A dosimetric study of electron beam therapy vs. high-dose-rate mould brachytherapy in adjuvant treatment of non-melanoma skin carcinomas of the head and neck region

Purpose
Radiation for superficial tumours of the head and neck region can be given either by brachytherapy or electrons. Brachytherapy (BT), due to rapid dose fall-off and minor set-up errors, should be superior to external beam radiotherapy (EBRT) for treatment of lesions in difficult locations such as the nose and earlobe. The present study is a dosimetric comparison of computed tomography (CT)-based mould brachytherapy treatment plans with 3D conformal electron beam therapy in the treatment of non-melanoma skin cancers (NMSC).

Material and methods
From December 2017 to November 2018 10 patients with NMSC of the head and neck region (forehead, nose, cheek) who underwent adjuvant radiation with HDR brachytherapy (BT) with a surface mould individual applicator were enrolled for analysis. We evaluated dose coverage by minimal dose to 90% of planning target volume (PTV, D₉₀), volumes of PTV receiving 90-150% of prescribed dose (PD) (VPTV_90-150), conformal index for 90% and 100% of PD (COIN₉₀, COIN₁₀₀), dose homogeneity index (DHI), dose nonuniformity ratio (DNR), and exposure of organs at risk (OARs) (eyes, lens, underlying bone and skin). Prospectively, we created CT-based treatment plans for electron beam therapy. We compared conformity (COIN₉₀, COIN₁₀₀), dose coverage of PTV (D₉₀, VPTV₉₀, VPTV₁₀₀), volumes of body receiving 10-90% of PD (V₁₀-V₉₀), doses to OARs (D_0.1cc and D_2cc) of BT and electron plans.

Results
We obtained mean BT-DHI 0.81, BT-DNR 0.608, Electron-DHI 1.25. We observed no significant differences in VPTV₉₀,₁₀₀ and D₉₀ between BT and electron beam. Mean BT-VPTV₁₂₅,₁₅₀ were significantly higher than Electron-VPTV₁₀₀,₁₂₅. COIN₉₀ was superior for BT plans.

Conclusions
CT-based surface mould brachytherapy results in better conformity of superficial lesions on small, irregular surfaces such as the nose and inner canthus than electrons with a slightly higher skin dose.