SU-GG-T-155: Carbon Fiber Couch Effects on Skin Doses for Volumetric Arcs

Abstract

Purpose: To evaluate carbon fiber couch (CFC) dosimetric effects on delivered skindose as well as the dose and photon energy interplay for volumetric modulated arc (VMAT) treatments. Method and Materials: A CFC (BrainLab) was incorporated into a commercial TPS (Pinnacle) by auto‐contouring. A retrospective investigation on five lung and five prostate patient plans was performed. Targets and OARs, together with a skin contour of 0.3 cm thickness in contact with the CFC, were delineated in each plan. For each patient two VMAT plans were generated: a single arc with 6MV photons, and two or three arcs with 18MV photons for the posterior arc(s) and 6 MV photons for the anterior arc. Both plans were normalized such that 95% of the PTV was covered by the same dose, escalated to the maximum allowed by the OAR constraints. CFC effects were tallied by the highest dose to 1% of the skin volume. Results: If 18MV rather than 6MV photons are used in the arcs traversing the CFC the skindose reduction ranges from 12% to more than 80%. In addition, the estimated skindoses range from ∼30% to more then ∼83% of the prescription doses for the mixed energy 6MV/18MV plans, implying even higher fraction of the prescription dose for the 6MV plans. Conclusions: The results indicate that mixed energy VMAT plans would result in a substantial skin sparing of more than ∼80% compared to VMAT plans with only 6MV arc(s). The increase in the treatment time due to the use of additional arcs is insignificant. The high skindoses in some cases (83% of the prescription) suggest that in hypofractionated SRS/SRT the CFC skin effect needs to be considered and promptly evaluated when arc delivery is used. Conflict of Interest: Research sponsored in part by Philips Radiation Oncology Systems.