Semester of Graduation
Master of Science (MS)
Physics and Astronomy
Introduction: As noted in National Council on Radiation Protection and Measurements (NCRP) Report 151, the medical physicist or other qualified expert has the responsibility to keep abreast of any new technology or treatment method that could potentially impact structural shielding design. Volumetric arc therapy (VMAT) became prevalent after the publication of Report 151 and thus was not explicitly addressed in Report 151. If the shielding-related characteristics of VMAT differ enough from the expectations of Report 151, especially in the circumstance of a vault utilized exclusively for VMAT, a shielding design based on Report 151 could potentially be inadequate. The goal of this work was (1) to assess the characteristics of VMAT deliveries that potentially impact shielding design, and (2) to determine if Report 151, as published and incorporating only minimal conservative design choices, will result in a safe shielding design for a dedicated VMAT treatment vault. Three shielding parameters were characterized: workload, VMAT modulation factor, and use factors.Materials and Methods: A secondary verification system, MobiusLog (Mobius Medical Systems, LP, Houston, TX) was used to obtain real-time mechanical tracking of VMAT deliveries on five linear accelerators at three facilities over a 4-month period. After anonymization, these log files were analyzed by individual accelerators and by type of treatment site (e.g., chest, head & neck, prostate), as well as collectively. The fractionation scheme for each patient was used to compute weekly workloads and VMAT modulation factors. Use factors were determined from reported gantry positions during treatment, for 90° (conventional), 45° (IMRT), 30°, and 15° angular binning intervals, and compared to Report 151. Using the VMAT-specific parameters, shielding was designed for a sample vault; these barriers were compared to barriers designed with parameters from Report 151.Results: Workloads derived from the log files agreed with independent clinic records, indicating the log files contained complete delivery information. Composited over all accelerators and treatment sites, the use factors were essentially uniformly distributed around the full circle; deviations from uniform were most noticeable at the smaller binning intervals and for the 0° (beam directed at floor) and 180° (beam directed at ceiling) intervals. The use factors per treatment site were also relatively uniformly distributed, except for lung and chest sites. The sample vault design showed that the Report 151 use factors occasionally underestimated barrier requirements compared to log file-based VMAT use factors or the assumption of uniform gantry rotation. The inclusion of reasonable conservative margins on calculations may allow Report 151-based barriers to be adequately safe. A tapered ceiling barrier that is designed using Report 151’s 45° (IMRT) interval use factors will not yield a safe shielding design. The VMAT-specific data should not significantly impact the design calculations for secondary barriers or doors, primarily because these barriers are independent of use factor.Conclusion: The VMAT-specific use factors reported in this work, as well as the assumption of uniformly-distributed use factors, consistently led to primary barrier thicknesses that were at least as safe as those calculated from published Report 151 use factors. Shielding calculations based on Report 151 can produce adequate primary barriers for a dedicated VMAT vault only if the qualified expert incorporates sufficient conservative overestimates, i.e., enough to compensate for the larger VMAT use factors, in some barriers. A tapered ceiling barrier in a dedicated VMAT vault should be designed using either VMAT-specific use factors or assumed uniformly-distributed use factors, not the published 45° (IMRT) interval use factors of Report 151. The assumption of uniformly distributed use factors is reasonable for designing a general-purpose VMAT vault, but treatment site-specific use factors should be used to design a vault that will be used heavily for only one (or a few) treatment sites, such as chest or lung.
Dieguez, Ana Lucia, "Structural Shielding Considerations for VMAT" (2021). LSU Master's Theses. 5380.
Matthews, Kenneth L. II