Giant Atypical Neurofibroma of the Calf in Neurofibromatosis Type 1: Case Report and Literature Review

TL;DR

A 33-year-old man with Neurofibromatosis Type 1 developed a large, atypical neurofibroma in his calf, which was surgically removed and showed no recurrence after 30 months.

Contribution

This case report adds to the understanding of atypical neurofibromatous neoplasms in NF1 and their management.

Findings

The tumor showed hypercellularity, moderate atypia, and a high Ki-67 index, consistent with ANNUBP.

Wide surgical resection was performed, and no local recurrence was observed at 30-month follow-up.

The case emphasizes the importance of pathology in diagnosing high-risk precursor lesions in NF1.

Abstract

Background and Clinical Significance: Neurofibromatosis type 1 (NF1) predisposes individuals to various peripheral nerve sheath tumors (PNSTs), including benign neurofibromas, malignant peripheral nerve sheath tumors (MPNSTs), and intermediate lesions known as atypical neurofibromatous neoplasms of uncertain biologic potential (ANNUBP), previously often termed atypical neurofibroma. These atypical lesions are considered premalignant precursors to MPNST. Case Presentation: We present the case of a 33-year-old male with NF1 who developed a rapidly growing, painful mass in his right calf. Clinical examination revealed signs consistent with NF1. Magnetic resonance imaging showed a large, heterogeneous mass in the lateral compartment. Biopsy revealed a neurofibroma with hypercellularity, moderate atypia, scarce S100 positivity, focal CD34 positivity, and an elevated Ki-67 proliferation index of 10–12%, consistent with ANNUBP. The patient underwent wide surgical resection, including the fibula and peroneal muscles. At the 30-month follow-up, there was no local recurrence, though the patient had a mild residual limp. Discussion: This case highlights the clinical presentation, diagnostic features, and management considerations for ANNUBP in NF1, emphasizing the importance of recognizing warning signs and the role of pathology in guiding treatment for these high-risk precursor lesions.

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