SU‐E‐T‐670: Radiotherapy Vault Shielding Evaluation Method for a Flattening Filter‐Free (FFF) Linac‐Practical Considerations and Recommendations

Abstract

Purpose:To date, there isn't formal approach for flattening filter‐free (FFF) linac vault shielding evaluation, thus, we propose an extension to NCRP#151 to accommodate the recent large number of FFF linac installations.Methods and Materials: We extended the approach in NCRP#151 to design two Truebeam vaults in our new cancer center for hypofractionated treatments. Monte Carlo calculations have characterized primary, scattered, leakage and neutron radiations from FFF‐modes. These calculations have shown that: a) FFF primary beam is softer on the central‐axis compared to flattening filtered (FF), b) the lateral dose profile is peaked on the central axis and less integral target current is required to generate the same tumor dose with the FF beam. Thus, the TVLs for FFF mode are smaller than those of the FF mode and the scatter functions of the FF mode (NCRP#151) may not be appropriate for FFF‐mode, c) the neutron source strength and fluence for 18X‐FFF is smaller than 18X‐FF, but it is not of a concern here, no 18X‐FFF‐mode is available on the linac under investigation.Results:These barrier thickness are smaller (12% reduction on the average) than those computed for conventional FF mode with same realistic primary workload since, the primary TVLs used here are smaller and the WL is smaller than the conventional (almost half reduced), keeping the TADR in tolerance.Conclusions:A comprehensive method for shielding barrier calculations based on dedicated data for FFF‐mode linacs is highly desired. Meanwhile, we provide an extension to NCRP#151 to accommodate the shielding design of such installations. It is also shown that if a vault is already designed for IMRT/VMAT and SABR hypofractionated treatments with FFF‐mode linac, the vault can also be used for a FFF mode linac replacement, leaving some leeway for slightly higher workload on the FFF linac.