Prone Breast Radiotherapy on the MR-Linac: A Technical Note on Skin-Couch Contact Dose Effects.

Accelerated partial breast irradiation (APBI) delivers a higher dose to a smaller breast volume with equivalent tumour control to whole breast irradiation and improved cosmesis. The MR-Linac offers superior soft tissue visualisation and adaptive planning, but limited bore size may result in skin-couch contact, potentially affecting skin dose. This retrospective pilot study evaluated the effect of MR-Linac couch contact on skin dose. Three patients with left-sided breast cancer positioned prone with unavoidable skin-couch contact were included. MR images were acquired using a 1.5 T MR simulator and contoured for target and organs-at-risk. Step-and-shoot IMRT plans (30Gy in 5 fractions) were optimised and recalculated with and without magnetic field presence. Dose to the whole skin rind (Skin_Rind) and couch-contact region (Skin_Rind_Ant) were compared. All plans met target and OAR constraints. Mean Skin_Rind dose was consistently higher with magnetic field presence, while dose to Skin_Rind_Ant was consistently lower. These findings contrast with conventional linac data, suggesting magnetic field and interface effects may alter skin dose deposition.