Dosimetric comparison of surface mould HDR brachytherapy with VMAT.

INTRODUCTION: The aim of this study was to investigate the dosimetric differences between surface mould high-dose-rate (HDR) brachytherapy and external beam volumetric-modulated arc therapy (VMAT) for two treatment sites.

METHODS: Previously treated HDR brachytherapy surface mould scalp (n = 4) and lower leg (n = 3) treatments were retrospectively analysed. The VMAT plans were optimised using an additional 3-mm setup margin on the clinical target volume (CTV) of the previously treated HDR plans. The HDR plans were calculated and normalised using the TG-43 formalism and recalculated with Acuros BV (AC).

RESULTS: On average, the mean brain and normal tissue doses were reduced by 44.8% and 27.4% for scalp and lower leg VMAT cases, respectively, when compared to AC calculated HDR plans. For VMAT plans, the average dose to a 1-mm thick skin structure deep to the target volume was not any lower than that in AC HDR plans. On average, the CTV coverage was 13.8% and 9.6% lower for scalp cases with AC dose calculation than with TG-43 and 8.3% and 5.3% lower for lower leg cases if 0- or 1-cm backscatter material was applied above the catheters, respectively.

CONCLUSIONS: VMAT is a feasible treatment option in the case of extensive skin malignancies of the scalp and lower leg. Uncertainties related to delivered dose with HDR brachytherapy when using the TG-43 dose calculation model or possible air gaps between the mould and skin favour the use of VMAT. The potential soft tissue deformation needs to be considered if VMAT is used.