Posterior Skin Dose Considerations for Rectal Cancer Treatment with Volumetric Modulated Arc Therapy in the Supine Orientation

Background: One method for the radiation therapy of rectal cancer is to set patients supine and treat them with volumetric modulated arc therapy (VMAT). The posterior skin dose is of concern due to undesirable bolusing from mounting surfaces the patient lays upon, namely the carbon fiber couch (CFC). The posterior skin dose may be mitigated by positioning the patient on top of a low-density material that separates the patient from the CFC. Purpose: Our objective was to determine the reduction in the posterior surface dose when a mattress or foam board is used to prop the patient away from the CFC. Materials and Methods: Three clinical rectal cancer patient VMAT plans were selected. A solid water phantom with optically stimulated luminescence dosimeters (OSLDs) placed at the posterior surface was mounted using three setups: directly on the CFC, with a mattress on the CFC, and with a 10 cm thick foam board on the CFC. The three VMAT plans were delivered to this phantom, with OSLDs measuring the posterior surface dose with each setup. In the treatment planning system (TPS), the CFC only, mattress, and foam board setups were simulated on the patient’s anatomy with posterior surface doses reported. Results: The OSLD measurements in the phantom showed that the mattress reduced the posterior surface dose on average by 1.3%, and the foam board reduced the dose by 8.3%. The TPS estimates demonstrated that, on average, the mattress reduced the surface dose by 15.8%, and the foam board reduced the dose by 33.0%. It is likely that the TPS had limitations accurately modeling the surface dose, so OSLD measurements were closer to clinical reality. Conclusions: The mattress does not reduce the posterior skin dose enough to warrant its use as a skin sparing device. The CFC produces a bolusing effect that can be reduced by separating the patient from the CFC with a 10 cm thick foam board.