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Shielding Requirements for a Halcyon Linac in a High-Throughput Clinic

R Kaderka*, K McConnell, D Scanderbeg, R Manger, UC San Diego, La Jolla, CA

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To assess shielding requirements for Varian’s Halcyon™ in a clinic with high patient load. Halcyon™ supports streamlined workflows that enable up to 80 treatments/day in our clinic. The o-ring design differs from traditional c-arm linacs and, as observed with tomotherapy units, this may influence shielding needs. This study investigates the influence of changed workload and design on primary and secondary barriers.

Methods: Halcyon™ is an o-ring mounted linac capable of delivering 6X flattening filter-free beams. NCRP151 was used to calculate primary and secondary barriers. Calculations require geometric parameters, use and occupancy factors, as well as workload estimates. Geometric parameters were taken from architectural blueprints. All treatment records for 2019 were datamined to determine primary and leakage workload. Per NCRP151, we assumed the secondary barrier for the linac leakage component sufficient to shield the scatter component. Halcyon™ contains a beam stopper, which was verified in earlier studies to reduce the primary workload by a factor 10³.

Results: The highest number of patients treated in one day was 80, which was then used for the primary workload with 3 Gy/patient. 20,339,892 monitor units were delivered in 2019, so a conservative average of 4000 Sv/week was used for the leakage workload. For our vault, the primary barriers were found to require 2.43,1.73, and 0.60 tenth-value layers (TVLs) of shielding. The same walls required 3.55,2.86, and 1.73 TVLs of leakage shielding, satisfying the NCRP two-source rule. Other secondary barriers required 2.39-3.65 TVLs.

Conclusion: Our findings confirm that Halcyon™ secondary barriers are sufficient to shield primary beam as leakage is the dominant contributor. Even in a busy radiotherapy clinic with up to 80 patients/day, a maximum of about 4TVLs are required for shielding. This substantially reduces construction costs for clinics looking to buy a Halcyon™ or the upcoming Halcyon™-based Ethos™ system.

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