Effect of topical agents on skin surface dose in volumetric modulated arc therapy for head and neck cancer

In volumetric modulated arc therapy (VMAT), the effect of an increase in skin surface dose due to topical agents might be negligible. We investigated the bolus effects of three types of topical agents in VMAT for head and neck cancer (HNC). Topical agents of different thicknesses (0.1, 0.5 and 2 mm) were prepared. When each topical agent was set, the surface doses were measured for the anterior static field and VMAT, with and without a thermoplastic mask. No significant differences were observed among the three topical agents. For topical agent thicknesses of 0.1, 0.5 and 2 mm, the increases in surface dose for the anterior static field without the thermoplastic mask were 7–9, 30–31 and 81–84%, respectively. With the thermoplastic mask, the corresponding increases were 5, 12–15 and 41–43%, respectively. The increases in surface dose for VMAT without the thermoplastic mask were 5–8, 16–19 and 36–39%, respectively, and those with the thermoplastic mask were 4, 7–10 and 15–19%, respectively. The rate of increase in surface dose with the thermoplastic mask was smaller than that without the thermoplastic mask. The increase in surface dose with topical agents of clinical standard thickness (0.02 mm) was estimated to be 2% with the thermoplastic mask. The increase in surface dose with topical agents in dosimetric simulation, compared with control situation, is not significant in clinical conditions for HNC patients.