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Case Reports

Abstract

A 21-year-old female with a known history of Neurofibromatosis Type 1 (NF1) presented with a 0.6 cm slowly enlarging, firm, skin-colored papule on the lower back. Excisional biopsy revealed a dermal, neurofibromatous-type neoplasm with a central focus showing scattered nuclear atypia, hypercellularity, and fascicular architecture. Mitoses were absent, and no necrosis was identified. Immunohistochemical studies showed diffuse S100 positivity throughout the entire lesion. CD34 showed preserved fibroblastic network in the peripheral conventional neurofibroma, but loss in the central atypical focus. The histologic features exceeded those of a conventional neurofibroma but were insufficient for a diagnosis of malignant peripheral nerve sheath tumor (MPNST); therefore, the lesion was diagnosed as an atypical neurofibromatous neoplasm of uncertain biologic potential (ANNUBP), a recently recognized entity occurring almost exclusively in NF1 patients. This case highlights the importance of recognizing ANNUBP as a distinct diagnostic category with potential for malignant transformation, necessitating accurate histologic evaluation and careful clinical follow-up.