Comparison of Skin Dose for Brain Cancer Patients Between a 1.5 T MR-Linac and a Conventional Linac

Abstract

The MR-Linac (MRL) is a new technology, and due to the presence of the electron return effect (ERE) elicited by the strong magnetic field, there is concern about there being elevated surface dose due to the ERE. The hypothesis is that the skin doses for the MRL is greater than for the conventional Linac. Six brain cancer patients being treated on our MRL were included in the present study and were planned with an IMRT technique using the MRL’s Monte Carlo based treatment planning system (TPS-MRL). Each patient had a back-up IMRT 6 MV plan for a conventional (no magnetic field) Linac generated on our standard TPS, which used a collapsed cone-based dose engine (TPS-conventional). These back-up plans were clinically delivered during an MRL preventative maintenance day, which provided an opportunity to compare skin dose measurements between MRL and conventional Linac. For one fraction on each Linac, an optically stimulated luminescence dosimeter (OSLD) was placed at the same location on the skin proximal to the target. Within TPS-MRL and TPS-conventional, a dose point was created at the OSLD location and at a skin depth equivalent to the water equivalent thickness of the OSLD (0.6 mm). Five of six patients had higher OSLD readings on the MRL than for the conventional Linac, ranging from 11.5%-24.8% higher (the one exception had the MRL 7.5% less than the conventional Linac). The OSLD measurement for the MRL ranged from 11.8-45.7 Gy; the OSLD doses for the conventional Linac ranged from 10.0-39.4 Gy. The root mean square error between measurement and TPS was 7.3% and 17.4% for MRL and conventional, respectively. This may be explained because the conventional Linac TPS uses a superposition-convolution algorithm, and the MRL TPS uses a theoretically more accurate Monte Carlo algorithm. Skin dose, as determined in the present study with in vivo OSLD measurements, were higher on the MRL than a conventional Linac for brain patients. Furthermore, the skin dose as estimated by the TPS-MRL is more accurately modeled compared to the TPS-conventional. Although the exact cause(s) of the apparent skin dose enhancement can only be speculated, the potential implications of the measurements in the present study include enhanced adverse skin reactions on the MRL compared to conventional Linac. However, the fact that the MRL-TPS models the surface dose with a Monte Carlo dose engine can enable clinicians to incorporate strategies for reducing surface dose at the planning stage.