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DOE OSTI2015

SU-E-T-19: A Comparison of the Dosimetric Effects of Brass Mesh and Superflab Boluses

Purpose: We compared the dosimetric effects of brass mesh (Whiting and Davis, Attleboro Falls, MA) and Superflab (CNMC, Nashville, TN) boluses to verify equivalence between the two. Brass mesh bolus may be a convenient alternative to traditional bolus as it better conforms to the skin surface. Methods: Measurements were taken using a 6MV 10×10cm field produced by an Infinity linear accelerator (Elekta, Stockholm, Sweden) in a solid water phantom using a parallel plate ion chamber (Model 96035, Keithley Instruments, Cleveland, OH). Measurements compared 0.5cm and 1cm of Superflab to one to six layers of brass bolus mesh. Measurements were performed at depths from 0cm (‘skin’) to 10cm. Oblique beams were not studied. Results: Four layers of brass mesh were equivalent to 0.5cm Superflab within 5% at all depths. Six layers of brass compares most favorably with 0.5cm Superflab, with doses at all depths within 3%. Six layers of brass were not fully equivalent to 1cm Superflab, although the agreement was within 5% for depths greater than 3mm. Surface dose was 12% lower than 1cm Superflab. Surface dose can be up to 34% different between Superflab and brass mesh, but is less than 5% different with 4–6 layers of brass when compared to 0.5cm Superflab. To achieve surface dose agreement better than 5% compared to 1cm Superflab, more than 6 layers would be needed. The attenuation at depth was a maximum of 0.17cm per layer. Conclusion: Between four and six layers of brass mesh can be equivalent to 0.5cm Superflab, depending on the level of agreement desired. Equivalence within 5% at all depths to 1cm Superflab was not achieved even with six layers. This data agrees with measurements taken by Utsunomiya et al. (2010). More point measurements at shallower depths should be taken prior to clinical implementation of brass bolus mesh.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

SU-F-J-21: Clinical Evaluation of Surface Scanning Systems in Different Treatment Locations

Purpose: To reduce imaging dose in fractionated IGRT, the ability of optical surface imaging systems (OSIS) to detect setup errors was tested. Therefore, clinical studies to evaluate for different treatment locations setup corrections derived by OSIS in comparison with x-ray image guidance in fractionated radiation therapy was performed. Methods: The setup correction accuracy of an OSIS system (AlignRT, VisionRT, London, UK) will be analysed for the 4 tumour locations Pelvis, Upper Abdomen, Thorax and Breast, 20 patients for each location in comparison to a different system (Sentinel, C-RAD, SE). For each patient, the setup corrections of the cone-beam computed tomography (CBCT) of an Elekta Versa HD linear accelerator (Elekta, Crawley, UK) is considered as gold-standard and then compared with those of the OSIS for the first ten fractions retrospectively. There were no clinical decisions made based on the surrogate system. For the OSIS, the reference surface is highly important as it represents the actual ground truth. It can be obtained either with the system itself or the surface structure delineated in the planning CT can be imported via DICOM interface. In this paper, the first results for the treatment region thorax are presented. The reference image modalities were compared. Results: Table 1 displays the difference between the setup corrections obtained with OSIS and CBCT in lateral (LAT), longitudinal (LNG) and vertical (VRT) direction for the DICOM reference image. While the median deviations are within a few millimeters, some outliers showed large deviations. Generally, the mean deviation as well as the spread was smallest in lateral and largest in vertical direction. Conclusion: Although the system allows fast, simple and non-invasive determination of setup corrections, it should be evaluated treatment region dependant. Therefore, the study is ongoing. The application of OSIS may help to reduce the imaging dose for the patient. We gratefully acknowledge the support by our colleagues from the Radiological University Clinic Heidelberg, where the study was performed. This work was funded by the Federal Ministry of Education and Research (BMBF) Germany, grant number 01IB13001B.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

SU-E-T-199: How Number of Control Points Influences the Dynamic IMRT Plan Quality and Deliverability

Purpose: To study the influence of number of control points on plan quality and deliverability. Methods: Five previously treated patients of carcinoma of rectum were selected. Planning target volume (PTV) and organs at risk (OARs) i.e. bladder and bowel were contoured. Dynamic IMRT plans (6MV, 7-fields, 45Gy/25 fractions and prescribed at 95% isodose) were created in Eclipse (Varian medical system, Palo Alto, CA) treatment planning system (TPS) for Varian CL2300C/D linear-accelerator. Base plan was calculated with 166 control points, variable mode (Eclipse Default). For generating other plans, all parameters were kept constant, only number of control points (Fixed mode) was varied as follows: 100, 166 and 200. Then, plan quality was analyzed in terms of maximum and mean dose received by the PTV and OARs. For plan deliverability, TPS calculated fluence was verified with I’matriXX (IBA Dosimetry, Germany) array and compared with TPS dose-plane using gamma index criteria of 3% dose difference and 3mm distance to agreement (DTA). Total number of monitor units (MU) required to deliver a plan was also noted. Results: The maximum variation for the PTV maximum with respect to eclipse default control point (166) was 0.28% (0.14Gy). Similarly, PTV mean varied only up to 0.22 %( 0.11Gy). Bladder maximum and bladder mean varied up to 0.51% (0.24Gy) and 0.16% (0.06Gy). The variation for the bowel maximum and bowel mean was also only 0.39% (0.19Gy) and 0.33% (0.04Gy). Total MU was within 0.32 % (4MU). Average gamma pass rate using different control points for five patients are 98.75±0.33%, 99.37±0.09%, 99.29±0.12%, 98.14±0.13% and 99.25±0.14% respectively. Conclusion: Slight variation (<1%) in PTV and OARs maximum and mean doses was observed with varying number of control points. Monitor unit was also not varied much. Reducing number of control points did not showed any comprise in plan deliverability in terms of gamma index pass rate.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

Volumetric Radiosurgery for 1 to 10 Brain Metastases: A Multicenter, Single-Arm, Phase 2 Study

Purpose: Interest is growing in treating multiple brain metastases with radiosurgery. We report on the effectiveness and tolerability of volumetric radiosurgery (VRS). Methods and Materials: We enrolled patients with a ≥6-month estimated life expectancy and 1 to 10 brain metastases with a diameter of ≤3 cm at 5 cancer centers. Volumetric radiosurgery was delivered in 5 fractions with 98% target coverage, prescribed as 95% of 50 Gy (47.5 Gy in 5 fractions) to the metastases with no margin and 95% of 40 Gy (38 Gy in 5 fractions) to their 2-mm planning target volumes, concurrent with 20 Gy to the whole brain planning target volume. The treatment was delivered with daily image guidance using conventional linear accelerators and volumetric modulated arc therapy. A magnetic resonance imaging scan was obtained every 3 months. The primary endpoint was the 3-month objective response in the brain according to the Response Evaluation Criteria in Solid Tumors, version 1.1. The principal secondary endpoint was 1-year actuarial control of treated metastases. Toxicities were graded using the Common Terminology Criteria for Adverse Events, version 4.0. The present study is registered with (ClinicalTrials.gov) ( (clinicaltrials.gov) identifier (NCT01046123)). Results: From July 2010 to May 2013, 60 patients underwent VRS with 47.5 Gy in 5 fractions for 12 metastases in the thalamus and basal ganglia (deep metastases) and 207 non-deep metastases. The median follow-up period was 30.5 months, and the median survival was 10.1 months. For the 43 patients assessable at 3 months, the objective response in the brain was 56%. The treated metastases were controlled in 88% of patients at 1 year and 84% at 3 years. Overall survival did not differ for patients with 4 to 10 versus 1 to 3 metastases (hazard ratio 1.18, P=.6). The crude incidence of severe radionecrosis (grade 3-5) was 25% (3 of 12) per deep metastasis, 1.9% (4 of 219) per non-deep metastasis, and 10% (6 of 60) per patient. Conclusions: For non-deep brain metastases, 47.5 Gy in 5 fractions was tolerable. Volumetric radiosurgery was effective for long-term control of treated brain metastases.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2015

SU-E-T-449: Hippocampal Sparing Radiotherapy Using Intensity Modulated Radiotherapy and Volumetric Modulated Arc Therapy

Purpose: The hippocampus sparing during the cranial irradiation has become interesting because it may mitigate radiation-induced neurocognitive toxicity. Herein we report our preliminary study for sparing the hippocampus with and without tilling condition for patient with brain metastases. Methods: Ten patients previously treated with whole brain were reviewed. Five patients tilted the head to around 30 degrees and others were treated without tilting. Treatment plans of linear accelerator (Linac)-based volumetric modulated arc therapy (VMAT) and intensity modulated radiotherapy (IMRT) were generated for prescription dose of 30 Gy in 15 fractions. Hippocampal avoidance regions were created with 5-mm volumetric expansion around the hippocampus. Whole brain, hippocampus and hippocampal avoidance volume were 1372cm3, 6cm3 and 30cm3 and hippocampal avoidance volume was 2.2% of the whole brain planned target volume in average. Organs at risk (OARs) are hippocampus, eyes, lens, and cochleae. Coverage index (CVI), conformity index (CI), homogeneity index (HI) and mean dose to OARs were used to compare dose characteristic of tilted and non-tilted cases. Results: In IMRT, when CI, CVI and HI of whole brain were 0.88, 0.09 and 0.98 in both tilted and non-tilted cases, absorbed dose of hippocampal avoidance volume in tilted cases were 10% lower than non-tilted cases. Doses in other OARs such as eyes, lens, and cochleae were also decreased about 20% when tilting the head. When CI, HI and CVI in VMAT were 0.9, 0.08 and 0.99, the dose-decreased ratio of OARs in both with and without tilting cases were almost the same with IMRT. But absolute dose of hippocampal avoidance volume in VMAT was 30% lower than IMRT. Conclusion: This study confirms that dose to hippocampus decreases if patients tilt the head. When treating the whole brain with head tilted, patients can acquire the same successful treatment Result and also preserve their valuable memory.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

SU-F-T-579: Extrapolation Techniques for Small Field Dosimetry Using Gafchromic EBT3 Film

Purpose: The purpose of this project is to test an experimental approach using an extrapolation technique for Gafchromic EBT3 film for small field x-ray dosimetry. Methods: Small fields from a Novalis Tx linear accelerator with HD Multileaf Collimators with 6 MV was used. The field sizes ranged from 5 × 5 to 50 × 50 mm2 MLC fields and a range of circular cones of 4 to 30 mm2 diameters. All measurements were performed in water at an SSD of 100 cm and at a depth of 10 cm. The relative output factors (ROFs) were determined from an extrapolation technique developed to eliminate the effects of partial volume averaging in film scan by scanning films with high resolution (1200 DPI). The size of the regions of interest (ROI) was varied to produce a plot of ROFs versus ROI which was then extrapolated to zero ROI to determine the relative output factor. The results were compared with other solid state detectors with proper correction, namely, IBA SFD diode, PTW 60008 and PTW 60012 diode. Results: For the 4 mm cone, the extrapolated ROF had a value of 0.658 ± 0.014 as compared to 0.642 and 0.636 for 0.5 mm and 1 mm2 ROI analysis, respectively. This showed a change in output factor of 2.4% and 3.3% at this comparative ROI sizes. In comparison, the 25 mm cone had a difference in measured output factor of 0.3% and 0.5% between 0.5 and 1.0 mm, respectively compared to zero volume. For the fields defined by MLCs a difference of up to 2% for 5×5 mm2 was observed. Conclusion: A measureable difference can be seen in ROF based on the ROI when radiochromic film is used. Using extrapolation technique from high resolution scanning a good agreement can be achieved.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

Response of TLD-100 in mixed fields of photons and electrons

Purpose: Thermoluminescent dosimeters (TLDs) are routinely used for dosimetric measurements of high energy photon and electron fields. However, TLD response in combined fields of photon and electron beam qualities has not been characterized. This work investigates the response of TLD-100 (LiF:Mg,Ti) to sequential irradiation by high-energy photon and electron beam qualities. Methods: TLDs were irradiated to a known dose by a linear accelerator with a 6 MV photon beam, a 6 MeV electron beam, and a NIST-traceable {sup 60}Co beam. TLDs were also irradiated in a mixed field of the 6 MeV electron beam and the 6 MV photon beam. The average TLD response per unit dose of the TLDs for each linac beam quality was normalized to the average response per unit dose of the TLDs irradiated by the {sup 60}Co beam. Irradiations were performed in water and in a Virtual Water Trade-Mark-Sign phantom. The 6 MV photon beam and 6 MeV electron beam were used to create dose calibration curves relating TLD response to absorbed dose to water, which were applied to the TLDs irradiated in the mixed field. Results: TLD relative response per unit dose in the mixed field was less sensitive than the relative response in the photon field and more sensitive than the relative response in the electron field. Application of the photon dose calibration curve to the TLDs irradiated in a mixed field resulted in an underestimation of the delivered dose, while application of the electron dose calibration curve resulted in an overestimation of the dose. Conclusions: The relative response of TLD-100 in mixed fields fell between the relative response in the photon-only and electron-only fields. TLD-100 dosimetry of mixed fields must account for this intermediate response to minimize the estimation errors associated with calibration factors obtained from a single beam quality.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

SU-E-J-49: Design and Fabrication of Custom 3D Printed Phantoms for Radiation Therapy Research and Quality Assurance

Purpose The rapid proliferation of affordable 3D printing techniques has enabled the custom fabrication of items ranging from paper weights to medical implants. This study investigates the feasibility of utilizing the technology for developing novel phantoms for use in radiation therapy quality assurance (QA) procedures. Methods A phantom for measuring the geometric parameters of linear accelerator (LINAC) on-board imaging (OBI) systems was designed using SolidWorks. The design was transferred to a 3D printer and fabricated using a fused deposition modeling (FDM) technique. Fiducials were embedded in the phantom by placing 1.6 mm diameter steel balls in predefined holes and securing them with silicone. Several MV and kV images of the phantom were collected and the visibility and geometric accuracy were evaluated. A second phantom, for use in the experimental evaluation of a high dose rate (HDR) brachytherapy dosimeter, was designed to secure several applicator needles in water. The applicator was fabricated in the same 3D printer and used for experiments. Results The general accuracy of printed parts was determined to be 0.1 mm. The cost of materials for the imaging and QA phantoms were $22 and $5 respectively. Both the plastic structure and fiducial markers of the imaging phantom were visible in MV and kV images. Fiducial marker locations were determined to be within 1mm of desired locations, with the discrepancy being attributed to the fiducial attachment process. The HDR phantom secured the applicators within 0.5 mm of the desired locations. Conclusion 3D printing offers an inexpensive method for fabricating custom phantoms for use in radiation therapy quality assurance. While the geometric accuracy of such parts is limited compared to more expensive methods, the phantoms are still highly functional and provide a unique opportunity for rapid fabrication of custom phantoms for use in radiation therapy QA and research.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

Video-rate optical dosimetry and dynamic visualization of IMRT and VMAT treatment plans in water using Cherenkov radiation

Purpose: A novel technique for optical dosimetry of dynamic intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) plans was investigated for the first time by capturing images of the induced Cherenkov radiation in water. Methods: A high-sensitivity, intensified CCD camera (ICCD) was configured to acquire a two-dimensional (2D) projection image of the Cherenkov radiation induced by IMRT and VMAT plans, based on the Task Group 119 (TG-119) C-Shape geometry. Plans were generated using the Varian Eclipse treatment planning system (TPS) and delivered using 6 MV x-rays from a Varian TrueBeam Linear Accelerator (Linac) incident on a water tank doped with the fluorophore quinine sulfate. The ICCD acquisition was gated to the Linac target trigger pulse to reduce background light artifacts, read out for a single radiation pulse, and binned to a resolution of 512 × 512 pixels. The resulting videos were analyzed temporally for various regions of interest (ROI) covering the planning target volume (PTV) and organ at risk (OAR), and summed to obtain an overall light intensity distribution, which was compared to the expected dose distribution from the TPS using a gamma-index analysis. Results: The chosen camera settings resulted in 23.5 frames per second dosimetry videos. Temporal intensity plots of the PTV and OAR ROIs confirmed the preferential delivery of dose to the PTV versus the OAR, and the gamma analysis yielded 95.9% and 96.2% agreement between the experimentally captured Cherenkov light distribution and expected TPS dose distribution based upon a 3%/3 mm dose difference and distance-to-agreement criterion for the IMRT and VMAT plans, respectively. Conclusions: The results from this initial study demonstrate the first documented use of Cherenkov radiation for video-rate optical dosimetry of dynamic IMRT and VMAT treatment plans. The proposed modality has several potential advantages over alternative methods including the real-time nature of the acquisition, and upon future refinement may prove to be a robust and novel dosimetry method with both research and clinical applications.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2015

SU-E-T-226: Correction of a Standard Model-Based Dose Calculator Using Measurement Data

Purpose: To propose a hybrid method that combines advantages of the model-based and measurement-based method for independent dose calculation. Modeled-based dose calculation, such as collapsed-cone-convolution/superposition (CCCS) or the Monte-Carlo method, models dose deposition in the patient body accurately; however, due to lack of detail knowledge about the linear accelerator (LINAC) head, commissioning for an arbitrary machine is tedious and challenging in case of hardware changes. On the contrary, the measurement-based method characterizes the beam property accurately but lacks the capability of dose disposition modeling in heterogeneous media. Methods: We used a standard CCCS calculator, which is commissioned by published data, as the standard model calculator. For a given machine, water phantom measurements were acquired. A set of dose distributions were also calculated using the CCCS for the same setup. The difference between the measurements and the CCCS results were tabulated and used as the commissioning data for a measurement based calculator. Here we used a direct-ray-tracing calculator (ΔDRT). The proposed independent dose calculation consists of the following steps: 1. calculate D-model using CCCS. 2. calculate D-ΔDRT using ΔDRT. 3. combine Results: D=D-model+D-ΔDRT. Results: The hybrid dose calculation was tested on digital phantoms and patient CT data for standard fields and IMRT plan. The results were compared to dose calculated by the treatment planning system (TPS). The agreement of the hybrid and the TPS was within 3%, 3 mm for over 98% of the volume for phantom studies and lung patients. Conclusion: The proposed hybrid method uses the same commissioning data as those for the measurement-based method and can be easily extended to any non-standard LINAC. The results met the accuracy, independence, and simple commissioning criteria for an independent dose calculator.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2012

Experimental validation of deterministic Acuros XB algorithm for IMRT and VMAT dose calculations with the Radiological Physics Center's head and neck phantom

Purpose: The purpose of this study was to verify the dosimetric performance of Acuros XB (AXB), a grid-based Boltzmann solver, in intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT). Methods: The Radiological Physics Center (RPC) head and neck (H and N) phantom was used for all calculations and measurements in this study. Clinically equivalent IMRT and VMAT plans were created on the RPC H and N phantom in the Eclipse treatment planning system (version 10.0) by using RPC dose prescription specifications. The dose distributions were calculated with two different algorithms, AXB 11.0.03 and anisotropic analytical algorithm (AAA) 10.0.24. Two dose report modes of AXB were recorded: dose-to-medium in medium (D{sub m,m}) and dose-to-water in medium (D{sub w,m}). Each treatment plan was delivered to the RPC phantom three times for reproducibility by using a Varian Clinac iX linear accelerator. Absolute point dose and planar dose were measured with thermoluminescent dosimeters (TLDs) and GafChromic registered EBT2 film, respectively. Profile comparison and 2D gamma analysis were used to quantify the agreement between the film measurements and the calculated dose distributions from both AXB and AAA. The computation times for AAA and AXB were also evaluated. Results: Good agreement was observed between measured doses and those calculated with AAA or AXB. Both AAA and AXB calculated doses within 5% of TLD measurements in both the IMRT and VMAT plans. Results of AXB{sub Dm,m} (0.1% to 3.6%) were slightly better than AAA (0.2% to 4.6%) or AXB{sub Dw,m} (0.3% to 5.1%). The gamma analysis for both AAA and AXB met the RPC 7%/4 mm criteria (over 90% passed), whereas AXB{sub Dm,m} met 5%/3 mm criteria in most cases. AAA was 2 to 3 times faster than AXB for IMRT, whereas AXB was 4-6 times faster than AAA for VMAT. Conclusions: AXB was found to be satisfactorily accurate when compared to measurements in the RPC H and N phantom. Compared with AAA, AXB results were equal to or better than those obtained with film measurements for IMRT and VMAT plans. The AXB{sub Dm,m} reporting mode was found to be closer to TLD and film measurements than was the AXB{sub Dw,m} mode. AXB calculation time was found to be significantly shorter (x 4) than AAA for VMAT.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2014

Validation of a Monte Carlo model for the MLC-Elekta Precise with MCNP6 - Paper 76

Cancer treatment using radiotherapy techniques needs an accurate prediction of the dose delivered to the patient at each position. Treatment planning systems (TPS) allow the optimization of the dose delivered to the tumor volume while minimizing the dose absorbed by the surrounding healthy tissue. Commercial treatment planning systems use empirical deterministic algorithms that do not correctly estimate the dose in the surroundings of heterogeneities. This lack of accuracy is taken into account as bigger security margins in the treatment, reducing the potential effect of the treatment. However, Monte Carlo (MC) simulation techniques have proved to predict the dose distribution more accurately and also representing experimental data better. Nevertheless, the clinical application of these techniques has been limited by the long calculation times they require. As high performance computers decrease in cost MC simulations become more feasible to be used in TPS. The present work improves a simplified model of the MLC-Elekta Precise linear accelerator (linac or unit from now on) previously developed and correctly validated by the group. A multileaf collimator (MLC) has been modelled according to the manufacturer specifications and added to the previous model to allow higher conformity in the studies. The full geometry has been entered to the MCNP6 Monte Carlo code to simulate the transport of particles through the linac. The geometry also includes a cubic water phantom for dosimetry studies and validation with experimental data. The experimental data employed for the dose distribution comparison has been measured at the linac from the linac Hospital Universitari Clinic de Valencia (Spain). Additionally, particles are stored in a phase space surface before their entrance in the water to be analyzed and for performance optimization. Given the results presented, we conclude that this first approach to validate a model of the Elekta Precise linac from the Hospital Cl nic Universitari de Valancia represents well the depth-dose distribution, but fails at representing the dose profiles. It has been shown that they predict too much penumbra, probably caused by the filtration between the leaves. In order to solve this problems, further work is needed to improve the simulated model. This work will mainly focus on the multileaf collimator. Also, the codes developed by the group to analyze phase space files produced by MCNP6 have proven highly useful. As work progress and the codes are debugged, they will be published so that the community can take advantage of them.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2014

SU-E-T-68: Clinical Implementation of Total Skin Electron Beam Therapy: A New- York Presbyterian Hospital Experience

Purpose: Total skin electron beam therapy (TSET) is used in the treatment of rare skin diseases such as mycosis fungoides, the most common type of cutaneous T-cell lymphoma. We report our experience with clinical implementation of TSET. Methods: A modified six-dual-field irradiation technique was chosen to deliver TSET. A Varian Trilogy linear accelerator with a nominal 6 MeV beam using high dose rate total skin electron mode (HDTSe) was employed. The recommendations of AAPM task group report 23 were followed for the commissioning. An acrylic plate (energy degrader) of 3.2 mm depth was mounted on the HDTSe applicator. The nominal source to skin distance was set at 450 cm. The optimum tilt angle of the gantry was determined using NACP-02 ionization chamber embedded in certified therapy grade solid water. Percent depth dose measurements were performed using ionization chamber and radiochromic films embedded in solid water and anthropomorphic phantom. For absolute dose measurements, TG-51 formalism was employed. The dose distribution on the entire skin was measured by irradiating the anthropomorphic phantom, with TLDs attached, mimicking the real treatment. Results: The 3.2 mm acrylic plate mounted on the HDTSe applicator degraded the energy of the electron beam to 4.1 MeV in the treatment plane, located at an SSD of 450 cm. The optimum tilt angle was found to be ±20°. A single-dual field had a longitudinal uniformity, measured at a depth of dose maximum, of ±7% over a length of about 200 cm. For the entire treatment the multiplication factor was found to be 2.86. On the surface of the phantom, the dose varied from 108% to 93% of the prescription dose. Conclusion: We have successfully commissioned TSET meeting the guidelines of the TG report 23, and treated our first patient on February 25, 2014.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2016

A new form of the calibration curve in radiochromic dosimetry. Properties and results

Purpose: This work describes a new form of the calibration curve for radiochromic dosimetry that depends on one fit parameter. Some results are reported to show that the new curve performs as well as those previously used and, more importantly, significantly reduces the dependence on the lot of films, the film orientation on the scanner, and the time after exposure. Methods: The form of the response curve makes use of the net optical densities ratio against the dose and has been studied by means of the Beer–Lambert law and a simple modeling of the film. The new calibration curve has been applied to EBT3 films exposed at 6 and 15 MV energy beams of linear accelerators and read-out in transmission mode by means of a flatbed color scanner. Its performance has been compared to that of two established forms of the calibration curve, which use the optical density and the net optical density against the dose. Four series of measurements with four lots of EBT3 films were used to evaluate the precision, accuracy, and dependence on the time after exposure, orientation on the scanner and lot of films. Results: The new calibration curve is roughly subject to the same dose uncertainty, about 2% (1 standard deviation), and has the same accuracy, about 1.5% (dose values between 50 and 450 cGy), as the other calibration curves when films of the same lot are used. Moreover, the new calibration curve, albeit obtained from only one lot of film, shows a good agreement with experimental data from all other lots of EBT3 films used, with an accuracy of about 2% and a relative dose precision of 2.4% (1 standard deviation). The agreement also holds for changes of the film orientation and of the time after exposure. Conclusions: The dose accuracy of this new form of the calibration curve is always equal to or better than those obtained from the two types of curves previously used. The use of the net optical densities ratio considerably reduces the dependence on the lot of films, the landscape/portrait orientation, and the time after exposure. This form of the calibration curve could become even more useful with new optical digital devices using monochromatic light.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

Real-Time 3D Image Guidance Using a Standard LINAC: Measured Motion, Accuracy, and Precision of the First Prospective Clinical Trial of Kilovoltage Intrafraction Monitoring–Guided Gating for Prostate Cancer Radiation Therapy

Purpose: Kilovoltage intrafraction monitoring (KIM) is a new real-time 3-dimensional image guidance method. Unlike previous real-time image guidance methods, KIM uses a standard linear accelerator without any additional equipment needed. The first prospective clinical trial of KIM is underway for prostate cancer radiation therapy. In this paper we report on the measured motion accuracy and precision using real-time KIM-guided gating. Methods and Materials: Imaging and motion information from the first 200 fractions from 6 patient prostate cancer radiation therapy volumetric modulated arc therapy treatments were analyzed. A 3-mm/5-second action threshold was used to trigger a gating event where the beam is paused and the couch position adjusted to realign the prostate to the treatment isocenter. To quantify the in vivo accuracy and precision, KIM was compared with simultaneously acquired kV/MV triangulation for 187 fractions. Results: KIM was successfully used in 197 of 200 fractions. Gating events occurred in 29 fractions (14.5%). In these 29 fractions, the percentage of beam-on time, the prostate displacement was >3 mm from the isocenter position, reduced from 73% without KIM to 24% with KIM-guided gating. Displacements >5 mm were reduced from 16% without KIM to 0% with KIM. The KIM accuracy was measured at <0.3 mm in all 3 dimensions. The KIM precision was <0.6 mm in all 3 dimensions. Conclusions: Clinical implementation of real-time KIM image guidance combined with gating for prostate cancer eliminates large prostate displacements during treatment delivery. Both in vivo KIM accuracy and precision are well below 1 mm.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2013

Flattening filter removal for improved image quality of megavoltage fluoroscopy

Purpose: Removal of the linear accelerator (linac) flattening filter enables a high rate of dose deposition with reduced treatment time. When used for megavoltage imaging, an unflat beam has reduced primary beam scatter resulting in sharper images. In fluoroscopic imaging mode, the unflat beam has higher photon count per image frame yielding higher contrast-to-noise ratio. The authors’ goal was to quantify the effects of an unflat beam on the image quality of megavoltage portal and fluoroscopic images.Methods: 6 MV projection images were acquired in fluoroscopic and portal modes using an electronic flat-panel imager. The effects of the flattening filter on the relative modulation transfer function (MTF) and contrast-to-noise ratio were quantified using the QC3 phantom. The impact of FF removal on the contrast-to-noise ratio of gold fiducial markers also was studied under various scatter conditions.Results: The unflat beam had improved contrast resolution, up to 40% increase in MTF contrast at the highest frequency measured (0.75 line pairs/mm). The contrast-to-noise ratio was increased as expected from the increased photon flux. The visualization of fiducial markers was markedly better using the unflat beam under all scatter conditions, enabling visualization of thin gold fiducial markers, the thinnest of which was not visible using the unflat beam.Conclusions: The removal of the flattening filter from a clinical linac leads to quantifiable improvements in the image quality of megavoltage projection images. These gains enable observers to more easily visualize thin fiducial markers and track their motion on fluoroscopic images.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2016

Deep Inspiration Breath Hold—Based Radiation Therapy: A Clinical Review

Several recent developments in linear accelerator–based radiation therapy (RT) such as fast multileaf collimators, accelerated intensity modulation paradigms like volumeric modulated arc therapy and flattening filter-free (FFF) high-dose-rate therapy have dramatically shortened the duration of treatment fractions. Deliverable photon dose distributions have approached physical complexity limits as a consequence of precise dose calculation algorithms and online 3-dimensional image guided patient positioning (image guided RT). Simultaneously, beam quality and treatment speed have continuously been improved in particle beam therapy, especially for scanned particle beams. Applying complex treatment plans with steep dose gradients requires strategies to mitigate and compensate for motion effects in general, particularly breathing motion. Intrafractional breathing-related motion results in uncertainties in dose delivery and thus in target coverage. As a consequence, generous margins have been used, which, in turn, increases exposure to organs at risk. Particle therapy, particularly with scanned beams, poses additional problems such as interplay effects and range uncertainties. Among advanced strategies to compensate breathing motion such as beam gating and tracking, deep inspiration breath hold (DIBH) gating is particularly advantageous in several respects, not only for hypofractionated, high single-dose stereotactic body RT of lung, liver, and upper abdominal lesions but also for normofractionated treatment of thoracic tumors such as lung cancer, mediastinal lymphomas, and breast cancer. This review provides an in-depth discussion of the rationale and technical implementation of DIBH gating for hypofractionated and normofractionated RT of intrathoracic and upper abdominal tumors in photon and proton RT.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2014

WE-G-17A-06: A Water Calorimeter for Use in MRI Linacs

Purpose: At VSL, Dutch Metrology Institute, a new water calorimeter was developed with the purpose to replace the existing primary standard for absorbed dose to water in the Netherlands. The new water calorimeter is designed to be operable in medium- to high energy photon beams, electrons, protons as well as MRI integrated linear accelerators. VSL has operated a water calorimeter since 2001. This calorimeter formed the basis for the NCS-18 dosimetry protocol, which is commonly applied by medical physicists in the Netherlands and Belgium. Methods: The unit Gray is the unit of interest for measurement of the absorbed dose to water. Water calorimetry involves the measurement of a small temperature rise (0.24 mK/Gy) with an uncertainty of less than 1 μK/Gy at a temperature of 4 °C. Using extensive multi-physics simulations the new calorimeter's thermal performance was simulated before it was constructed at the end of 2013. With the advent of radiotherapy treatment units incorporating MR imaging the performance of the thermistor temperature sensors were characterized in a 1.5 T magnetic field. Results: A change of thermistor resistance was observed of less than 0.004% as a Result of the magneto-resistance effect in a 1.5 T magnetic field. Although a magneto-resistance effect was detectable, the effect on the temperature response in the water calorimeter was found to be negligible. Conclusion: With the realization of the new calorimeter operable in MRI linacs and designed for use in a variety of beam modalities, VSL is ready for accurate dosimetry in new advanced radiotherapy modalities. Due to the small form factor the calorimeter can be used on location in the actual therapy beam inside a 68 cm linac bore. This work was supported by EMRP grant HLT06. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.

07 ISOTOPE AND RADIATION SOURCES↗