Out‐of‐field dose assessment for a 1.5 T MR‐Linac with optically stimulated luminescence dosimeters

PURPOSE: To assess the out‐of‐field surface and internal dose of the 1.5 T MR‐Linac compared to the conventional external beam linac using optically stimulated luminescence dosimeters (OSLDs), and evaluate the out‐of‐field dose calculation accuracy of the Monaco treatment planning system (TPS) of the 1.5T MR‐Linac. METHODS: A cubic solid water phantom, with OSLDs on the surface, was vertically irradiated by MR‐Linac square fields with different sizes. In addition, OSLDs were arranged out of the beam edges in four directions. An anthropomorphic adult phantom, with 125 cm(3) simulated volume, was irradiated in four orthogonal directions by both MR‐Linac and conventional linac at the head, thoracic, and pelvic sites. Out‐of‐field doses were measured by OSLDs on both the surface and internal emulational organs at risk (OARs). The results were compared to the simulated dose from Monaco TPS. RESULTS: At different field sizes (5 × 5 to 20 × 20 cm(2)) and distances (1 to 10 cm) to beam edge, the out‐of‐field surface dose measured on MR‐Linac varied from 0.16 % (10 cm to 5 × 5 cm(2) edge) to 7.02 % (1 cm to 20 × 20 cm(2) edge) of the maximum dose laterally and from 0.14 % (10 cm to 5 × 5 cm(2) edge) to 8.56 % (1 cm to 20 × 20 cm(2) edge) of the maximum dose longitudinally. Compared to the OSLDs measured data, the Monaco TPS presented an overestimate of the out‐of‐field dose of OARs at 0–2 % isodose area on both surface and internal check points, and the overestimation gets greater as the distance increases. The underestimation was found to be 0–35% at 2–5% isodose area on both surface and internal check points. Compared to the conventional linac, MR‐Linac delivered higher average values of out‐of‐field dose on surface check points (20%, 19%, 21%) and internal simulated OARs (42%, 37%, 9%) of the anthropomorphic phantom at head, thoracic, and pelvic irradiations, respectively. CONCLUSIONS: Compared to the conventional linac, MR‐Linac has the same out‐of‐field dose distribution. However, considering the absolute dose values, MR‐Linac delivered relatively higher out‐of‐field doses on both surface and internal OARs. Additional radiation shielding to patients undergoing MR‐Linac may provide protection from out‐of‐field exposure.