Updated dosimetric comparison of HDR interventional radiotherapy including ⁶⁰Co sources versus VMAT and electron beam...

Non-melanoma skin cancers (NMSCs), primarily basal cell carcinoma and squamous cell carcinoma, are the most common malignancies worldwide and frequently arise on the scalp, where complex curvature and anatomical constraints make treatment planning challenging. While surgery remains the standard approach, radiotherapy, particularly high-dose-rate interventional radiotherapy (HDR-IRT, modern brachytherapy), has become an important alternative for patients who are not ideal surgical candidates. Previous dosimetric analyses have demonstrated that ¹⁹²I HDR-IRT provides superior target coverage and dose conformity compared with external beam techniques such as volumetric modulated arc therapy (VMAT) and electron beam therapy (EBT). However, the reproducibility of these findings across centres and radioactive sources, including ⁶⁰Co, has not been systematically validated. This study aimed to independently validate prior dosimetric results by independently replanning the same cohort of 25 patients with scalp NMSC using ⁶⁰Co HDR-IRT. The objective was to assess whether the dosimetric advantages of HDR-IRT remain consistent when different operators, institutions, and isotopes are employed, and to compare ⁶⁰Co HDR-IRT with VMAT and EBT under identical clinical conditions. A retrospective dosimetric comparison was conducted using the original CT datasets from 25 patients previously analyzed with ¹⁹²Ir HDR-IRT. At an independent institution, new plans were generated for ⁶⁰Co HDR-IRT, VMAT, and EBT using the same contours, applicator geometry, prescription doses, and V_95% constraints. HDR-IRT plans were constructed in Oncentra Brachy v4.6 based on TG-43 formalism; VMAT and EBT plans were generated in Eclipse TPS v16.1 using 6 MV photons and 6 MeV electrons, respectively. Dosimetric parameters (V_90%, V_95%, V_100%, D_90%, D_mean, D_max) and brain D2cc values were extracted from DVHs. Statistical comparisons were performed with the Mann-Whitney U test (p < 0.05). All techniques provided comparable V_90% (> 99%) and V_95% values, ensuring consistent baseline coverage. However, HDR-IRT using Co-60 demonstrated significantly higher V_100%, D_90%, and D_mean values than VMAT and EBT across all prescription groups (p < 0.05). This reflects the proximity of the source to the target and the steep dose gradient inherent to brachytherapy. VMAT produced the most homogeneous dose distributions but at the expense of slightly reduced high-dose coverage. EBT performed adequately for small, flat lesions but showed pronounced limitations in large or curved anatomies, with increased variability and occasional hotspots in the brain. Brain D_2cc doses remained comparable across modalities (p > 0.05), confirming adequate protection of critical structures. This external validation confirms that HDR-IRT retains its dosimetric superiority over VMAT and EBT even when replanned by different operators, at a different institution, and using an alternative isotope (⁶⁰Co instead of ¹⁹²Ir). The findings demonstrate that HDR-IRT is a robust and reproduciblemodality for scalp NMSC, capable of achieving excellent target coverage and rapid dose fall-off without compromising organ-at-risk sparing. These results strengthen the evidence supporting HDR-IRT as a reliable option for treating superficial scalp cancers across diverse clinical settings.