F DeBlois

ImaSim, a software tool for basic education of medical x-ray imaging in radiotherapy and radiology

Introduction: X-ray imaging is an important part of medicine and plays a crucial role in radiotherapy. Education in this field is mostly limited to textbook teaching due to equipment restrictions. A novel simulation tool, ImaSim, for teaching the fundamentals of the x-ray imaging process based on ray-tracing is presented in this work. ImaSim is used interactively via a graphical user interface (GUI). Materials and methods: The software package covers the main x-ray based medical modalities: planar kilo voltage (kV), planar (portal) mega voltage (MV), fan beam computed tomography (CT) and cone beam CT (CBCT) imaging. The user can modify the photon source, object to be imaged and imaging setup with three-dimensional editors. Objects are currently obtained by combining blocks with variable shapes. The imaging of three-dimensional voxelized geometries is currently not implemented, but can be added in a later release. The program follows a ray-tracing approach, ignoring photon scatter in its current implementation. Simulations of a phantom CT scan were generated in ImaSim and were compared to measured data in terms of CT number accuracy. Spatial variations in the photon fluence and mean energy from an x-ray tube caused by the heel effect were estimated from ImaSim and Monte Carlo simulations and compared. Results: In this paper we describe ImaSim and provide two examples of its capabilities. CT numbers were found to agree within 36 Hounsfield Units (HU) for bone, which corresponds to a 2% attenuation coefficient difference. ImaSim reproduced the heel effect reasonably well when compared to Monte Carlo simulations. Discussion: An x-ray imaging simulation tool is made available for teaching and research purposes. ImaSim provides a means to facilitate the teaching of medical x-ray imaging.

SU‐FF‐T‐40: Brachytherapy TPS QA Using EBT Model GafChromic Film

Purpose: We describe a method for quality assurance (QA) test that assesses all steps of conformal CT‐based high dose rate (HDR) brachytherapy. A procedure was developed to compare dose distribution generated by the treatment planning system and measured by the EBT model GAFCHROMIC™ film within a dedicated brachytherapy QA phantom. Method and Materials: A phantom consists of 3 catheters embedded in a surface plane of 2 cm thick slab of solid water (SW) mimicking the Fletcher GYN applicator set. Additional slabs of solid water were added above and below this slab to provide full scattering conditions. A single sheet of EBT model GAFCHROMIC™ film was placed 2 cm below the plane containing the catheters. Dose distribution, mimicking the GYN treatment, was calculated in OnCentra Brachy treatment planning software and dose distribution within the film plane was exported in DICOM format. Same plan was delivered to the phantom containing the film, and dose distribution was measured using our novel extended range radiochromic film dosimetry protocol, which employs all three channels of the film absorptionspectrum and can measure dose in the range from 0.02 to 100 Gy. Calculated and measured dose distributions were compared within FilmQA software in terms of gamma function. Comparison was made over region of interest (ROI), defined by region containing isodose ⩾ 5% of prescription dose.Results: Based on recommendations given in the commissioning of brachytherapytreatment planning systems section from IAEA TRS‐430 report, we have established a 5% dose/2 mm distance criterion for gamma function. We found that 95% of points within ROI were passing this criterion. Conclusions: QA method described provides a comprehensive verification of all the steps involved in CT‐based HDR brachytherapy. We recommend the method to be used during commissioning process and once a year as a part of brachytherapy QA program.

MO‐D‐303A‐06: ImaSim, An Animated Tool for Teaching Imaging

Purpose: To enhance the teaching and the self study of diagnostic and radiotherapyphotonimaging by the creation of an interactive educational software package (SP) based on a simulation environment. Various imaging modalities found in a radiology or radiation oncology department are included in the SP, namely conventional kV planar imaging, MV portal imaging,CTimaging and cone beam CTimaging.Materials and Methods: The SP aims at faithfully reproducing the physics behind these modalities while keeping the operation simple and straightforward. Simulated geometries are split into three parts in a modular approach: (1) source geometry, (2) object geometry and (3) detector geometry. (1) kV simulations are based on a validated theoretical model while MV outputs are obtained from the literature. Photons simulated fall into the energy range encompassing radiology and radiation oncology (1 keV ‐ few MeV). (2) They are transported through user created virtual three‐dimensional objects of various materials. Photon attenuation is modeled, while scattering is ignored to reduce calculation times. (3) Virtual detectors, with various energy responses are used to collect photons. The user can interactively vary many parameters. Results: The SP enables a user to quickly demonstrate and study principles associated with the creation of a radiological image in a classroom or in a self‐learning setting. Simulations are managed via an aesthetically appealing and intuitive user interface that simplifies user control to facilitate the learning process. Many imaging phenomena can be studied with the aid of the SP, namely the heel effect, focal spot blurring, contrast vs. photon energy, CT and CBCT reconstruction, backprojection filtering, etc. The SP will be made available to the community by early 2010. Conclusion: This work, by rendering accurate image creation easily accessible, has the potential to enhance textbook based teaching and heighten student interest in medicalphotonimaging.

SU‐FF‐I‐160: SpekCalc: A Free and User‐Friendly Software Program for Calculating X‐Ray Tube Spectra

Purpose: To develop a free and easy‐to‐use software program to model the x‐ray spectra emitted from x‐ray tubes. Such a program is useful for the education of physicists, technicians and technologists working in radiation physics but also as a research tool. Method and Materials: A Graphical User Interface (GUI) was created using REALbasic (REAL software, Inc.). This GUI, designated the name SpekCalc, allows the user to calculate and display the x‐ray spectra emitted from tungsten‐anode x‐ray tubes. The underlying theoretical description for the bremsstrahlung and characteristic x‐ray production is taken from a recent published model. Results: The user selects the tube potential in kVp, the take‐off angle and the amount of filtration. At a click of a button the resulting spectrum is calculated, displayed and can be saved for later use. Beam quality parameters such as the half‐value‐layer, in mm of aluminum and copper, and the mean beam energy, in keV, are also presented to the user. The range of potentials that can be modeled is wide (40–300 kVp) making Spekcalc useful to both the diagnostic imaging and keV photonradiotherapy fields. Filtration can be selected in mm, for 7 materials: aluminium,copper, tungsten, tin, beryllium, water and air. This allows an in‐depth exploration of the filtration effects of materials of differing atomic number. Over 300 individuals have downloaded this program thus‐far, for reasons as diverse as teaching in universities, to learn interactively about x‐ray tubes, for research in radiology and radiotherapy.Conclusion: A useful educational tool for physicists, technicians and technologists has been created, in the form of freely‐available software utility to calculate x‐ray tube spectra.

SU-E-T-462: Impact of the Radiochromic Film Energy Response On Dose Measurements of Low Energy Electronic Brachytherapy Sources

Purpose: We investigated the effect of the EBT3 GafChromicTM film model absorbed dose energy response when used for percent depth dose (PDD) measurements in low-energy photon beams. Methods: We measured PDDs in water from a Xoft 50 kVp source using EBT3 film, and compared them to PDD measurements acquired with a PTW-TN34013 parallel-plate ionization chamber. For the x-ray source, we simulated spectra using the EGSnrc (BEAMnrc) Monte Carlo code, and calculated Half Value Layer (HVL) at different distances from the source in water. Pieces of EBT3 film were irradiated in air and calibration curves were created in terms of air-kerma in air ((Kair)air) for different beam qualities. Pieces of EBT3 film were positioned at distances of 2–6 cm from the Xoft source in a water phantom using a custom-made holder, and irradiated at the same time. As scatter is incorporated in the measured film signal in water, measured (Kair)wat was subsequently converted into absorbed dose to water by the ratio of mass energy absorption coefficients following the AAPM TG-61 dosimetry protocol. Results: Our results show that film calibration curves obtained at beam qualities near the effective energy of the Xoft 50 kVp source in water lead to variation in absorbed dose energy dependence of the response of around 3%. However, if the calibration curve was established at MV beam quality, the error in absorbed dose could be as large as 15%. We observed agreement within 1% between PDD measurements using EBT3 film model (using a calibration curve obtained at 80 kVp, HVL=2.18 mm Al, Eeff=29.5 keV) and the parallel-plate ionization chamber. Conclusion: Accurate dose measurements using radiochromic films at low photon energies require that the radiochromic film dosimetry system be calibrated at corresponding low energies, as large absorbed dose errors are expected for calibrations performed at MV beam qualities.

Sci—Fri AM(2): Brachy—01: Commissioning of Brachytherapy TPS Using EBT Model GafChromic Film

Method for quality assurance (QA) of conformal CT‐based high dose rate (HDR) brachytherapy is described, based on comparing dose distributions generated by treatment planning system and measured by EBT model GAFCHROMIC film within dedicated brachy‐therapy QA phantom. Phantom consists of 3 catheters embedded in surface plane of 2 cm thick slab of solid water (SW) mimicking the Fletcher GYN applicator set. Additional slabs of solid water were added above and below this slab to provide full scattering conditions. Single sheet of film was placed 2 cm below the plane containing catheters. Dose distribution was calculated with OnCentra Brachy treatment planning software and exported in DICOM format. Same plan was delivered to the phantom containing the film, and dose distribution was measured using our novel extended range radiochromic film dosimetry protocol, employing all three channels of the film absorptionspectrum allowing dose measurements in the range 0.02–100 Gy. Calculated and measured dose distributions were compared with FilmQA software in terms of gamma function, over region of interest (ROI) defined by region containing isodose ≥ 5% of prescription dose. Based on recommendations given in IAEA TRS‐430 report, we established a 5% dose/2 mm distance criterion for gamma function. We found that 95% of points within ROI were passing this criterion. QA method described provides a comprehensive verification of all the steps involved in CT‐based HDR brachytherapy. We recommend the method to be used during commissioning process and once a year as part of brachytherapy QA program.

WE-FG-202-04: Decomposition of FDG-PET Based Differential Uptake Volume Histograms in Rectal Cancer Patients

PURPOSE The goal of this study is to test the possible use of the analytical decomposition of differential uptake volume histograms (dUVHs) obtained from FDG-PET/CT data to isolate sub-volumes within a tumor known as biological target volumes (BTVs). METHODS A retrospective study was conducted on a cohort of 20 histo-pathologically confirmed rectal adenocarcinoma patients having PET/CT scans for staging. All patients (T3N0) underwent pre-operative endorectal brachytherapy. After surgery, patients were restaged: 10 patients were T0N0 and 10 were restaged as remaining T3N0. The extent of the disease was sampled in order to create dUVHs; subsequently decomposed into the fewest number of analytical Gaussian functions. RESULTS With the assumption that each function fit corresponded to a single sub-volume within the tumor, six sub-volumes were found to consistently emerge. The first two sub-volumes were influenced by contouring and were not considered in the analysis. For the T3N0 population, abundances for volumes V3-V6 were 63.6%±11.3%, 25.7%±8.4%, 6.1%±4.9%, and 4.7%±2.6%. For the T0N0 population, they were 50.2%±6.8%, 33.4%±4.3%, 11.8%±7.6%, and 4.7%±2.4%. The two populations were compared using two tailed T-tests: volumes 3 and 4 were statistically different with p values of 0.021 and 0.056 respectively. V6 was located at 8.63 ± 2.2 for T0N0 and 6.14 ± 0.78 for T3N0 group (p=0.016). CONCLUSION We described a method for dUVH decomposition using FDG-PET images of rectal adenocarcinoma patients that subsequently went for pre-operative brachytherapy. In addition to extracting different sub-volumes corresponding to different FDG uptake levels, we observed different abundances of two sub-volumes as well as positions of the maximum uptake between the two patient groups. In addition to opening the door to further investigation into underlying physiological phenotypes of segmented subvolumes and their use for biological radiotherapy treatment planning, this method may also provide parameters that could correlate to clinical outcomes in radiotherapy patients.

EP-1399: Radiochromic film based dose measurements during radiotherapy 4D CT-simulation

Purpose/Objective: The aim of this study was two folds: (i) to implement a software that automatically assess multileaf collimator (MLC) performance in dynamic mode (ii) to analyse MLC performance over a long period of continuous operation. Materials and Methods: This study was carried out with a Trilogy linac performing Rapidarc® treatments equipped with a Millenium 120 MLC (Varian) . Qualimagiq software solution was used (v5.4.1, Qualiformed, La Roche-Sur-Yon, France). Tests were as follows : (i) four sliding window at gantry 0°, 90°, 270° and 180° (CQ1); (ii) four picket fence at gantry 0°, 90°, 270° and 180° (CQ2) ; (iii) two picket fence with gantry rotation with and without intentional errors (CQ3 and CQ4) ; (iv) dose rate versus gantry speed (CQ5) ; (v) dose rate versus leafs velocities (CQ6). Images were acquired at 6MV with an AS1000 portal imager (EPID) and transferred to the software for data processing and analysis. All these tests were performed once a week for 9 months. Images were corrected from EPID mechanical slack. Acceptable tolerance levels and their uncertainties were taken from publications, manufacturer recommendations, repeatability and short-term reproducibility study. Results: Weekly images acquisition and associated analysis take about 25 min and 5 min respectively. CQ1, CQ2, CQ4, CQ5 and CQ6 agreed with recommendations. For CQ3, leafs positions were over 0.2mm limit. Further investigations showed that both leaf bank shifted during gantry rotation (0,74mm±0,04mm, clockwise and counter clockwise). Conclusions: A simple tool to understand and to master for assessing periodically MLC performance in dynamic mode was implemented. Apart from the CQ3 test, they were all within tolerances. Thanks to this tool, we have understood why CQ3 was out of tolerance. A sag in MLC carriage due to the gravity effect happens during gantry rotation. This phenomenon would not have been detected with log files analysis and will be further investigated on other accelerators (2100C/D and True Beam, Varian).

Poster — Thur Eve — 20: CTDI Measurements using a Radiochromic Film-based clinical protocol

The purpose of the study was evaluating accuracy and reproducibility of a radiochromic film-based protocol to measure computer tomography dose index (CTDI) as a part of annual QA on CT scanners and kV-CBCT systems attached to linear accelerators. Energy dependence of Gafchromic XR-QA2 ® film model was tested over imaging beam qualities (50 – 140 kVp). Film pieces were irradiated in air to known values of air-kerma (up to 10 cGy). Calibration curves for each beam quality were created (Film reflectance change Vs. Air-kerma in air). Film responses for same air-kerma values were compared. Film strips were placed into holes of a CTDI phantom and irradiated for several clinical scanning protocols. Film reflectance change was converted into dose to water and used to calculate CTDIvol values. Measured and tabulated CTDIvol values were compared. Average variations of ±5.2% in the mean film reflectance change were observed in the energy range of 80 to 140 keV, and 11.1% between 50 and 140 keV. Measured CTDI values were in average 10% lower than tabulated CTDI values for CT-simulators, and 44% higher for CBCT systems. Results presented a mean variation for the same machine and protocol of 2.6%. Variation of film response is within ±5% resulting in ±15% systematic error in dose estimation if a single calibration curve is used. Relatively large discrepancy between measured and tabulated CTDI values strongly support the trend towards replacing CTDI value with equilibrium dose measurement in the center of cylindrical phantom, as suggested by TG- 111.

SU-F-BRE-11: Neutron Measurements Around the Varian TrueBeam Linac.

PURPOSE With the emergence of flattening filter free (FFF) photon beams, several authors have noted many advantages to their use. One such advantage is the decrease in neutron production by photonuclear reactions in the linac head. In the present work we investigate the reduction in neutrons from a Varian TrueBeam linac using the Nested Neutron Spectrometer (NNS, Detec). The neutron spectrum, total fluence and source strength were measured and compared for 10 MV with and without flattening filter and the effect of moderation by the room and maze was studied for the 15 MV beam. METHODS The NNS, similar to traditional Bonner sphere detectors but operated in current mode, was used to measure the neutron fluence and spectrum. The NNS was validated for use in high dose rate environments using Monte Carlo simulations and calibrated at NIST and NRC Canada. Measurements were performed at several positions within the treatment room and maze with the linac jaws closed to maximize neutron production. RESULTS The measurements showed a total fluence reduction between 35-40% in the room and maze when the flattening filter was removed. The neutron source strength Qn was calculated from in-room fluence measurements and was found to be 0.042 × 102 n/Gy, 0.026 × 102 n/Gy and 0.59 × 1012 n/Gy for the 10 MV, the 10 MV FFF and 15 MV beams, respectively. We measured ambient equivalent doses of 11 mSv/hr, 7 mSv/hr and 218 mSv/hr for the 10 MV, 10 MV FFF and 15 MV by the head. CONCLUSION Our measurements revealed a decrease in total fluence, neutron source strength and equivalent dose of approximately 35-40% across the treatment room for the FFF compared to FF modes. This demonstrates, as expected, that the flattening filter is a major component of the neutron production for the TrueBeam. The authors greatly acknowledge support form the Canadian Nuclear Commission and the Natural Sciences and Engineering Research Council of Canada through the CREATE program. Co-authors Dubeau and Witharana are employees of Detec (Gatineau, Quebec), manufacturer of the Nested Neutron Spectrometer.