Pigmented (melanotic) neurofibroma - A clinicopathologic and immunohistochemical analysis of 19 lesions from 17 patients

Neurofibromas with melanin-laden pigmented cells are rare, accounting for l ess than 1% of all neurofibromas accessioned to the Soft Tissue Registry of the Armed Forces Institute of Pathology between the years 1970 and 1996. T his study analyzes the clinicopathologic features associated with 19 specim ens removed from 17 patients. Eleven males and six females, ranging in age from 2 to 61 years (median, 28 years), participated in the study. Nine of 1 5 patients whose race was provided were black. Eight patients (47%) are kno wn to have neurofibromatosis, and two others (12%) are strongly suspected o f having this disorder; two patients have similarly affected family members . Eight patients were noted to have multiple skin tumors, and in each of tw o cases, two pigmented neurofibromas were available for review. Two patient s had hypertrichosis and cutaneous hyperpigmentation resembling a hairy nev us, and one had a cafe au lait spot directly overlying a pigmented neurofib roma. Tumors ranged in size from 1.7 to 50 cm in greatest dimension and inv olved the buttock or leg (n = 6), head or neck (n = 8), trunk (n = 2), wris t or hand(n = 2), and an unspecified site (n = 1). The neurofibromas exhibi ted diffuse (n = 15), combined diffuse and plexiform (n = 2), combined diff use and intraneural epithelioid(n = 1), and nonspecific (n = 1) growth patt erns. The process involved the skin (n = 14), subcutis (n = 18), and/or ske letal muscle (n = 3). Wagner-Meissner-like bodies were identified in 11 tum ors, and mitoses (average, less than one mitosis per 10 high-power fields) were present in three lesions. All examples contained scattered pigmented c ells with dendritic, tadpole-shaped, spindled or epithelioid morphology. Th ese cells were positive with Fontana-Masson (nine of nine) and Warthin-Star ry (pH, 3.2; four of four) stains, and were depigmented with a melanin blea ch method (two of two). An iron stain was negative. The tumors had immunore activity for S-100 protein (11 of 11), HMB-45 (10 of 11), Melan-A (four of four), tyrosinase (four of four), and CD34 (four of four). Although recurre nces are documented, none of the tumors are known to have undergone maligna nt transformation. A pigmented neurofibroma can be confused with a pigmente d dermatofibrosarcoma protuberans (Bednar tumor) because the melanin-laden cells of both processes are similar. However, the latter entity exhibits a more extensive storiform growth, has greater immunoreactivity for CD34, and lacks a diffuse proliferation of S-100 protein positive Schwann cells.