% and 3 mm gamma analysis. In some of the treatment fractions, daily setup film showed variation in dose distribution in the junction area.Conclusion:It is critical to measure dose distribution in the arc junction area and use additional quality assurance measures to verify daily setup for CSI patient where one or more junctions are present. EBT3 film prove to be a good tool to achieve this task considering flexibility associated with the film such as symmetry, self‐developing and ease of use

SU‐E‐T‐498: Initial Characterization of the New EBT XD Gafchromic Film

PurposeTo assess the dosimetric accuracy of the new EBT XD (eXtended Dose) Gafchromic film and to compare the lateral scan effect between EBT XD and EBT3 film.Methods:All films were exposed in solid water using a 6 MV beam and were analyzed with FilmQA Pro software and an Epson 10000 XL scanner. Calibration was performed using films exposed to 8 known doses from 0–3000 cGy. To assess dosimetric accuracy, a 15×18 cm2 60° EDW delivered a dose range of 200–2900 cGy to 4 separate EBT XD films. Comparison to treatment planning system (TPS) predictions was made using a gamma analysis with 2%/2mm passing criteria. To assess the lateral scan effect, 21 × 21 cm2 open fields delivered doses of 1000, 2000, and 3000 cGy isocentrically to both EBT3 and EBT XD films at 10 cm depth. Films were centered and scanned such that the widest parts of the exposed film extended lateral to the center of the scanner bed in order to maximize the lateral scan effect. Ratios of measured to TPS predicted doses for EBT3 and EBT XD film were calculated at 5 cm and 8 cm lateral from the center of the scanner bed to quantitatively assess the lateral scan effect.ResultsThe average gamma passing rate for the EDW was 98.9% (range 98.7 – 99.1%). EBT XD film exhibited minimal lateral scan effect (&lt;1%) up to 30 Gy. In contrast, EBT3 showed an under‐response of 13% and 25% at lateral positions of 5 cm and 8 cm respectively for the 30 Gy exposure.ConclusionEBT XD can accurately measure dose over a wide dynamic range. The non‐polarizing structure of its active ingredient essentially eliminates the lateral scan effect up to at least 30 Gy. EBT XD film is therefore well suited for SRS/SBRT verification measurements.

EP-1419: Portal dosimetry verification of stereotactic intensity modulated fields

Images from Vidar scans were directly analyzed on a Tomotherapy DQA station. Images from the EPSON scanner were acquired and processed using FilmQAPro software with and without corrections for the characteristic response nonuniformity normal to the scan direction and transferred back to Tomotherapy DQA station for plan-film analysis. Gamma factor maps for five treatment plans (brain, abdomen, vulva and two prostates) were visually compared for EDR2 and EBT3 films scanned on Vidar as well as EBT3 films scanned on EPSON using the FilmQAPro software with and without the nonuniformity corrections 1. C. Fiandra, et al., Med. Phys. 33 4314-9

EP-1420: Comparison of AAA and collapsed cone algorithms for treatment planning of lung SABR

Images from Vidar scans were directly analyzed on a Tomotherapy DQA station. Images from the EPSON scanner were acquired and processed using FilmQAPro software with and without corrections for the characteristic response nonuniformity normal to the scan direction and transferred back to Tomotherapy DQA station for plan-film analysis. Gamma factor maps for five treatment plans (brain, abdomen, vulva and two prostates) were visually compared for EDR2 and EBT3 films scanned on Vidar as well as EBT3 films scanned on EPSON using the FilmQAPro software with and without the nonuniformity corrections 1. C. Fiandra, et al., Med. Phys. 33 4314-9

Gafchromic film dosimetry: Four years experience using FilmQA Pro software and Epson flatbed scanners

Purposes: To assess performance of FilmQA Pro software for pre-treatment patient-specific quality assurance (QA), using radiochromic films and two commercial flatbed scanners. To evaluate a novel multichannel approach compared to the classical red channel evaluation. Material and methodsPatient films (mostly EBT2 films, one box of EBT3) were digitalized using successively two flatbed scanners: the A4-size Epson V750 and the A3-size Epson 10000XL. Prior to patient dose verification, basic characteristics of films and scanners were investigated. Patient films were analyzed using FilmQA Pro software, which enables to use the signal from all three colour channels (Red, Green, Blue). ResultsCompared to the red channel evaluation, multichannel evaluation presents better passing rates with regard to local gamma index. As expected, we obtained better results using A3-size scanner compared to A4-size scanner, especially when considering large region of interest. An observation of great interest was made for both scanners: after intensive use, a tilting in the blue transmittance profiles appeared in the lamp direction, making multichannel analysis unsuitable for accurate dose evaluation.

Radiochromic film calibration for low-energy seed brachytherapy dose measurement.

Radiochromic film dosimetry is typically performed for high energy photons and moderate doses characterizing external beam radiotherapy (XRT). The purpose of this study was to investigate the accuracy of previously established film calibration procedures used in XRT when applied to low-energy, seed-based brachytherapy at higher doses, and to determine necessary modifications to achieve similar accuracy in absolute dose measurements. Gafchromic EBT3 film was used to measure radiation doses upwards of 35 Gy from 75 kVp, 200 kVp, 6 MV, and (∼28 keV) I-125 photon sources. For the latter irradiations a custom phantom was built to hold a single I-125 seed. Film pieces were scanned with an Epson 10000XL flatbed scanner and the resulting 48-bit RGB TIFF images were analyzed using both FilmQA Pro software andMATLAB. Calibration curves relating dose and optical density via a rational functional form for all three color channels at each irradiation energy were determined with and without the inclusion of uncertainties in the measured optical densities and dose values. The accuracy of calibration curve variations obtained using piecewise fitting, a reduced film measurement area for I-125 irradiation, and a reduced number of dose levels was also investigated. The energy dependence of the film lot used was also analyzed by calculating normalized optical density values. Slight differences were found in the resulting calibration curves for the various fitting methods used. The accuracy of the calibration curves was found to improve at low doses and worsen at high doses when including uncertainties in optical densities and doses, which may better represent the variability that could be seen in film optical density measurements. When exposing the films to doses > 8 Gy, two-segment piecewise fitting was found to be necessary to achieve similar accuracies in absolute dose measurements as when using smaller dose ranges. When reducing the film measurement area for the I-125 irradiations, the accuracy of the calibration curve was degraded due to the presence of localized film heterogeneities. No degradation in the calibration curves was found when reducing the number of calibration points down to only 4, but with piecewise fitting, 6 calibration points as well as a blank film are required. Variations due to photon energy in film optical density of up to 3% were found above doses of 2 Gy. A modified procedure for performing EBT3 film calibration was established for use with low-energy brachytherapy seeds and high dose exposures. The energy dependence between 6 MV and I-125 photons is significant such that film calibrations should be done with an appropriately low-energy source when performing low-energy brachytherapy dose measurements. Two-segment piecewise fitting with the inclusion of errors in measured optical density as well as dose was found to result in the most accurate calibration curves. Above doses of 1 Gy, absolute dose measurements can be made with an accuracy of 1.6% for 6 MV beams and 5.7% for I-125 seed exposures if using the I-125 source for calibration, or 2.3% if using the 75 kVp photon beam for calibration.

SU‐E‐T‐269: Quality Assurance of Spine Volumetric Modulated Arc Therapy with Flattening Filter Free Beams Using Gafchromic EBT3 Films

Purpose:We implemented the Gafchromic film‐based patient specific QA of volumetric modulated arc therapy (VMAT) with flattening‐filter free (FFF) beams for spine metastases and validated the accuracy of fast arc delivery.Methods:EBT3 films and a homemade cylindrical QA phantom were employed for dosimetric verification of VMATs. For 14 FFF VMAT plans (10 with 10‐MV FFF beams and 4 with 6‐MV FFF beams), the doses were recalculated on the phantom and delivered by a TrueBeam STx accelerator equipped with a high‐definition 120 leaf MLC. The EBT3 films were scanned using an Epson 10000XL scanner through the FilmQA Pro software. All the irradiated film images were converted to dose map using a calibration response curve. The resulting dose map of film measurement was compared with treatment plan and evaluated using gamma analysis with dose tolerance of 2% within 2 mm. In addition, the point‐dose measurement in the phantom using an ion chamber was evaluated as a reference in a ratio of measured and planned doses.Results:The gamma pass rates averaged over all FFF plans for composite‐field measurements were 96.0 ± 3.6% (88.9%–99.5%). When adopting a tolerance level of 3% – 3 mm, the gamma pass rates were improved with the ranges from 98% to 100%. In addition, dose profiles and dose distributions showed that spinal cord was protected by the rapid dose fall‐off and by delivering the treatment with high precision. In point‐dose measurements, the average differences between the measured and planned doses were 0.5% ± 1.0% of the prescription dose.Conclusion:We demonstrated that Gafchromic EBT3 film would be an effective patient‐specific QA tool, especially for VMAT of spine SBRT with treatment of small fields and highly gradient dose distributions. The results of film QA verified that the dosimetric accuracy of spine SBRT utilizing RapidArc with FFF beams in our institution is reliable.

A dosimetric evaluation of the Eclipse AAA algorithm and Millennium 120 MLC for cranial intensity-modulated radiosurgery

The aim of this study is to assess the accuracy of a convolution-based algorithm (anisotropic analytical algorithm [AAA]) implemented in the Eclipse planning system for intensity-modulated radiosurgery (IMRS) planning of small cranial targets by using a 5-mm leaf-width multileaf collimator (MLC). Overall, 24 patient-based IMRS plans for cranial lesions of variable size (0.3 to 15.1cc) were planned (Eclipse, AAA, version 10.0.28) using fixed field-based IMRS produced by a Varian linear accelerator equipped with a 120 MLC (5-mm width on central leaves). Plan accuracy was evaluated according to phantom-based measurements performed with radiochromic film (EBT2, ISP, Wayne, NJ). Film 2D dose distributions were performed with the FilmQA Pro software (version 2011, Ashland, OH) by using the triple-channel dosimetry method. Comparison between computed and measured 2D dose distributions was performed using the gamma method (3%/1mm). Performance of the MLC was checked by inspection of the DynaLog files created by the linear accelerator during the delivery of each dynamic field. The absolute difference between the calculated and measured isocenter doses for all the IMRS plans was 2.5% ± 2.1%. The gamma evaluation method resulted in high average passing rates of 98.9% ± 1.4% (red channel) and 98.9% ± 1.5% (blue and green channels). DynaLog file analysis revealed a maximum root mean square error of 0.46mm. According to our results, we conclude that the Eclipse/AAA algorithm provides accurate cranial IMRS dose distributions that may be accurately delivered by a Varian linac equipped with a Millennium 120 MLC.

Semi-3D dosimetry of high dose rate brachytherapy using a novel Gafchromic EBT3 film-array water phantom

There is a need to modernise clinical brachytherapy dosimetry measurement beyond traditional point dose verification to enable appropriate quality control within 3D treatment environments. This is to keep pace with the 3D clinical and planning approaches which often include significant patient-specific optimisation away from 'standard loading patterns'. A multi-dimension measurement system is required to provide assurance of the complex 3D dose distributions, to verify equipment performance, and to enable quality audits. However, true 3D dose measurements around brachytherapy applicators are often impractical due to their complex shapes and the requirement for close measurement distances. A solution utilising an array of radiochromic film (Gafchromic EBT3) positioned within a water filled phantom is presented. A calibration function for the film has been determined over 0 to 90Gy dose range using three colour channel analysis (FilmQAPro software). Film measurements of the radial dose from a single HDR source agree with TPS and Monte Carlo calculations within 5 % up to 50 mm from the source. Film array measurements of the dose distribution around a cervix applicator agree with TPS calculations generally within 4 mm distance to agreement. The feasibility of film array measurements for semi-3D dosimetry in clinical HDR applications is demonstrated.