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 V 150 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 V 150 , 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 V 150 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,) 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.