Cellular neurothekeoma: detailed characterization in a series of 133 cases.

Cellular neurothekeomas are distinctive benign cutaneous tumors of uncertain histogenesis. As relatively few cases have been reported, their clinical features and morphologic spectrum remain incompletely defined, and the significance of atypical histologic features is uncertain. This study examined the clinicopathologic and immunohistochemical features of 133 cellular neurothekeomas received between 1987 and 2003. There was a 1.8:1 female predominance, with a mean age of 25 years (84% <40 y). Mean tumor size was 1.1 cm (range: 0.3 to 6 cm; 90% <2 cm). The tumors arose most often on the upper limb (35%) or head and neck (33%). Fifty-two percent of the tumors were limited to the dermis, and 48% also involved superficial subcutaneous tissue. In 30% of cases, neurothekeoma was suggested by the referring pathologist; the most common other diagnoses offered were plexiform fibrohistiocytic tumor, benign fibrous histiocytoma, and a low-grade sarcoma. Histologically, most cases were poorly marginated; 33 (25%) infiltrated fat, and 10 (8%) entrapped skeletal muscle (all but 1 situated on the face). Nearly all tumors had a lobulated or micronodular architecture and were composed of nests and bundles of epithelioid to spindled cells with palely eosinophilic cytoplasm, often separated by dense hyaline collagen; 17 (13%) showed focally sheetlike areas, and 5 (4%) were notably plexiform. Myxoid stroma was observed in 38 (29%) tumors; 11 (8%) were predominantly myxoid. Five (4%) showed marked stromal hyalinization. Osteoclastic giant cells were seen in 20 (15%) cases. The mean mitotic rate was 3 per 10 high power fields; 28 (21%) had > or =5 per 10 high power fields. Most tumors showed mild cytologic atypia in the form of nuclear variability and small nucleoli; 33 (25%) contained notably pleomorphic cells. All tumors were reactive for NKI-C3, 110/123 (89%) expressed neuron-specific enolase, 73/127 (57%) showed at least focal staining for smooth muscle actin, and only 1 was focally desmin positive. All tumors were negative for S-100 protein. Follow-up ranged from 5 to 146 months (mean 44 mo). Ten tumors recurred locally (7 situated on the face), after a mean of 18 months; tumor had been marginally excised or had involved excision margins in all cases with available information. No other clinical or pathologic features correlated with recurrence. Cellular neurothekeomas have a predilection for the upper limbs and head and neck of pediatric and young adult females and rarely recur following incomplete excision. There is no good evidence that these lesions show nerve sheath differentiation and the nomenclature will likely change when the tumor cell lineage is better defined. Atypical histologic features (including pleomorphism, infiltration of subcutis, and a high mitotic rate) seem to have no clinical significance.