Highly conformal CT based surface mould brachytherapy for non-melanoma skin cancers of earlobe and nose

Purpose: 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 like nose and earlobe. Evidences in this field are scarce. We describe computed tomography (CT) based surface mould BT for non-melanoma skin cancers (NMSC), and compare its conformity, dose coverage, and tissue sparing ability to EBRT.

Material and methods: We describe procedure of preparation of surface mould applicator and dosimetry parameters of BT plans, which were implemented in 10 individuals with NMSC of nose and earlobe. 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) (VPTV₉₀-150), conformal index for 90 and 100% of PD (COIN₉₀, COIN₁₀₀), dose homogeneity index (DHI), dose nonuniformity ratio (DNR), exposure of organs. Prospectively, we created CT-based photons and electrons plans. We compared conformity (COIN₉₀, COIN₁₀₀), dose coverage of PTV (D₉₀, VPTV₉₀, VPTV₁₀₀), volumes of body receiving 10-90% of PD (V₁₀-V₉₀) of EBRT and BT plans.

Results: We obtained mean BT-DHI = 0.76, BT-DNR = 0.23, EBRT-DHI = 1.26. We observed no significant differences in VPTV₉₀ and D₉₀ between BT and EBRT. Mean BT-VPTV₁₀₀ (89.4%) was higher than EBRT-VPTV₁₀₀ (71.2%). Both COIN₉₀ (BT-COIN₉₀ = 0.46 vs. EBRT-COIN₉₀ = 0.21) and COIN₁₀₀ (BT-COIN₁₀₀ = 0.52 vs. EBRT-COIN₁₀₀ = 0.26) were superior for BT plans. We observed more exposure of normal tissues for small doses in BT plans (V₁₀, V₂₀), for high doses in EBRT plans (V₇₀, V₉₀).

Conclusions: Computed tmography-based surface mould brachytherapy for superficial lesions on irregular surfaces is a highly conformal method with good homogeneity. Brachytherapy is superior to EBRT in those locations in terms of conformity and normal tissue sparing ability in high doses.