SU-E-T-407: Investigation of the Need for Bolus for Superficial Head and Neck Tumors Treated with 3D, RapidArc, and IMRT

Abstract

Purpose: To determine whether or not the multiple oblique and tangential beam angles of RapidArc and IMRT treatment plans eliminate the need for bolus when treating superficial tumors. This study uses film measurements to evaluate the efficacy of IMRT and RapidArc planning techniques employed to achieve adequate surface dose without bolus. Methods: Gafchromic EBT2 QD+ film was used to measure skin dose for plans created using Eclipse 8.9 and delivered via a Varian 21iX linac. Dose was delivered to a cylindrical Catphan (CP504) and the film measurements were evaluated using in‐house software. Measurements were compared to results from the treatment planning system calculated using Varian's Anisotropic Analytical Algorithm (8.9.08). Results: The Gafchromic film measurements showed that the 7‐field IMRT plan with strategically placed beams tangential to the phantom surface did increase skin dose 63% compared with a non‐bolused 3D plan. RapidArc delivery increased the skin dose by an additional 13% beyond IMRT. However, even with RapidArc, the skin dose obtained was only 44% of the prescribed dose (2Gy) showing that bolus is still required. Attempts to modify planning technique by adding skin flash to lessen the dose falloff and expanding the external contour to match had no effect on skin dose. Incidentally, we found that the 5mm bolus often used for head and neck plans only brought the surface dose to 78% of the prescription dose (in the case of 3D) which may still be clinically inadequate. Conclusions: IMRT and RapidArc treatment plans delivered without bolus do increase skin dose compared with their 3D counterparts, but the effect is not great enough avoid using bolus when prescription surface dose is required. While planning techniques to lessen dose fall off at the skin may be useful for managing setup uncertainty, they do not increase surface dose in any way.