In Situ Onycholemmal Carcinoma in a Young Adult with Germline RB1 Mutation: A Case Report

INTRODUCTION: In situ onycholemmal carcinoma, also termed atypical or malignant onychopapilloma, is an exceptionally rare nail unit neoplasm that is clinically difficult to distinguish from its benign counterpart. We report the youngest documented case of this entity and the first occurring in a patient with hereditary retinoblastoma due to a germline RB1 mutation. CASE PRESENTATION: A 20-year-old woman with hereditary retinoblastoma presented with a 2-year history of progressive longitudinal erythronychia of the left second fingernail. Longitudinal excisional biopsy revealed an atypical epithelial proliferation with onycholemmal keratinization, diffuse Ki-67 expression, and negative p16 staining, consistent with in situ onycholemmal carcinoma. The lesion was successfully treated with Mohs micrographic surgery, and no recurrence was observed after 6 months. CONCLUSION: This case expands the clinical spectrum of in situ onycholemmal carcinoma by demonstrating its occurrence in a young patient with a germline RB1 pathogenic variant. The findings raise the possibility of a role for RB pathway dysregulation in tumor pathogenesis and underscore the importance of maintaining a low threshold for biopsy of evolving longitudinal erythronychia, particularly in patients with inherited cancer predisposition syndromes.