Diagnosing an Atypical, Melan-A-Positive Granular Cell Tumor with Extensive Intraneural and Perineural Spread: Case Report

Introduction: This case report demonstrated an uncommon manifestation of Melan-A positive granular cell tumors (GCTs), typically benign and originating from Schwann cells. Challenges related to histologic variants, infiltrative growth patterns, perineural invasion, and Melan-A expression are discussed. The study covers GCTs histogenesis, clinical associations, differential diagnosis with melanoma, and criteria for identifying malignant GCTs. A suggested immunohistochemical panel, including Inhibin-A and PRAME, is proposed for Melan-A positive GCTs.

Case Presentation: A 27-year-old male presented with two tender subcutaneous nodules (approximately 0.5 cm each) on the left second digit's dorsal aspect and the right hand's proximal palm. Clinical differentials included cyst, neuroma, and fibromatosis. The patient had a previous benign GCT on the back. Personal and family medical histories were unremarkable.

Histologic Examination: Biopsy of the left dorsal second digit revealed a GCT with classic appearance, similar to the patient's previous back nodule. The palm nodule biopsy showed a circumscribed elongated lesion with intraneural and perineural spread. Immunohistochemical evaluation demonstrated positivity for S-100, Inhibin-a, and Melan-A, and negativity for HMB-45 and PRAME.

Conclusions: This report highlights a unique case of multiple GCTs, one with extensive intraneural and perineural involvement and rare Melan-A positivity in a patient with a prior GCT. The rarity of Melan-A positivity in GCTs poses diagnostic challenges, due to it resembling melanoma. Use of a comprehensive immunohistochemical panel, including S-100, Inhibin, Melan-A, HMB-45, and PRAME, demonstrated efficacy in resolving diagnostic dilemmas.