Dose distribution comparison in volumetric-modulated arc therapy plans for head and neck cancers with and without an external body contour extended technique.

Abstract

This study compared volumetric-modulated arc therapy (VMAT) plans for head and neck cancers with and without an external body contour extended technique (EBCT). Dose calculation algorisms for VMAT have limitations in the buildup region. Three VMAT plans were enrolled, with one case having a metal artifact from an artificial tooth. The proper dose was calculated using Eclipse version 11.0. The body contours were extended 2 cm outward from the skin surface in three-dimensional space, and the dose was recalculated with an anisotropic analytical algorithm (AAA) and Acuros XB (AXB). Monitor units (MUs) were set, and the dose distributions in the planning target volume (PTV), clinical target volume, and organ at risk (OAR) and conformity index (CI) with and without an EBCT were compared. The influence of a metal artifact outside of the thermoplastic head mask was also compared. The coverage of PTV by the 95% dose line near the patient's skin was increased drastically by using an EBCT. Plan renormalization had a negligible impact on MUs and doses delivered to OARs. CI of PTV with a 6-MV photon beam was closer to 1 than that with a 10-MV photon beam when both AAA and AXB were used in all cases. Metal artifacts outside the head mask had no effect on dose distribution. An EBCT is needed to estimate the proper dose at object volumes near the patient's skin and can improve the accuracy of the calculated dose at target volumes.