A gate evaluation of the sources of error in quantitative90 Y PET.

PURPOSE: Accurate reconstruction of the dose delivered by 90 Y microspheres using a postembolization PET scan would permit the establishment of more accurate dose-response relationships for treatment of hepatocellular carcinoma with 90 Y. However, the quality of the PET data obtained is compromised by several factors, including poor count statistics and a very high random fraction. This work uses Monte Carlo simulations to investigate what impact factors other than low count statistics have on the quantification of90 Y PET. METHODS: PET acquisitions of two phantoms-a NEMA PET phantom and the NEMA IEC PET body phantom-containing either 90 Y or 18 F were simulated using gate. Simulated projections were created with subsets of the simulation data allowing the contributions of random, scatter, and LSO background to be independently evaluated. The simulated projections were reconstructed using the commercial software for the simulated scanner, and the quantitative accuracy of the reconstruction and the contrast recovery of the reconstructed images were evaluated. RESULTS: The quantitative accuracy of the 90 Y reconstructions were not strongly influenced by the high random fraction present in the projection data, and the activity concentration was recovered to within 5% of the known value. The contrast recovery measured for simulated 90 Y data was slightly poorer than that for simulated 18 F data with similar count statistics. However, the degradation was not strongly linked to any particular factor. Using a more restricted energy range to reduce the random fraction in the projections had no significant effect. CONCLUSIONS: Simulations of 90 Y PET confirm that quantitative 90 Y is achievable with the same approach as that used for 18 F, and that there is likely very little margin for improvement by attempting to model aspects unique to 90 Y, such as the much higher random fraction or the presence of bremsstrahlung in the singles data.

The evaluation of a 2D diode array in "magic phantom" for use in high dose rate brachytherapy pretreatment quality assurance.

PURPOSE: High dose rate (HDR) brachytherapy is a treatment method that is used increasingly worldwide. The development of a sound quality assurance program for the verification of treatment deliveries can be challenging due to the high source activity utilized and the need for precise measurements of dwell positions and times. This paper describes the application of a novel phantom, based on a 2D 11 × 11 diode array detection system, named "magic phantom" (MPh), to accurately measure plan dwell positions and times, compare them directly to the treatment plan, determine errors in treatment delivery, and calculate absorbed dose. METHODS: The magic phantom system was CT scanned and a 20 catheter plan was generated to simulate a nonspecific treatment scenario. This plan was delivered to the MPh and, using a custom developed software suite, the dwell positions and times were measured and compared to the plan. The original plan was also modified, with changes not disclosed to the primary authors, and measured again using the device and software to determine the modifications. A new metric, the "position-time gamma index," was developed to quantify the quality of a treatment delivery when compared to the treatment plan. The MPh was evaluated to determine the minimum measurable dwell time and step size. The incorporation of the TG-43U1 formalism directly into the software allows for dose calculations to be made based on the measured plan. The estimated dose distributions calculated by the software were compared to the treatment plan and to calibrated EBT3 film, using the 2D gamma analysis method. RESULTS: For the original plan, the magic phantom system was capable of measuring all dwell points and dwell times and the majority were found to be within 0.93 mm and 0.25 s, respectively, from the plan. By measuring the altered plan and comparing it to the unmodified treatment plan, the use of the position-time gamma index showed that all modifications made could be readily detected. The MPh was able to measure dwell times down to 0.067 ± 0.001 s and planned dwell positions separated by 1 mm. The dose calculation carried out by the MPh software was found to be in agreement with values calculated by the treatment planning system within 0.75%. Using the 2D gamma index, the dose map of the MPh plane and measured EBT3 were found to have a pass rate of over 95% when compared to the original plan. CONCLUSIONS: The application of this magic phantom quality assurance system to HDR brachytherapy has demonstrated promising ability to perform the verification of treatment plans, based upon the measured dwell positions and times. The introduction of the quantitative position-time gamma index allows for direct comparison of measured parameters against the plan and could be used prior to patient treatment to ensure accurate delivery.

Poster - Thur Eve - 01: Development of simple and fast EBT2 film calibration procedure using PDD table.

Standard calibration procedure for EBT films is laborious and time-consuming. The objective of this work was to develop a simple and fast approach of EBT2 film calibration using PDD tables. EBT2 sheet is cut into 3 stripes of 5×25.5cm2 . The strips were exposed to dose of 600, 200 and 70cGy at dmax each while placed horizontally in the middle of a 30×30×30cm3 solid water phantom. Varian 21EX 6MV 10×10cm2 beam was used with the gantry rotated to 90° and SSD of 100cm to the phantom surface. After at least 24 hours, the films were digitized with flatbed scanner (Epson10000XL), according to a modified ISP scanning protocol. All images were analysed using an in-house Matlab code and ImageJ software. The net-optical densities against depths in the solid phantom were calibrated using PDD tables measured with ionization chamber for same machine. For verification, another calibration curve was generated for the same film batch following the same calibration protocol. Seven pieces of films were exposed to known doses and these doses were reconstructed using two derived calibration curves. The proposed approach was 3.6 times faster than the standard considering the number of films used in each methods, 3 stripes compared to11 pieces. The mean relative dose difference calculated for these films using the PDD calibration and the standard methods was 1.0±1.2% and 0.5±2.2% with maximum relative differences of 3.0% and 4.7% respectively. Our results show that PDD calibration approach is much easier, faster and predicts dose more reproducibly and accurately than the standard approach.

Sci-Fri PM: Delivery - 12: Scatter-B-Gon: Implementing a fast Monte Carlo cone-beam computed tomography scatter correction on real data.

A fast and accurate MC-based scatter correction algorithm was implemented on real cone-beam computed tomography (CBCT) data. An ACR CT accreditation phantom was imaged on a Varian OBI CBCT scanner using the standard-dose head protocol (100 kVp, 151 mAs, partial-angle). A fast Monte Carlo simulation developed in the EGSnrc framework was used to transport photons through the uncorrected CBCT scan. From the simulation output, the contribution from both primary and scattered photons for each projection image was estimated. Using these estimates, a subtractive scatter correction was performed on the CBCT projection data. Implementation of the scatter correction algorithm on real CBCT data was shown to help mitigate scatter-induced artifacts, such as cupping and streaking. The scatter corrected images were also shown to have improved accuracy in reconstructed attenuation coefficient values. In three regions of interest centered on material inserts in the ACR phantom, the reconstructed CT numbers agreed with clinical CT scan data to within 35 Hounsfield units after scatter correction. These results suggest that the proposed scatter correction algorithm is successful in improving image quality in real CBCT images. The accuracy of the attenuation coefficients extracted from the corrected CBCT scan renders the data suitable for adaptive on the fly dose calculations on individual fractions, as well as vastly improved image registration.

Sci-Fri AM: Imaging - 08: Comparison of single and dual layer detector blocks for pre-clinical MRI-PET.

Our group is developing a small animal PET scanner which would fit in a 7 Tesla MRI scanner to provide simultaneous PET and MR acquisitions and images. There is very little room for the PET detectors and we must use nonmagnetic materials. This presentation describes preliminary work with two PET detectors consisting of LYSO blocks each with two crystal layers: 49 1.67*1.67*6.0 mm on the bottom layer and 36 4.0 mm deep crystals on the top layer. These are mounted on SensL 4*4 arrays of silicon photo-multipliers whose outputs are multiplexed to provide three signals: Energy, X, Y. These detectors were mounted on translation stages and scanned past a 0.25 mm diameter 370 MBq Na-18 source embedded in tissue equivalent plastic. The results were compared with similar single layer crystal blocks with 10 mm deep crystals to assess the advantage of dual layer crystals to reduce radial blurring in a PET scanner with a diameter of only 64 mm. The ability to identify correctly each crystal is reported as its resolvability index, (RI), defined as the (crystal response FWHM)/(crystal's separation) in the crystal identification matrix. For the dual layer block RI =0.44, and for dual layer block RI=0.22. The coincidence response resolution for the single layer block varied from 1.23±0.05 mm at the centre of the scanner to 3.09±0.10 mm at 15.8 mm radius, while the dual layer block varied from 1.31±0.06 to 1.96±0.51 mm over the same range, confirming the Monte-Carlo simulations showing reduced radial blurring.

Poster - Thur Eve - 18: Characterization of a camera and LED lightbox imaging system for radiochromic film dosimetry.

Radiation therapy treatment modalities continue to develop and have become increasingly complex. With this, dose verification and quality assurance (QA) is of great importance to ensure that a prescribed dose is accurately and precisely delivered to a patient. Radiochromic film dosimetry has been adopted as a convenient option for QA, because it is relatively energy independent, is near tissue equivalent, and has high spatial resolution. Unfortunately, it is not always easy to use. In this study, preliminary work towards developing a novel method of imaging radiochromic film is presented. The setup consists of a camera mounted vertically above a lightbox containing red LEDs, interfaced with computer image acquisition software. Imaging results from this system will be compared with imaging performed using an Epson Expression 10000XL scanner (a device in common clinical use). The lightbox imaging technique with camera readout is much faster relative to a flatbed scanner. The film measurements made using the camera are independent of film orientation, and show reduced artifacts, so that there are fewer corrections required compared to the use of flatbed scanners. Optical scatter also appears to be less of an issue with this design than with the flat bed scanner. While further work needs to be done to optimize the lightbox imaging system, the lightbox system shows great promise for a rapid, simple, and orientation independent setup, improving on existing film scanning systems.

SU-E-T-107: Uncertainty Estimate of a Practical EBT-2 Film Dosimetry Approach: Transpose-And-Scan Technique.

PURPOSE: To estimate the uncertainty of a practical EBT2 film dosimetry approach that has been established at our institution and used for routine patient-specific plan verifications, particularly for SBRT and RapidArc, as well as planning system commissioning. Our technique is unique from other common dosimetry protocols with respect to calibration, irradiation and scanning. METHODS: Film dosimetry for patient-specific quality assurance of 29 patient plans were retrospectively reviewed. For each case, four films were irradiated; two for calibration and two for treatment plan. Each pair of two films were irradiated together in a phantom with one film transposed (rotated 180 degrees relative to the other) to compensate for asymmetric film response. After a minimum of 12 hrs post-irradiation, each film was scanned in four different orientations to mitigate non-uniform response of the scanner light and detector elements. The scanned 8 calibration and 8 plan images were averaged into one calibration and one plan film image, respectively. Each color channel of the calibration film was correlated to the reference dose matrix to produce a 3rd order polynomial calibration curve. Finally, each color channel of the plan film was converted to a dose map using the corresponding calibration curve. Average dose maps of the red and green channels were correlated to the treatment planning dose matrix, and the mean dose differences at the center of dose distributions (5×5mm̂2 area) as well as a gamma analysis were evaluated. RESULTS: The absolute dose differences were -0.8±1.7% (range=-4.5-3.0%). The gamma pass-rates (3%/3mm) were 94±7% (min.=74%). The pass rate increased to 99±3%(min.=87%) with the film scaled relatively to the plan doses. CONCLUSIONS: Based on a large number of cases, our approach appears to be robust to non-uniform film and scanner responses, and is shown to have an uncertainty (1SD) of less than 2% for absolute film dosimetry.

SU-E-T-118: Characterization of EBT3 Films in Photon and Proton Beams.

PURPOSE: To measure the calibration curves of EBT3 dosimetry films in photon and proton beams and to quantify the related uncertainties from one beam type to another. METHODS: EBT3 Gafchromic films have similar properties than EBT2 with a symmetric construction and a matte polyester substrate to prevent Newton's ring artefacts. Films from a same batch were exposed in three different beam qualities, an Elekta SL25 6 MV photon beam, a 100 MeV 5×5cm2 proton beam delivered by pencil-beam scanning dedicated system from IBA and a 60 MeV fixed proton beam (2.5cm in diameter) at Clatterbridge Center for Oncology (CCO), UK. The films were read using an EPSON 10000 XL/PRO scanner. Film calibration curves were acquired for all modalities within a range of 0.05 to 20 Gy. Influence of increasing linear-energy transfer (LET) on film response was investigated by comparing dose measured by EBT3 to a silicon diode detector in depth for a fully-modulated beam using the CCO beam line (homogeneous dose with distal end at 3.1cm in water). A comprehensive uncertainty budget (reproducibility, uniformity'¦) was estimated on films irradiated by Elekta SL25. RESULTS: The main source of uncertainty was the non-uniformity of the scanner response. By placing all the irradiated films at the center of the scanner, the uncertainty could be reduced from 5.8% to 1.9% (1 sigma). For all beams and energies, the calibration curves were matched within uncertainties. Along the fully-modulated depth dose curve, diode and EBT3 measurement were in a 4% agreement point-to-point, indicating films weak dependence with LET. CONCLUSIONS: The weak influence of LET, beam type and energy on film response as well as its small uncertainty make EBT3 suitable for relative dosimetry and a promising candidate for measuring correction factors (quality, recombination,'¦) for reference dosimetry with ion chambers of non-standard beams (e.g pencil-beam scanning proton-therapy). â€Å“This work is supported by the Walloon Region under the project name InVivoIGT, convention number 1017266.â€.

SU-E-T-96: Energy Dependence of the New GafChromic- EBT3 Film's Dose Response-Curve.

PURPOSE: To study and compare the dose response curves of the new GafChromic EBT3 film for megavoltage and kilovoltage x-ray beams, with different spatial resolution. METHODS: Two sets of EBT3 films (lot#A101711-02) were exposed to each x-ray beam (6MV, 15MV and 50kV) at 8 dose values (50-3200cGy). The megavoltage beams were calibrated per AAPM TG-51 protocol while the kilovoltage beam was calibrated following the TG-61 using an ionization chamber calibrated at NIST. Each film piece was scanned three consecutive times in the center of Epson 10000XL flatbed scanner in transmission mode, landscape orientation, 48-bit color at two separate spatial resolutions of 75 and 300 dpi. The data were analyzed using ImageJ and, for each scanned image, a region of interest (ROI) of 2×2cm2 at the field center was selected to obtain the mean pixel value with its standard deviation in the ROI. For each energy, dose value and spatial resolution, the average netOD and its associated uncertainty were determined. The Student's t-test was performed to evaluate the statistical differences between the netOD/dose values of the three energy modalities, with different color channels and spatial resolutions. RESULTS: The dose response curves for the three energy modalities were compared in three color channels with 75 and 300dpi. Weak energy dependence was found. For doses above 100cGy, no statistical differences were observed between 6 and 15MV beams, regardless of spatial resolution. However, statistical differences were observed between 50kV and the megavoltage beams. The degree of energy dependence (from MV to 50kV) was found to be function of color channel, dose level and spatial resolution. CONCLUSIONS: The dose response curves for GafChromic EBT3 films were found to be weakly dependent on the energy of the photon beams from 6MV to 50kV. The degree of energy dependence varies with color channel, dose and spatial resolution. GafChromic EBT3 films were supplied by Ashland Corp. This work was partially supported by DGAPA-UNAM grant IN102610 and Conacyt Mexico grant 127409.

SU-E-T-106: Evaluation of Sensitivity and Uniformity of New Radiochromic Film with Two Commercial Scanners.

PURPOSE: A newly introduced radiochromic film, the GAFCHROMIC EBT3, has been expected as much useful device for the IMRT dosimetry. The purpose of this study was to investigate the sensitivity and the uniformity of the films between an Epson ES-10000G flatbed scanner and a Vidar DosimetryPRO Advantage (Red) scanner. METHODS: Doses ranging from 1 cGy to 1600 cGy with 15-MV photon beam was irradiated to the film in a solid water phantom, respectively. All of the films were then digitized after irradiation using both two scanners. Sensitivities, local fluctuations of the film with two scanners were evaluated. Local fluctuations were defined as the relative (percent) standard deviation of the film response in ROIs (3 cmx3 cm). RESULTS: As to the Vidar scanner, the sensitivity of the film was higher for low dose range (below <400 cGy). While, as to the Epson scanner, the sensitivity using the red color channel was higher than others for low dose range. At high dose range (above >400 cGy), the green color channel had higher sensitivity than others. The Vidar scanner exhibited the lower local fluctuations than the Epson scanner for all dose ranges. For the Epson scanner, the red color channel had the lower local fluctuations than the green and blue color channel for all dose ranges. CONCLUSIONS: This study shows the characteristics of the new EBT3 films, in conjunction with the Epson ES-10000G flatbed scanner and the Vidar DosimetryPRO Advantage (Red) scanner.

SU-E-T-135: Investigation of Commercial-Grade Flatbed Scanners and a Medical- Grade Scanner for Radiochromic EBT Film Dosimetry.

PURPOSE: To evaluate the characteristics of commercial-grade flatbed scanners and medical-grade scanners for radiochromic EBT film dosimetry. METHODS: Performance aspects of a Vidar Dosimetry Pro Advantage (Red), Epson 750 Pro, Microtek ArtixScan 1800f, and Microtek ScanMaker 8700 scanner for EBT2 Gafchromic film were evaluated in the categories of repeatability, maximum distinguishable optical density (OD) differentiation, OD variance, and dose curve characteristics. OD step film by Stouffer Industries containing 31 steps ranging from 0.05 to 3.62 OD was used. EBT films were irradiated with dose ranging from 20 to 600 cGy in 6×6 cm2 field sizes and analyzed 24 hours later using RIT113 and Tomotherapy Film Analyzer software. Scans were performed in transmissive mode, landscape orientation, 16-bit image. The mean and standard deviation Analog to Digital (A/D) scanner value was measured by selecting a 3×3 mm2 uniform area in the central region of each OD step from a total of 20 scans performed over several weeks. Repeatability was determined from the variance of OD step 0.38. Maximum distinguishable OD was defined as the last OD step whose range of A/D values does not overlap with its neighboring step. RESULTS: Repeatability uncertainty ranged from 0.1% for Vidar to 4% for Epson. Average standard deviation of OD steps ranged from 0.21% for Vidar to 6.4% for ArtixScan 1800f. Maximum distinguishable optical density ranged from 3.38 for Vidar to 1.32 for ScanMaker 8700. A/D range of each OD step corresponds to a dose range. Dose ranges of OD steps varied from 1% for Vidar to 20% for ScanMaker 8700. CONCLUSIONS: The Vidar exhibited a dose curve that utilized a broader range of OD values than the other scanners. Vidar exhibited higher maximum distinguishable OD, smaller variance in repeatability, smaller A/D value deviation per OD step, and a shallower dose curve with respect to OD.

SU-E-T-117: A Point Densitometer Scanner for EBT2 Film Dosimetry.

PURPOSE: To create and characterize a device that can accurately and reproducibly digitize Gafchromic(R) EBT2 film for dosimetry. METHODS: We have constructed a point densitometer scanner that allows the acquisition of 1 dimensional optical density profiles of EBT2 dosimetry film at submillimeter resolution. An LED with peak output at 636nm collimated and focused with an aspheric lens was used as a light source. A large area photodiode was used as a detector. A custom transimpedence amplifier was developed to optimize the detector signal response for optical densities corresponding to a clinically relevant dose range (0 - 3Gy). A custom motorized translational stage was developed allowing 1 dimensional scans and manual fine adjustment in the lateral direction. The entire scanner was contained in a light tight enclosure to eliminate the effects of ambient light. Dose profiles were generated from EBT2 films exposed uniformly to varying doses using a superficial unit with the films placed on top of a solid water phantom. Dose profiles were generated from the same films using a flatbed scanner (Epson 10000XL) implementing two and three optical channel corrections for film inhomogeneity and the results were compared. RESULTS: Initial analysis of the point densitometer shows a beam FWHM of 0.7mm and position error of the translational stage at 0.02mm in the scan direction and 0.2mm in the lateral direction. Reproducibility of the scans has been shown to within 1%. Orientation dependence and measured dose homogeneity have been improved over corrected flatbed digitization techniques. CONCLUSIONS: Preliminary characterization of the point densitometer shows its potential as an improved quality assurance tool for EBT2 film dosimetry.

SU-E-T-128: Dosimetric Characteristics of Gafchromic EBT3 Films for Megavoltage Photon and Proton Beams.

PURPOSE: Gafchromic film for quantitative analysis was renewed from EBT2 to EBT3 film in November 2011. The purpose of this study is to investigate the relevant characteristics of EBT3 film for its application in dosimetric verification for IMRT/VMAT or proton therapy. METHOD: We investigated the characteristics of EBT3 film with comparison of previous EBT2 film. The experiments in this study composed two categories. At first, the photo spectroscopy for the irradiated film was compared between EBT2 and EBT3. The film 1 day after the irradiation was analyzed by a photo spectrometer (SR520: JASCO Corporation, Japan). Secondly, we investigated several calibration curves which obtained by same batch. The films were calibrated by irradiation the films to 13 dose steps. The irradiated films were scanned by a flatbed scanner (ES-10000XL, Epson-Seiko Corporation, Japan). The difference on scan orientation was evaluated alternate portrait and landscape directions. The photon and proton beams were delivered from Clinac 21EX (Varian) and Mitsubishi machine, respectively. RESULTS: The peak absorption wavelength of EBT3 film and its response at all active range were basically same with that of EBT2 film. The peak wavelength of photo absorption in EBT3 was observed at 585 and 634 nm. The fog optical density was increased due to the hazy matte polyester for active layer. However, there is no change the tendency of the calibration curve responding to megavoltage photon and proton beams. The scan orientation dependency of EBT3 film was observed with similar to EBT2 film. The optical density of portrait orientation was 10% higher than that of landscape orientation. CONCLUSION: The dosimetric characteristics of EBT3 film were basically same with EBT2 film. With regard to the matte polyester, the creation of Newton's rings during scanning procedure was reduced. However, the suitable scan protocol should be used for accurate film dosimetry.

SU-E-T-142: Evaluation of the Energy Dependence of Gafchromic EBT3 Film for Electron Beams.

PURPOSE: To evaluate the energy dependence of Gafchromic EBT3 film for establishing a quality assurance method of bolus electron conformal radiotherapy. METHODS: We irradiated electron beam to EBT3 films, which were set in the water tank. The linear accelerator used was Varian Clinac 21EX. The energy of electron beams were 9 and 12 MeV. The irradiated field size was 10×10 cm2 and the source to surface distance was 100 cm. The depths of measurement were 22 (depth of dose maximum; dmax), 31, and 37 mm for 9 MeV and 28 (dmax), 43, and 50 mm for 12 MeV. The irradiated doses were 25, 50, 75, 100, 150, 200, and 300 cGy. EBT3 films were readout with a flat-bed scanner 48 hours after irradiation, and the optical density (OD) curve was obtained for each beam energy and depth. The OD curves were approximated by a third-order polynomial. The doses were evaluated at netOD 0.1 and 0.3 from the approximated curves. RESULTS: The differences of the evaluated doses from those for 9 MeV at 22 mm depth were from 2 to 14 % for netOD=0.1, and from 1 to 13 % for netOD=0.3, respectively. The netOD curves of dmax for both energies showed good agreement, while large discrepancy was found in the deeper depths. CONCLUSIONS: The dependence of dose response of EBT3 film on electron beam energy was small at dmax, while it increased at deeper depth in the present study. It can be considered that the discrepancy was caused by setup error because dose gradient was steeper at the deeper region. In future work, we will perform more precise measurement with a solid phantom to evaluate the energy dependence of EBT3 film.

SU-E-T-121: Investigating the Optimal Scanning Resolution for Radiochromic EBT-2 Films Using an Epson 10000XL Flatbed Scanner.

PURPOSE: The purpose of this work is to determine the optimal scanner resolution of an Epson 10000XL scanner for the analysis of radiochromic EBT-2 films. Using Fourier analysis and the Nyquist-Shanon sampling theory, the highest frequency component required to sufficiently reproduce a previously measured step dose profile was investigated. METHODS: A setup was created, in which one half of a 6×6cm2 EBT-2 film was shielded on exposure using a 15×5×10cm3 lead block to obtain sharp step dose profiles. The film itself was placed between two 6cm RW3 stacks on top of which the lead block was placed. Using a Siemens Primus linear accelerator operating at 6/15MV nominal energies, the setup was exposed to 400MUs at 6MV and 500MUs at 15MV respectively. Preliminary investigations were performed without RW3 between the lead and film. Initial image acquisition was performed at 600dpi to minimize information loss. Using the average of five line profiles, a uniformity correction algorithm provided by the manufacturer was implemented prior to the Fast Fourier Transform (FFT) operation. In an iterative process, all frequency components above a cut-off frequency wcut were successively removed and the original image reconstructed with the inverse FFT operation. The goodness of fit was evaluated by comparing the change in penumbra width on image reconstruction. RESULTS: The minimum scanning resolution required to analyze the step dose profiles created without build-up material was 52dpi for 6MV and 30dpi for 15MV. By adding build-up material, in the areas of secondary electron equilibrium the required resolution reduces to 12dpi for 6MV and 8dpi for 15MV. CONCLUSIONS: For sufficient image reproduction within any information loss, resolutions as low as 52dpi at 6MV and 30dpi at 15MV are sufficient for evaluating EBT-2 films. This is in compliance with 50dpi recommended by the manufacturer.

SU-E-T-120: Minimum Absorbed Dose Limit for Gafchromic EBT2 Film Response after Exposure to Low-Energy Photons.

PURPOSE: To investigate the accuracy of the absorbed dose measured with Gafchromic EBT2 film in low-energy photon radiation fieldsMethods: Six EBT2 film (lot # F06110901) pieces (1cm2 ) per dose were exposed to x-rays of 50 kV, 80 kV, 120 kV and 60Co gamma rays from a Leksell Gamma Knife at dose values from 50 mGy to 100 Gy. The x-ray beams were calibrated following the AAPMTG-61 protocol using ionization chambers calibrated at NIST or Wisconsin University depending on the beam quality, while the 60Co gamma was calibrated in water using MD-V2-55 film. Each film piece was scanned once using a HP Scanjet 7650 document flatbed scanner in transmission mode, 48-bit color at 300 dpi spatial-resolution. The data analysis was made through the ImageJ. The measured light intensity for the red channel with its associate standard deviation was used to evaluate the netOD and its standard combined uncertainty. The absorbed dose as a function of the netOD was fitted using the logistic model and the relative combined uncertainties were evaluated for each energy photon beam. RESULTS: EBT2 film response curve depends on the low-energy photons and the degree of energy-dependence is a function of absorbed dose. The absorbed dose relative combined uncertainty as a function of the absorbed dose indicates that the minimum absorbed dose limit is also energy dependent. Lower is the energy photon; more accurate is the measurement at low dose value. This can be explain by the fact that comparing to high energy photons, low energy photons can produce locally enough ionization density to create more color centre in the same film area. CONCLUSIONS: Minimum absorbed dose limit of Gafchromic EBT2 films were found to be energy dependent. The response curve depends on the low-energy photons and the degree of energy-dependence is a function of absorbed dose This work is partially supported by DGAPA-UNAM grant IN102610 and Conacyt Mexico grant 127409.

SU-E-T-122: Dose Response Analysis of Radiochromic Films in Regions of Low Dose Using Separation Color Components.

PURPOSE: To evaluate the dose response of EBT2 films in regions of low dose using the decomposition of the image's color channels (RGB, Red, Green and Blue). METHODS: Doses ranging from 1 Gy to 60 Gy were used to calibrate the dose response of Gafchromic ® EBT2 films irradiated in 6MV photons beams. Segments of film with dimensions of 8.5 cm × 8.5 cm were used. Another segment of film with dimensions 8.5 cm × 20.25 cm was also irradiated with a maximum dose of 4Gy to determine the percentage depth dose (PDD). The films were digitized by a LaserJet M1132 MFP - HP ® scanner in standard resolution of 150dpi and analyzed by a routine created in MatLab to convert the image to gray levels as well as assess the desired color components from the image. RESULTS: The green component presented the higher sensitivity (17.8 a.u./Gy) when the separated color channels and the shades of gray analysis are compared. The red component presented the highest signal to noise ratio in the low dose range (63% at 1Gy). The blue component presented low sensitivity (0.66 a.u./Gy) in the entire dose range. A linear fitting (r=0.998) was used to the green and gray components until a dose of 4 Gy. The red component presented a non-linear behavior in the entire dose range. The useful dose range found was from 1 Gy to 15 Gy. The maximum differences between the reference PDD, measured with ionization chamber in a water phantom, and the PDDs determined with film were 6%, 9% and 14% for the green, gray and red components, respectively. CONCLUSIONS: This work results show that the use of radiochromic films on planning verification procedures in low dose ranges can be benefit from the analysis of the image's separated color components.

SU-E-T-115: A Novel Cylindrical 3D Water Scanner for Beam Data Collection: II. Dosimetric Characteristics.

PURPOSE: To study the dosimetric characteristics of a commercial three-dimensional water scanner (3D SCANNER, Sun Nuclear Corp). The novel cylindrical scanner uses a compact electrometer mounted on the side of the tank, eliminating the need for chamber extension cables. The electrometer has a wide dynamic range, requiring no gain adjustment as scanning conditions change, e.g., field size, dose rate, wedge field, etc. METHODS: Measurements from the 3D SCANNER were compared against those from another commercial scanner (Blue Phantom, IBA Dosimetry). Comparable collection intervals and scanning speeds were used on both systems. Profile and depth measurements were performed for open field beams (6 and 18 MV; 10-cm and 30-cm squared fields; depths of dmax, 10, and 30 cm) and wedged fields (6 and 18 MV; 30-cm squared field; depth of 10 cm). Electron beam profile and depth measurements were performed for a 20-cm squared applicator (6, 12, and 20 MeV; depth of dmax). The root mean square (RMS) values were determined for each scanner's measurements. RESULTS: The measured field sizes were within 0.06 cm and 0.05 cm for photon and electron beams, respectively. The photon D10/D20 ratios differed by < 0.4% and the electron I50 values were within 0.02 cm. The RMS of the profiles was approximately 0.1% for both the 3D SCANNER and Blue Phantom measurements. Between the two scanners, all measurements agreed within RMS, excluding the penumbra region. Two exceptions occurred: a 0.5% difference was observed in the shoulder of the 12 MeV profiles, and a 0.8% difference was observed in the 6 MeV depth measurements near the surface. CONCLUSIONS: The quality of the scanned profile and depth measurements from the 3D SCANNER is comparable to that of another commercial scanner. Both scanning systems have similar levels of scanning noise. This work was partially supported by a research grant from Sun Nuclear Corp., Melbourne, FL.

SU-E-T-417: Tomotherapy Isocentricity: Is it Good Enough for Stereotactic Radiosurgery?

PURPOSE: To evaluate the isocenter accuracy of radiation beam relative to the Tomotherapy linear accelerator's mechanical axis using a simplified Winston Lutz test, and to determine if Tomotherapy system is accurate for SRS applications. METHODS: BrainLab Winston Lutz pointer was setup on the Tomotherapy couch to the isocenter according to the green lasers. Four static plans were created to deliver radiation with 1cm jaw opening, all MLC closed except the central two leaves, and gantry at 0, 90, 180 and 270 degree, respectively. Gafchromic EBT films were taped on a solid water slab and placed on opposite side of gantry at a fixed distance for each plan. After the exposure, the films were scanned using Vidar scanner along with a 1 cm scale drawn (Figure 1-A, 2-A). The images were first processed by ImageJ, using Find Edges function to enhance the visibility of the boundaries of the circles, from a metal ball in the pointer, and the rectangles defined by two central leaves and the jaws (Figure 1-B, 2-B). Then two diagonal lines were drawn on each rectangle, and a cross on each circle, with the cross sections representing the centers of the rectangles and circles, respectively (Figure 1-C, 2-C). RESULTS: The displacements of the centers of the circles from the centers of the rectangles for the first setup (films were 11cm from the isocenter) were 0.50, 0.707, 1.0 and 0.707 millimeters for each gantry angle, respectively, with an average of 0.73mm. The displacements of second setup (films were 25cm from the isocenter) were 0.54, 1.07, 0.87 and 0.54 millimeters, with an average of 0.75mm. CONCLUSIONS: The preliminary data show that the radiation isocenter agrees with mechanical isocenter well within one millimeter tolerance required for SRS treatments, which indicated Tomotherapy system is suitable for such applications.

SU-E-T-398: Verification of Gamma Knife EXtend System Based Fractionated Treatment Planning Using EBT2 Film.

PURPOSE: To present EBT2 film verification of treatment planning with the eXtend System, a relocatable frame system for multiple-fraction or serial multiple-session radiosurgery. METHODS: A human head shaped phantom simulated the verification process for fractionated Gamma Knife (GK) treatment. Phantom preparation for eXtend Frame based treatment planning involved creating a dental impression, fitting the phantom to the frame system, acquiring a stereotactic computed tomography (CT) scan. A CT scan (Siemens, Emotion6) of the phantom was obtained with following parameters: Tube Voltage - 110 kV, Tube Current - 280 mA, pixel size - 0.5 mm × 0.5 mm and 1 mm slice thickness. A treatment plan with two 8 mm collimator shots and three sector blocking in each shot was made. Dose prescription of 4.0 Gy at 100% was delivered for the first fraction out of the two fractions planned. Gafchromic EBT2 film (ISP Wayne, NJ) was used as 2D verification dosimeter in this process. Films were cut and placed inside the film insert of the phantom for treatment dose delivery. Meanwhile a set of films from the same batch were exposed from 0 Gy to 12 Gy doses for calibration purpose. EPSON (Expression 10000XL) scanner was used for scanning the exposed films in transparency mode. Scanned films were analyzed with in-house made Matlab codes. RESULTS: Gamma index analysis of film measurement in comparison with TPS calculated dose resulted in high pass rates >90% for different tolerance criteria of 2%/2mm, 1%/1mm, and 0.5%/0.5mm. The isodose overlay and linear dose profiles of film measured and computed dose distribution on sagittal and coronal plane was in close agreement. CONCLUSIONS: Through this study we propose a treatment verification QA method for eXtend frame based fractionated Gamma Knife radiosurgery using EBT2 film. Acknowledgement: Authors acknowledge the help of Andre Micke, ISP for sharing his expertise on EBT2 film.