Room: ePoster Forums
Purpose: To date, there has not been a formal approach for vault shielding evaluation to accommodate the new technology ring-geometry linac of HalcyonTM with an in-line single 6MV photon flattening filter-free (FFF) beam and a beam-stopper at the opposite end.
Methods: We developed a method for performing shielding calculations for a new vault or for providing shielding adequacy assessment for an existing vault that will accommodate a Halcyon linac. The method addressed shielding considerations for a FFF linac beam, with inclusion of the specific features of the Halcyon beam and MLC system. Due to it specific features, Halcyon with a primary beam transmission factor of less than 0.1% @1m from the isocenter, is positioned optimally for a linac replacement option in a radiotherapy vault. Thus, there is an increased importance of the leakage radiation compared to the primary and patient scatter components, which is against the common practice for a standard linac vault shielding assessment. The formalism is using the NCRP#151 approach modified appropriately for the special situation. Comparisons of primary, leakage and patient scatter components were made based on the intended clinical use of this lianc, being primarily a fast IMRT/VMAT delivery solution.
Results: We have shown that, for Halcyon the leakage barrier thickness is larger by 0.5TVL for 100% conventional treatments and by 1TVL for 100% IMRT, pointing to an interplay of the importance between leakage and primary. The patient scatter contribution was the smallest than the primary and leakage.
Conclusion: A comprehensive method for shielding barrier calculations for a the new Halcyon linac platform was developed. Based on our findings, we recommend the evaluation of both primary and leakage for Halcyon installations for potential interplay of their importance depending on the vault dimensions and intending use of the unit.