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A Monte Carlo calculation model of electronic portal imaging device for transit dosimetry through heterogeneous media.

PURPOSE To develop and evaluate a fast Monte Carlo (MC) dose calculation model of electronic portal imaging device (EPID) based on its effective atomic number modeling in the XVMC code. METHODS A previously developed EPID model, based on the XVMC code by density scaling of EPID structures, was modified by additionally considering effective atomic number (Zeff) of each structure and adopting a phase space file from the EGSnrc code. The model was tested under various homogeneous and heterogeneous phantoms and field sizes by comparing the calculations in the model with measurements in EPID. In order to better evaluate the model, the performance of the XVMC code was separately tested by comparing calculated dose to water with ion chamber (IC) array measurement in the plane of EPID. RESULTS In the EPID plane, calculated dose to water by the code showed agreement with IC measurements within 1.8%. The difference was averaged across the in-field regions of the acquired profiles for all field sizes and phantoms. The maximum point difference was 2.8%, affected by proximity of the maximum points to penumbra and MC noise. The EPID model showed agreement with measured EPID images within 1.3%. The maximum point difference was 1.9%. The difference dropped from the higher value of the code by employing the calibration that is dependent on field sizes and thicknesses for the conversion of calculated images to measured images. Thanks to the Zeff correction, the EPID model showed a linear trend of the calibration factors unlike those of the density-only-scaled model. The phase space file from the EGSnrc code sharpened penumbra profiles significantly, improving agreement of calculated profiles with measured profiles. CONCLUSIONS Demonstrating high accuracy, the EPID model with the associated calibration system may be used for in vivo dosimetry of radiation therapy. Through this study, a MC model of EPID has been developed, and their performance has been rigorously investigated for transit dosimetry.

SU-F-J-114: On-Treatment Imagereconstruction Using Transit Images of Treatment Beams Through Patient and Thosethrough Planning CT Images

PURPOSE To reconstruct patient images at the time of radiation delivery using measured transit images of treatment beams through patient and calculated transit images through planning CT images. METHODS We hypothesize that the ratio of the measured transit images to the calculated images may provide changed amounts of the patient image between times of planning CT and treatment. To test, we have devised lung phantoms with a tumor object (3-cm diameter) placed at iso-center (simulating planning CT) and off-center by 1 cm (simulating treatment). CT images of the two phantoms were acquired; the image of the off-centered phantom, unavailable clinically, represents the reference on-treatment image in the image quality of planning CT. Cine-transit images through the two phantoms were also acquired in EPID from a non-modulated 6 MV beam when the gantry was rotated 360 degrees; the image through the centered phantom simulates calculated image. While the current study is a feasibility study, in reality our computational EPID model can be applicable in providing accurate transit image from MC simulation. Changed MV HU values were reconstructed from the ratio between two EPID projection data, converted to KV HU values, and added to the planning CT, thereby reconstructing the on-treatment image of the patient limited to the irradiated region of the phantom. RESULTS The reconstructed image was compared with the reference image. Except for local HU differences>200 as a maximum, excellent agreement was found. The average difference across the entire image was 16.2 HU. CONCLUSION We have demonstrated the feasibility of a method of reconstructing on-treatment images of a patient using EPID image and planning CT images. Further studies will include resolving the local HU differences and investigation on the dosimetry impact of the reconstructed image.

SU‐E‐T‐160: Exit EPID Image Prediction Below Heterogeneous Phantoms Using Monte Carlo Codes

PURPOSE To predict accurate exit EPID images below lung or bone phantoms by appropriate Monte Carlo modeling. It has been evaluated that XVMC and EGSnrc Monte Carlo codes are suitable for dose calculation in tissue equivalent material at both patient and EPID levels. However, the EPID image prediction is challenging due to its complex structure and material composition of high atomic number. METHODS A 6MV beam and phantom were modeled using BEAMnrc and XVMC codes. The accuracy of exit dose was validated through comparison with 2-D ion chamber dose under a 2 cm build-up layer. After the validation, Varian aS1000 EPID has been modeled to calculate the EPID response. The material composition of EPID has been used for modeling in EGSnrc user code, DOSRZnrc, whereas homogeneous layer modeling by density scaling of the composition has been applied in XVMC. Dose images at 150 cm were calculated under combinations of plastic water, lung and bone flat phantoms for various field sizes. The results were compared to EPID measurement. RESULTS Ion chamber array measurements at EPID level agreed with the XVMC and BEAMnrc/DOSRZnrc calculations within ∼3%. EPID images calculated by DOSRZnrc were in good agreement (∼1%) with the measurement in all cases. The results from XVMC were within ∼2% difference in plateau region, but penumbra slopes were sluggish and dose response at off-axis was slightly lower. CONCLUSION EPID dose images were calculated through EPID composition modeling by XVMC and BEAMnrc/DOSRZnrc. Both Monte Carlo calculations agreed with the measurements in all settings, except penumbra slopes and off-axis responses in XVMC calculations. The EPID model by XVMC can be used for faster and less accurate calculations, whereas the other for more accurate, time spending calculations.

SU-F-J-118: On-Treatment 4D CT Reconstruction From Planning 4D CT Using Linear Amplitude Scaling

PURPOSE To reconstruct on-treatment 4D CT images from planning 4D CT images by adapting deformation vector field (DVF) of the planning CT to the on-treatment condition, while the adaptation is based on the scaling of two amplitudes that are motion characteristics at the times of treatment and planning CT acquisition, respectively. METHODS An anthropomorphic digital phantom (XCAT) was used to generate 4D image sets with 1-cm and 2-cm tumor motions simulating conditions of planning CT and treatment, respectively. DVFs were acquired from the planning CT image set. The DRR images were acquired simulating setup kV images from the two CT image sets. On the DRR images, tumor positions and their motion amplitudes were quantified. The DVFs were scaled linearly by the amplitude ratio between the treatment and the planning CT times, assuming the elasticity of lung. The scaled DVFs were used to resample the planning 4D CT images generating on-treatment 4D CT images. The on-treatment 4D CT images thus acquired were compared with the reference on-treatment images (2-cm motion). RESULTS The resampled images showed good agreement within 1 mm residual errors with the reference images. The normalized cross correlation was 0.995. CONCLUSION A linear model of amplitude scaling was developed to reconstruct on-treatment 4D CT images from planning 4D CT images using the setup KV images acquired during treatment. The model was validated on a digital phantom. For the model to fully work, a further research needs to be followed, that aims at utilizing a phase-specific CT image set that is geometrically identical between pretreatment and treatment conditions.

SU-E-T-251: Developing a Daily Proton Beam Monitoring System

Purpose: To develop a daily monitoring system for proton beam output check and beam uniformity check. Methods: Designed for continuously irradiated photon and electron beams with a field size of 20 cm x 20 cm, the daily output checker (Sun Nuclear, Inc.) is not suitable for monitoring proton beams with inter-pulse beam-off and a field size smaller than 14–16 cm in diameter. To allow such proton beam monitoring, the following tests were performed. 1. Absolute dose and array calibrations which accept continuous irradiation only, were performed using photon beams. 2. Five ion chambers within the central area of 8 cm x 8 cm were utilized to check constancy of output at the center of beam modulation and at distal edge and to check beam symmetry and flatness. 3. To simplify our evaluation, the array calibration was manually modified, such that all five chambers report equal values in spite of their differences in build-up thicknesses. 4. The chamber at the lower-right corner is placed under a buildup thickness that can offer dose measurement at the distal edge. This buildup thickness was determined by proton beam range measurements, which established buildup thickness for beam output measurement at the central chamber and range measurement at the corner chamber. 5. The beam-off delay which allows receipt of pulsed irradiation was activated and optimal delay times were determined for each proton beam at 149.6, 185.6, and 249.5 MeV. Results: The above system was tested by miss-steering proton beams and altering phantom thickness by 1 mm at a time. The system reliably monitored the beam with: 3% tolerance for beam flatness, symmetry and output. The range difference of 0.5 mm could be detected at all energies by setting a tolerance of 20%. Conclusion: A quick daily proton beam monitoring system was feasible.

SU-E-T-366: Time-Resolved EPID Dosimetry for Validating ArcIMRT

PURPOSE The purpose of this study is to demonstrate time-resolved dosimetry for verifying arc intensity modulated radiation therapy (arcIMRT) delivery, utilizing the continuous image acquisition of an electronic portal imaging device (EPID). METHODS An arcIMRT field was made of sliding, multi-leaf collimator (MLC) motion of X1 leaves traveling from -5 cm to +5 cm and back, while Y opening was kept at 10 cm. For this field, this travel was repeated three times during the gantry rotation of 180°. The images were continuously acquired while the EPID was irradiated with 240 MU at a constant dose rate of 300 MU/min. By summing 10 frames, and thus reducing the temporal resolution to 1 second, a longitudinal non-uniformity in the images was reduced to less than 3%. Dose images in EPID were also calculated by the XVMC code for MLC positions at 1 mm interval using the EPID model developed in our previous study. The calculated images at 1 sec resolution were then correlated to and compared with the images, validating the time-resolved dosimetry. RESULTS Over the period of 48 seconds, 481 EPID images were acquired. The MLCs traveled at 1.25 cm/sec. Thus, 10 images were assigned to 1.25 cm of MLC travel for the correlation and for dose evaluation. For all gantry angles, the agreement of the gamma tests was above 90%, given 3 mm distance to agreement and 3% dose difference, when MLCs moved forwardly to the X2 side. However, when they travel back, the pass rates were below 90% due to MLC lagging which was detected in time-resolved EPID dose imaging. CONCLUSION Time-resolved, four-dimensional dose validation of arcIMRT was demonstrated, showing the temporal information of dynamic radiation delivery. This can be used for the validation of 4D treatment delivery techniques.

SU‐E‐T‐55: A Study of Film Dosimetry for Routine Beam Profile and PDD Constancy Checks in Proton Beams

PURPOSE Radiographic film dosimetry suffers from energy dependence in Proton dosimetry, and thus is not suitable for absolute dosimetry. In this study, we investigate film dosimetry for the constancy check of percentage depth dose (PDD) and beam profile measurements in proton beams. METHODS From PDD measured by film and ion chamber (IC), calibration factors as a function of depth (IC/film) was obtained. These factors imply variable slopes (with energy and depth) of linear characteristic curves that relate film response to dose. They were used to convert day-to-day film measurements into dose. Film dosimetry of a 186 MeV proton beam was performed to investigate this hypothesis. In addition, Monte Carlos simulation of a 250 MeV proton beam was performed calculating proton fluence spectrum along the off-axis direction. By multiplying stopping powers of film emulsion and water, respectively, to the spectrum, doses to film and water were calculated. The ratio of film dose to water dose was evaluated across the off-axis distance to understand film response. RESULTS The measured and calibrated PDD approached to that of IC, but near the end of spread-out-bragg-peak (SBOP), a spurious peak is observed due to the mismatch of distal edge between calibration film and measurement film. The SBOP width was measurable within 1mm. The distal edge was reproducible within 1.5mm. Entrance dose was reproducible within 5.5%. The possible sources of such errors include developer uncertainty, film emulsion nonuniformity, and misalignment of film edge to the phantom surface. For off-axis evaluation, the dose ratio varied within 3%, and thus film is shown to be accurate for dosimetry across off-axis distance. CONCLUSIONS Radiographic film can be suitable for beam profile measurements and may be suitable for PDD constancy check for proton beams. Routine use will confirm such error which will be presented in this study.