Dosimetric evaluation of superficial radionuclide-based high-dose-rate brachytherapy for deep keratinocyte carcinoma: A case series

Purpose: Keratinocyte carcinoma (KC) is the most common malignancy in the United States. Current American Brachytherapy Society and GEC-ESTRO guidelines recommend interstitial high-dose-rate (HDR) brachytherapy for tumors exceeding 5 mm in depth. This case series evaluated the feasibility and efficacy of superficial radionuclide-based HDR brachytherapy for deep (> 5 mm) KC. Material and methods: Four patients with pathologically confirmed KC were treated at a single institution between March 2019 and November 2023. All received 40 Gy in 8 fractions, delivered twice weekly. Three patients with deeper lesions were treated using a custom, in-house multichannel flap applicator, with a 1 cm source-to-skin distance to achieve adequate dose at depth while minimizing V₁₅₀ to superficial tissues. One patient with a lip lesion was treated using a 0.5 cm source-to-skin distance to limit dose penetration. All patients underwent individualized CT-based planning. Dosimetric parameters including V₁₅₀, D_95%, mean dose, and maximum dose were analyzed. Results: The median patient age was 90.2 years (range, 81-95), and the median tumor depth was 7.6 mm (range, 5.6-10.8). The median V₁₅₀ was 0.0057 cc (range, 0-0.09), the median D_95% was 95.1% of the prescribed dose (range, 94.9-102.4%), and the median mean dose and maximum dose were 44.72 Gy (range, 41.61-48.38) and 71.24 Gy (range, 52.23-87.45), respectively. No grade ≥ 3 acute toxicities were observed, and cosmetic outcomes were favorable. Conclusions: Superficial radionuclide-based HDR brachytherapy is a safe and feasible modality for treating deep (> 5 mm) KC. A 1 cm source-to-skin distance applicator optimizes dose distribution for thicker lesions while maintaining acceptable toxicity and cosmesis. Further prospective studies are warranted to validate long-term efficacy.