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DOE OSTI2015

SU-E-T-67: A Quality Assurance Procedure for VMAT Delivery Technique with Multiple Verification Metric Using TG-119 Protocol

Purpose: A successful VMAT plan delivery includes precise modulations of dose rate, gantry rotational and multi-leaf collimator shapes. The purpose of this research is to construct routine QA protocol which focuses on VMAT delivery technique and to obtain a baseline including dose error, fluence distribution and mechanical accuracy during VMAT. Methods: The mock prostate, head and neck (HN) cases supplied from AAPM were used in this study. A VMAT plans were generated in Monaco TPS according to TG-119 protocol. Plans were created using 6 MV and 10 MV photon beams for each case. The phantom based measurement, fluence measurement and log files analysis were performed. The dose measurement was performed using 0.6 cc ion chamber, which located at isocenter. The fluence distribution were acquired using the MapCHECK2 mounted in the MapPHAN. The trajectory log files recorded inner 20 leaf pairs and gantry angle positions at every 0.25 sec interval were exported to in-house software developed by MATLAB and determined those RMS values. Results: The dose difference is expressed as a ratio of the difference between measured and planned doses. The dose difference for 6 MV was 0.91%, for 10 MV was 0.67%. In turn, the fluence distribution using gamma criteria of 2%/2 mm with a 50% minimum dose threshold for 6 MV was 98.8%, for 10 MV was 97.5%, respectively. The RMS values of MLC for 6 MV and 10 MV were 0.32 mm and 0.37 mm, of gantry were 0.33 degree and 0.31 degree. Conclusion: In this study, QA protocol to assess VMAT delivery accuracy is constructed and results acquired in this study are used as a baseline of VMAT delivery performance verification.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

SU-E-T-417: Experimental Verification of Robustness Optimized Intensity Modulated Proton Therapy Plans for Craniospinal Irradiation

Purpose: The robustness-optimized intensity modulated proton therapy (RO-IMPT) plans takes into account the setup errors to eliminate the hot/cold spots caused by setup errors at the junction for craniospinal (CSI) irradiation fields. The RO-IMPT requires a large area for the junction of two fields. This work is to verify the treatment planning system calculated dose with introduction of setup errors by two-dimensional (2D) dose measurement. Two fields of total lengths of 45 cm with junctions of 21 cm were measured. Methods: 2D dose distributions in the coronal plane were measured by a commercial 2D ion-chamber-array detector in a cubic solid water phantom. A total spine field consisting of an upper and a lower spine field was measured at the depths of 5.4 and 6.9 cm. The active area for the measurement was 24×24 cm{sup 2}2. The junction and the area outside the junction of the spine fields were measured separately to complete the verification for the total field. Six different amounts of set-up errors along the superior-inferior direction up to 1.4 cm were intentionally applied. The measured dose with and without set-up errors were compared with the calculated dose with and without set-up errors using Gamma-index analysis. 3%/3mm and 2%/2mm dose/distance agreement criteria were used. The point dose at the center of the junction was also compared. Results: For all fields with various introduced setup errors, 100 % of pixels passed the 3%/3mm criteria and on average 96% of pixels passed 2%/2mm criteria. Differences in the measured and calculated doses were within 1.1%. Conclusion: The efficacy of the robustness for the CSI RO-IMPT plans were verified by measurement. The measurement method can be used for patient treatment field specific QA for CSI RO-IMPT plans in our institution in the near future.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

SU-E-T-582: On-Line Dosimetric Verification of Respiratory Gated Volumetric Modulated Arc Therapy Using the Electronic Portal Imaging Device

Purpose: To investigate the clinical utility of on-line verification of respiratory gated VMAT dosimetry during treatment. Methods: Portal dose images were acquired during treatment in integrated mode on a Varian TrueBeam (v. 1.6) linear accelerator for gated lung and liver patients that used flattening filtered beams. The source to imager distance (SID) was set to 160 cm to ensure imager clearance in case the isocenter was off midline. Note that acquisition of integrated images resulted in no extra dose to the patient. Fraction 1 was taken as baseline and all portal dose images were compared to that of the baseline, where the gamma comparison and dose difference were used to measure day-to-day exit dose variation. All images were analyzed in the Portal Dosimetry module of Aria (v. 10). The portal imager on the TrueBeam was calibrated by following the instructions for dosimetry calibration in service mode, where we define 1 calibrated unit (CU) equal to 1 Gy for 10×10 cm field size at 100 cm SID. This reference condition was measured frequently to verify imager calibration. Results: The gamma value (3%, 3 mm, 5% threshold) ranged between 92% and 100% for the lung and liver cases studied. The exit dose can vary by as much as 10% of the maximum dose for an individual fraction. The integrated images combined with the information given by the corresponding on-line soft tissue matched cone-beam computed tomography (CBCT) images were useful in explaining dose variation. For gated lung treatment, dose variation was mainly due to the diaphragm position. For gated liver treatment, the dose variation was due to both diaphragm position and weight loss. Conclusion: Integrated images can be useful in verifying dose delivery consistency during respiratory gated VMAT, although the CBCT information is needed to explain dose differences due to anatomical changes.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2015

SU-E-T-600: Patient Specific IMRT Verification Using a Phosphor-Screen Based Geometric QA System: A Preliminary Study

Purpose: Raven QA (JPLC, MD) is a unified and comprehensive quality assurance system for QA of TG-142, which use a phosphor screen, a mirror system and a camera. It is to test if this device can be used for IMRT QA dosimetry. Methods: A lung IMRT case is used deliver dose to Raven QA. Accuracy of dose distribution of 5cm slab phantom using Eclipse planning system (Varian) has been confirmed both from a Monte Carlo Simulation and from a MapCheck (SunNuclear) measurement. Geometric distortion and variation of spatial dose response are corrected after background subtraction. A pin-hole grid plate is designed and used to determine the light scatter in the Raven QA box and the spatial dose response. Optic scatter model was not applied in this preliminary study. Dose is normalized to the response of the 10×10 field and the TMR of 5cm depth was considered. Results: Time to setup the device for IMRT QA takes less than 5 minutes as other commercially available devices. It shows excellent dose linearity and dose rate independent, less than 1 %. Background signal, however, changes for different field sizes. It is believed to be due to inaccurate correction of optic scatter. Absolute gamma (5%, 5mm) passing rate was higher than 95%. Conclusion: This study proves that the Raven QA can be used for a patient specific IMRT verification. Part of this study is supported by the Maryland Industrial Partnership Grant.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2015

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 methods Dose 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.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

MO-F-CAMPUS-T-03: Data Driven Approaches for Determination of Treatment Table Tolerance Values for Record and Verification Systems

Purpose: To determine appropriate couch tolerance values for modern radiotherapy linac R&V systems with indexed patient setup. Methods: Treatment table tolerance values have been the most difficult to lower, due to many factors including variations in patient positioning and differences in table tops between machines. We recently installed nine linacs with similar tables and started indexing every patient in our clinic. In this study we queried our R&V database and analyzed the deviation of couch position values from the acquired values at verification simulation for all patients treated with indexed positioning. Mean and standard deviations of daily setup deviations were computed in the longitudinal, lateral and vertical direction for 343 patient plans. The mean, median and standard error of the standard deviations across the whole patient population and for some disease sites were computed to determine tolerance values. Results: The plot of our couch deviation values showed a gaussian distribution, with some small deviations, corresponding to setup uncertainties on non-imaging days, and SRS/SRT/SBRT patients, as well as some large deviations which were spot checked and found to be corresponding to indexing errors that were overriden. Setting our tolerance values based on the median + 1 standard error resulted in tolerance values of 1cm lateral and longitudinal, and 0.5 cm vertical for all non- SRS/SRT/SBRT cases. Re-analizing the data, we found that about 92% of the treated fractions would be within these tolerance values (ignoring the mis-indexed patients). We also analyzed data for disease site based subpopulations and found no difference in the tolerance values that needed to be used. Conclusion: With the use of automation, auto-setup and other workflow efficiency tools being introduced into radiotherapy workflow, it is very essential to set table tolerances that allow safe treatments, but flag setup errors that need to be reassessed before treatments.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

WE-EF-303-09: Proton-Acoustic Range Verification in Proton Therapy

Purpose: We investigated proton-acoustic signals detection for range verification with current ultrasound instruments in typical clinical scenarios. Using simulations that included a realistic noise model, we determined the theoretical minimum dose required to generate detectable proton-acoustic signals. Methods: An analytical model was used to calculate the dose distributions and local pressure rise (per proton) for beams of different energy (100 and 160 MeV) and spot widths (1, 5, and 10 mm) in a water phantom. The acoustic waves propagating from the Bragg peak were modeled by the general 3D pressure wave equation and convolved with Gaussian kernels to simulate various proton pulse widths (0.1 – 10 ms). A realistic PZT ultrasound transducer (5 cm diameter) was simulated with a Butterworth band-pass filter, and ii) randomly generated noise based on a model of thermal noise in the transducer. The signal-to-noise ratio was calculated, determining the minimum number of protons and dose required per pulse. The maximum spatial resolution was also estimated from the signal spectrum. Results: The calculated noise in the transducer was 12–28 mPa, depending on the transducer center frequency (70–380 kHz). The minimum number of protons were on the order of 0.6–6 million per pulse, leading to 3–110 mGy dose per pulse at the Bragg peak, depending on the spot size. The acoustic signal consisted of lower frequencies for wider pulses, leading to lower noise levels, but also worse spatial resolution. The resolution was 1-mm for a 0.1-µs pulse width, but increased to 5-mm for a 10-µs pulse width. Conclusion: We have established minimum dose detection limits for proton-acoustic range validation. These limits correspond to a best case scenario with a large detector with no losses and only detector thermal noise. Feasible proton-acoustic range detection will require at least 10{sup 7} protons per pulse and pulse widths ≤ 1-µs.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

Evaluation of a real-time BeO ceramic fiber-coupled luminescence dosimetry system for dose verification of high dose rate brachytherapy

Purpose: The authors evaluate the capability of a beryllium oxide (BeO) ceramic fiber-coupled luminescence dosimeter, named radioluminescence/optically stimulated luminescence (RL/OSL) BeO FOD, for dosimetric verification of high dose rate (HDR) treatments. The RL/OSL BeO FOD is capable of RL and OSL measurements. Methods: The RL/OSL BeO FOD is able to be inserted in 6F proguide needles, used in interstitial HDR treatments. Using a custom built Perspex phantom, 6F proguide needles could be submerged in a water tank at 1 cm separations from each other. A second background fiber was required to correct for the stem effect. The stem effect, dose linearity, reproducibility, depth-dose curves, and angular and temperature dependency of the RL/OSL BeO FOD were characterised using an Ir-192 source. The RL/OSL BeO FOD was also applied to the commissioning of a 10 mm horizontal Leipzig applicator. Results: Both the RL and OSL were found to be reproducible and their percentage depth-dose curves to be in good agreement with those predicted via TG-43. A combined uncertainty of 7.9% and 10.1% (k = 1) was estimated for the RL and OSL, respectively. For the 10 mm horizontal Leipzig applicator, measured percentage depth doses were within 5% agreement of the published reference calculations. The output at the 3 mm prescription depth for a 1 Gy delivery was verified to be 0.99 ± 0.08 Gy and 1.01 ± 0.10 Gy by the RL and OSL, respectively. Conclusions: The use of the second background fiber under the current setup means that the two fibers cannot fit into a single 6F needle. Hence, use of the RL is currently not adequate for the purpose of in vivo brachytherapy dosimetry. While not real-time, the OSL is shown to be adequate for in vivo brachytherapy dosimetry.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

Helium ions for radiotherapy? Physical and biological verifications of a novel treatment modality

Purpose: Modern facilities for actively scanned ion beam radiotherapy allow in principle the use of helium beams, which could present specific advantages, especially for pediatric tumors. In order to assess the potential use of these beams for radiotherapy, i.e., to create realistic treatment plans, the authors set up a dedicated {sup 4}He beam model, providing base data for their treatment planning system TRiP98, and they have reported that in this work together with its physical and biological validations. Methods: A semiempirical beam model for the physical depth dose deposition and the production of nuclear fragments was developed and introduced in TRiP98. For the biological effect calculations the last version of the local effect model was used. The model predictions were experimentally verified at the HIT facility. The primary beam attenuation and the characteristics of secondary charged particles at various depth in water were investigated using {sup 4}He ion beams of 200 MeV/u. The nuclear charge of secondary fragments was identified using a ΔE/E telescope. 3D absorbed dose distributions were measured with pin point ionization chambers and the biological dosimetry experiments were realized irradiating a Chinese hamster ovary cells stack arranged in an extended target. Results: The few experimental data available on basic physical processes are reproduced by their beam model. The experimental verification of absorbed dose distributions in extended target volumes yields an overall agreement, with a slight underestimation of the lateral spread. Cell survival along a 4 cm extended target is reproduced with remarkable accuracy. Conclusions: The authors presented a simple simulation model for therapeutical {sup 4}He beams which they introduced in TRiP98, and which is validated experimentally by means of physical and biological dosimetries. Thus, it is now possible to perform detailed treatment planning studies with {sup 4}He beams, either exclusively or in combination with other ion modalities.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

SU-C-201-05: Silicon Array Dosimeter in Situ with Electronic Portal Image Device for Simultaneous Transit Dose and Image Verification in Radiotherapy

Purpose: To investigate an Electronic Portal Imaging Device (EPID) coupled to a 2D array dosimeter to provide simultaneous imaging and dose verification. Methods: The novel dual detector configuration comprised of a 2D diode array dosimeter, referred to as a Magic Plate (MP) placed directly on a standard EPID. Dose response of the MP was evaluated by measuring the detector’s response with respect to off-axis position and field size with 30 cm of solid water (SW) acting as a transit object in the beam. Measurements were performed with 3, 5, 10 and 15 mm SW build-up and compared to 2D ionisation chamber array (ICA) measurements and the PinnacleTM treatment planning system (TPS) at a source to detector distance of 150 cm with a 6 MV beam. Clinical dosimetric performance was evaluated by measuring a number of intensity-modulated radiation therapy (IMRT) beams in transit geometry. Imaging performance of the EPID was quantified by measuring the contrast-to-noise ratio (CNR) and spatial resolution. Images of a Rando phantom were used for qualitative assessment. Results: Measured MP off-axis and field size response agreed within 2% of TPS and ICA responses when measured using 15 mm SW build-up. Clinical IMRT beams had gamma pass rates of ≥95% at 3%/3mm criteria. Measured CNR and spatial resolution (f50) were 264.96, 210.6, and 0.41, 0.40 with build-up of 5 and 15 mm respectively for the dual detector configuration. CNR and spatial resolution of 643.9 and 0.41 were measured for standard EPID. CNR was quantitatively worse in the dual detector configuration. Differences in imaging performance were not visible in a qualitative assessment using a Rando phantom. Conclusion: Combining a prototype MP 2D dosimeter with a conventional EPID did not significantly detract from the performance of either device and has the potential for simultaneous on-line patient transit dosimetry and image assessment in radiation therapy. Cancer Institute NSW Australia(Research Equipment Grant 10/REG/1-20) and Cancer Council NSW (Grant ID RG 1-06)

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

SU-C-207A-04: Accuracy of Acoustic-Based Proton Range Verification in Water

Purpose: To determine the accuracy and dose required for acoustic-based proton range verification (protoacoustics) in water. Methods: Proton pulses with 17 µs FWHM and instantaneous currents of 480 nA (5.6 × 10{sup 7} protons/pulse, 8.9 cGy/pulse) were generated by a clinical, hospital-based cyclotron at the University of Pennsylvania. The protoacoustic signal generated in a water phantom by the 190 MeV proton pulses was measured with a hydrophone placed at multiple known positions surrounding the dose deposition. The background random noise was measured. The protoacoustic signal was simulated to compare to the experiments. Results: The maximum protoacoustic signal amplitude at 5 cm distance was 5.2 mPa per 1 × 10{sup 7} protons (1.6 cGy at the Bragg peak). The background random noise of the measurement was 27 mPa. Comparison between simulation and experiment indicates that the hydrophone introduced a delay of 2.4 µs. For acoustic data collected with a signal-to-noise ratio (SNR) of 21, deconvolution of the protoacoustic signal with the proton pulse provided the most precise time-of-flight range measurement (standard deviation of 2.0 mm), but a systematic error (−4.5 mm) was observed. Conclusion: Based on water phantom measurements at a clinical hospital-based cyclotron, protoacoustics is a potential technique for measuring the proton Bragg peak range with 2.0 mm standard deviation. Simultaneous use of multiple detectors is expected to reduce the standard deviation, but calibration is required to remove systematic error. Based on the measured background noise and protoacoustic amplitude, a SNR of 5.3 is projected for a deposited dose of 2 Gy.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

SU-C-207A-06: On-Line Beam Range Verification with Multiple Scanning Particle Beams: Initial Feasibility Study with Simulations

Purpose: To investigate the feasibility and requirement for intra-fraction on-line multiple scanning particle beam range verifications (BRVs) with in-situ PET imaging, which is beyond the current single-beam BRV with extra factors that will affect the BR measurement accuracy, such as beam diameter, separation between beams, and different image counts at different BRV positions. Methods: We simulated a 110-MeV proton beam with 5-mm diameter irradiating a uniform PMMA phantom by GATE simulation, which generated nuclear interaction-induced positrons. In this preliminary study, we simply duplicated these positrons and placed them next to the initial protons to approximately mimic the two spatially separated positron distributions produced by two beams parallel to each other but with different beam ranges. These positrons were then imaged by a PET (∼2-mm resolution, 10% sensitivity, 320×320×128 mm^3 FOV) with different acquisition times. We calculated the positron activity ranges (ARs) from reconstructed PET images and compared them with the corresponding ARs of original positron distributions. Results: Without further image data processing and correction, the preliminary study show the errors between the measured and original ARs varied from 0.2 mm to 2.3 mm as center-to-center separations and range differences were in the range of 8–12 mm and 2–8 mm respectively, indicating the accuracy of AR measurement strongly depends on the beam separations and range differences. In addition, it is feasible to achieve ≤ 1.0-mm accuracy for both beams with 1-min PET acquisition and 12 mm beam separation. Conclusion: This study shows that the overlap between the positron distributions from multiple scanning beams can significantly impact the accuracy of BRVs of distributed particle beams and need to be further addressed beyond the established method of single-beam BRV, but it also indicates the feasibility to achieve accurate on-line multi-beam BRV with further improved method.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

SU-F-J-116: Clinical Experience-Based Verification and Improvement of a 4DCT Program

Purpose: To demonstrate the role of continuous improvement fulfilled by the Medical Physicist in clinical 4DCT and CBCT scanning. Methods: Lung (SABR and Standard) patients’ 4D respiratory motion and image data were reviewed over a 3, 6 and 12 month period following commissioning testing. By identifying trends of clinically relevant parameters and respiratory motions, variables were tested with a programmable motion phantom and assessed. Patient traces were imported to a motion phantom and 4DCT and CBCT imaging were performed. Cos6 surrogate and sup-inf motion was also programmed into the phantom to simulate the long exhale of patients for image contrast tests. Results: Patient surrogate motion amplitudes were 9.9+5.2mm (3–35) at 18+6bpm (6–30). Expiration/Inspiration time ratios of 1.4+0.5second (0.6–2.9) showed image contrast effects evident in the AveCT and 3DCBCT images. Small differences were found for patients with multiple 4DCT data sets. Patient motion assessments were simulated and verified with the phantom within 2mm. Initial image reviews to check for reconstructed artefacts and data loss identified a small number of patients with irregularities in the automatic placement of inspiration and expiration points. Conclusion: The Physicist’s involvement in the continuous improvements of a clinically commissioned technique, processes and workflows continues beyond the commissioning stage of a project. Our experience with our clinical 4DCT program shows that Physics presence is required at the clinical 4DCT scan to assist with technical aspects of the scan and also for clinical image quality assessment prior to voluming. The results of this work enabled the sharing of information from the Medical Physics group with the Radiation Oncologists and Radiation Therapists. This results in an improved awareness of clinical patient respiration variables and how they may affect 4D simulation images and also may also affect the treatment verification images.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

SU-F-J-199: Predictive Models for Cone Beam CT-Based Online Verification of Pencil Beam Scanning Proton Therapy

Purpose: To utilize online CBCT scans to develop models for predicting DVH metrics in proton therapy of head and neck tumors. Methods: Nine patients with locally advanced oropharyngeal cancer were retrospectively selected in this study. Deformable image registration was applied to the simulation CT, target volumes, and organs at risk (OARs) contours onto each weekly CBCT scan. Intensity modulated proton therapy (IMPT) treatment plans were created on the simulation CT and forward calculated onto each corrected CBCT scan. Thirty six potentially predictive metrics were extracted from each corrected CBCT. These features include minimum/maximum/mean over and under-ranges at the proximal and distal surface of PTV volumes, and geometrical and water equivalent distance between PTV and each OARs. Principal component analysis (PCA) was used to reduce the dimension of the extracted features. Three principal components were found to account for over 90% of variances in those features. Datasets from eight patients were used to train a machine learning model to fit these principal components with DVH metrics (dose to 95% and 5% of PTV, mean dose or max dose to OARs) from the forward calculated dose on each corrected CBCT. The accuracy of this model was verified on the datasets from the 9th patient. Results: The predicted changes of DVH metrics from the model were in good agreement with actual values calculated on corrected CBCT images. Median differences were within 1 Gy for most DVH metrics except for larynx and constrictor mean dose. However, a large spread of the differences was observed, indicating additional training datasets and predictive features are needed to improve the model. Conclusion: Intensity corrected CBCT scans hold the potential to be used for online verification of proton therapy and prediction of delivered dose distributions.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

SU-F-T-67: Correction Factors for Monitor Unit Verification of Clinical Electron Beams

Purpose: Monitor units calculated by electron Monte Carlo treatment planning systems are often higher than TG-71 hand calculations for a majority of patients. Here I’ve calculated tables of geometry and heterogeneity correction factors for correcting electron hand calculations. Method: A flat water phantom with spherical volumes having radii ranging from 3 to 15 cm was created. The spheres were centered with respect to the flat water phantom, and all shapes shared a surface at 100 cm SSD. D{sub max} dose at 100 cm SSD was calculated for each cone and energy on the flat phantom and for the spherical volumes in the absence of the flat phantom. The ratio of dose in the sphere to dose in the flat phantom defined the geometrical correction factor. The heterogeneity factors were then calculated from the unrestricted collisional stopping power for tissues encountered in electron beam treatments. These factors were then used in patient second check calculations. Patient curvature was estimated by the largest sphere that aligns to the patient contour, and appropriate tissue density was read from the physical properties provided by the CT. The resulting MU were compared to those calculated by the treatment planning system and TG-71 hand calculations. Results: The geometry and heterogeneity correction factors range from ∼(0.8–1.0) and ∼(0.9–1.01) respectively for the energies and cones presented. Percent differences for TG-71 hand calculations drop from ∼(3–14)% to ∼(0–2)%. Conclusion: Monitor units calculated with the correction factors typically decrease the percent difference to under actionable levels, < 5%. While these correction factors work for a majority of patients, there are some patient anatomies that do not fit the assumptions made. Using these factors in hand calculations is a first step in bringing the verification monitor units into agreement with the treatment planning system MU.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

SU-G-201-05: Comparison of Different Methods for Output Verification of Eleckta Nucletron’s Valencia Skin Applicators

Purpose: The provided output factors for Elekta Nucletron’s skin applicators are based on Monte Carlo simulations. These outputs have not been independently verified, and there is no recognized method for output verification of the vendor’s applicators. The purpose of this work is to validate the outputs provided by the vendor experimentally. Methods: Using a Flexitron Ir-192 HDR unit, three experimental methods were employed to determine dose with the 30 mm diameter Valencia applicator: first a gradient method using extrapolation ionization chamber (Far West Technology, EIC-1) measurements in solid water phantom at 3 mm SCD was used. The dose was derived based on first principles. Secondly a combination of a parallel plate chamber (Exradin A-10) and the EIC-1 was used to determine air kerma at 3 mm SCD. The air kerma was converted to dose to water in line with TG-61 formalism by using a muen ratio and a scatter factor measured with the skin applicators. Similarly a combination of the A-10 parallel plate chamber and gafchromic film (EBT 3) was also used. The Nk factor for the A-10 chamber was obtained through linear interpolation between ADCL supplied Nk factors for Cs-137 and M250. Results: EIC-1 measurements in solid water defined the outputs factor at 3 mm as 0.1343 cGy/U hr. The combination of A-10/ EIC-1 and A-10/EBT3 lead to output factors of 0.1383 and 0.1568 cGy/U hr, respectively. For comparison the output recommended by the vendor is 0.1659 cGy/U hr. Conclusion: All determined dose rates were lower than the vendor supplied values. The observed discrepancy between extrapolation chamber and film methods can be ascribed to extracameral gradient effects that may not be fully accounted for by the former method.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

SU-G-IeP4-06: Feasibility of External Beam Treatment Field Verification Using Cherenkov Imaging

Purpose: Cherenkov light emission has been shown to correlate with ionizing radiation (IR) dose delivery in solid tissue. In order to properly correlate Cherenkov light images with real time dose delivery in a patient, we must account for geometric and intensity distortions arising from observation angle, as well as the effect of monitor units (MU) and field size on Cherenkov light emission. To test the feasibility of treatment field verification, we first focused on Cherenkov light emission efficiency based on MU and known field size (FS). Methods: Cherenkov light emission was captured using a PI-MAX4 intensified charge coupled device(ICCD) system (Princeton Instruments), positioned at a fixed angle of 40° relative to the beam central axis. A Varian TrueBeam linear accelerator (linac) was operated at 6MV and 600MU/min to deliver an Anterior-Posterior beam to a 5cm thick block phantom positioned at 100cm Source-to-Surface-Distance(SSD). FS of 10×10, 5×5, and 2×2cm{sup 2} were used. Before beam delivery projected light field images were acquired, ensuring that geometric distortions were consistent when measuring Cherenkov field discrepancies. Cherenkov image acquisition was triggered by linac target current. 500 frames were acquired for each FS. Composite images were created through summation of frames and background subtraction. MU per image was calculated based on linac pulse delay of 2.8ms. Cherenkov and projected light FS were evaluated using ImageJ software. Results: Mean Cherenkov FS discrepancies compared to light field were <0.5cm for 5.6, 2.8, and 8.6 MU for 10×10, 5×5, and 2×2cm{sup 2} FS, respectably. Discrepancies were reduced with increasing field size and MU. We predict a minimum of 100 frames is needed for reliable confirmation of delivered FS. Conclusion: Current discrepancies in Cherenkov field sizes are within a usable range to confirm treatment delivery in standard and respiratory gated clinical scenarios at MU levels appropriate to standard MLC position segments.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

MO-FG-CAMPUS-JeP1-01: Prompt Gamma Imaging with a Multi-Knife-Edge Slit Collimator: Evaluation for Use in Proton Beam Range Verification

Purpose: To evaluate and characterize a multi-slit collimated imaging system for use in prompt gamma range verification of proton therapy. Methods: Acrylic (PMMA) targets were irradiated with a 50 MeV proton beam. With the collimator placed 13 cm from the beam axis, photons of energy from 2–7 MeV were measured. Image reconstruction provided 2-dimensional distribution of gamma rays. Estimated Bragg peak location was compared with 1-dimensional profiles of photon images. Shifts in Bragg peak were simulated by physically moving the targets in 1 mm increments. Results: The imaging system measured prompt gamma emissions resulting from a 50 MeV proton beam, at currents up to 2 nA, incident on a PMMA target. Overall system detection efficiency was approximately 2.6×10{sup −5} gamma/proton. With delivery of 1×10{sup 11} protons, shifts of 1 mm in the target location were detected in 2D prompt gamma images and 1D profiles. With delivery of 1×10{sup 8} protons, shifts of approximately 3 mm were detectable. Conclusion: This work has characterized the performance of a prototype multi-slit collimated imaging system. The system can produce 2D images of prompt gamma distributions and detect shifts in Bragg peak location down to 1 mm. These results encourage further development and optimization of the system for clinical proton beam applications. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number: DENA0000979 through the Nuclear Science and Security Consortium.

60 APPLIED LIFE SCIENCES↗