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An anthropomorphic phantom has been developed by Varian Medical Systems for commissioning multileaf -collimator (MLC), stereotactic radiosurgery (SRS) treatments on Varian TrueBeam and Edge linear accelerators. Northwest Medical Physics Center (NMPC) has collected end-to-end data on these machines, at six independent clinical sites, to establish baseline dosimetric and geometric commissioning criteria for SRS measurements with this phantom. The Varian phantom is designed to accommodate four interchangeable target cassettes, each designed for a specific quality assurance function. End-to-end measurements utilized the phantom to verify the coincidence of treatment isocenter with a hidden target in a Winston-Lutz cassette after localization using cone-beam computed tomography (CBCT). Dose delivery to single target (2 cm) and singleisocenter, multitarget (2 and 1 cm) geometries was verified using ionization chamber and EBT3 film cassettes. A nominal dose of 16 Gy was prescribed for each plan using a site's standard beam geometry for SRS cases.Measurements were performed with three Millennium and three high-definition MLC machines at beam energies of 6-MV and 10-MV flattening-filter-free energies. Each clinical site followed a standardized procedure for phantom simulation, treatment planning, quality assurance, and treatment delivery. All treatment planning and delivery was performed using ARIA oncology information system and Eclipse treatment planning software. The isocenter measurements and irradiated film were analyzed using DoseLab quality assurance software; gamma criteria of 3%/1 mm, 3%/0.5 mm, and 2%/1 mm were applied for film analysis. Based on the data acquired in this work, the recommended commissioning criteria for endto-end SRS measurements with the Varian phantom are as follows: coincidence of treatment isocenter and CBCT-aligned hidden target < 1 mm, agreement of measured chamber dose with calculated dose ≤ 5%, and film gamma passing > 90% for gamma criteria of 3%/1 mm after DoseLab auto-registration shifts ≤ 1 mm in any direction.

Purpose: To determine the accuracy of the BrachyVision HDR treatment planning computer at predicting near source doses in the presence and absence of bone scatter in base of tongue treatments. Materials & Methods: A HDR treatment plan was developed to deliver a uniform planer dose at a depth of 1cm from a custom made planer geometry applicator with 5 catheters separated by 1cm. Plan dwell times were scaled to deliver doses of 10, 20, 50, 75, and 150Gy for a dose calibration curve for GrafChromic HD‐810 film. Dose measurements were performed at distances of 0mm, 1mm, 2mm, 3mm, 5mm, 7mm, & 10mm from the surface of the catheters. Three prescription doses were used for these measurements depending on distance from the catheters; 10Gy (0mm), 15Gy (1mm), & 25Gy (2, 5, 7, 10mm). Measurements were performed in solid water and in solid water directly against and 1mm away from a bone equivalent backscatter (Schedule 80 PVC). Doses were compared using 35 point doses (7 along each of 5 catheters) at 1cm increments. Results: Analysis of the films showed average percent differences of; 24±13%, ∼0±6%, 4±4%, 2±6%, 2±6%, & 4±3% (0, 1, 2, 5, 7, 10mm respectively) for solid water, 31%, 5%, 9%, 11%, 16%, & 14% (0, 1, 2, 5, 7, 10mm respectively) with similar standard deviations to solid water, for directly against the PVC. Conclusions: For this planar geometry are within a standard deviation of predicted doses for a homogeneous media except directly against the catheter. At a plane 5mm from the catheter the presence of high‐z back scatter produced an average increase in dose of 5%.

Purpose: This study aimed to investigate the high dose rate (HDR) 192Ir brachytherapy, including near source dosimetry, of a catheter‐based applicator from 0.5 mm to 1 cm along the transverse axis. Methods: Radiochromic film and Monte Carlo (MC) simulation were used to generate absolute dose for the catheter‐based applicator. Results from radiochromic film and MC simulation were compared directly to the treatment planning system (TPS) based on the AAPM Updated Task Group 43 (TG‐43U1) dose calculation formalism. Results: Difference between dose measured using radiochromic film along the transverse plane at 0.5 mm from the surface and the predicted dose by the TPS was 24%±13%. Dose difference between the MC simulation along the transverse plane at 0.5 mm from the surface and the predicted dose by the TPS was 22.1%±3%. For distances from 1.5 mm to 1 cm from the surface, radiochromic film and MC simulation agreed with TPS within an uncertainty of 3%. Conclusion: The TPS under‐predicts the dose at the surface of the applicator, i.e., 0.5 mm from the catheter surface, as compared to the measured and MC simulation predicted dose. MC simulation results demonstrated that 15% of this error is due to neglecting the beta particles and discrete electrons emanating from the sources and not considered by the TPS and 7% of the difference was due to the photon alone, potentially due to the differences in MC dose modeling, photon spectrum, scoring techniques, and effect of the presence of the catheter and the air gap. Beyond 1mm from the surface, the TPS dose algorithm agrees with the experimental and MC data within 3%.