Carbon fiber couch effects on skin dose for volumetric modulated arcs.
PURPOSE
The purpose of this study is to evaluate the dosimetric effect of carbon fiber couches (CFCs) on delivered skin dose as well as to explore potential venues for its minimization for volumetric modulated arc (VMAT) treatments.


METHODS
A carbon fiber couch (BrainLab) was incorporated in Pinnacle treatment planning system (TPS) by autocontouring. A retrospective investigation on five lung and five prostate patient plans was performed. Targets and organs at risk (OARs), together with a 0.3 cm thick skin contour interfacing the CFC, were outlined in each plan. For each patient, two VMAT plans were generated: a single arc with 6 MV photon energy and two or three arcs with 18 MV photon energy for the posterior arc(s) and 6 MV energy for the anterior arc (mixed energy plans). Both plans for each patient case were normalized such that 95% of the PTV was covered by the same prescription dose, ranging from 7600 to 7800 cGy. For each patient, the prescription doses were escalated to the maximum allowed by the OAR constraints. CFC bolus effects on skin doses were tallied by the highest dose to 1% of skin volume.


RESULTS
With the utilization of higher energy photons for the posterior arcs, the statistically significant differences in skin dose between the two plans were as high as 34% of the prescribed dose, where surface doses changed on average from 3800 to 2940 cGy for 6 MV and mixed energy plans, respectively. In addition, skin doses in excess of 68% and 80% of the prescription doses for mixed and 6 MV energy plans, respectively, were observed in individual cases.


CONCLUSIONS
The presented findings indicate that mixed energy VMAT plans would result in a substantial skin sparing of more than approximately 34% compared to VMAT plans with only 6 MV arc(s). Additionally, the high skin doses in some cases (81% of the prescription dose) suggest that in hypofractionated SRS/SRT treatments, the carbon fiber couch effects on skin doses need to be evaluated when arc delivery is considered as a treatment option.