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.