Cellular neurothekeoma: analysis of 37 cases emphasizing atypical histologic features.

Cellular neurothekeoma is a frequent source of diagnostic difficulty. In order to gain more insight into the range of histologic features of cellular neurothekeoma, we examined all cases from our institution, with a focus on describing atypical histologic features. Cases with sufficient histologic material for evaluation were retrieved. Cases were analyzed for demographics, growth pattern, myxoid stroma, cytologic atypia, mitotic rate, perineural invasion, and other histologic features. The 37 patients (16 M; 21 F) had a mean age of 31.0 years (range: 4-89). Tumors involved the head and neck (n=16), arms (n=11), trunk and shoulders (n=8), and foot (n=2). All cases had at least focal nesting of epithelioid to spindled tumors cells characteristic of cellular neurothekeoma. In many, alternate growth patterns were present and represented the dominant pattern in some. These patterns included fascicular (n=9), sheet-like (n=6), and corded (n=4). Myxoid stroma was present in 14 and was prominent in 5. Cytologic atypia was present in 19 patients, with 3 having severe atypia. Mean mitotic rate was 2.0/mm(2) (range 0-10 per mm(2)). Neurotropism was seen in four cases. Other unusual features included collagen trapping, giant cells, hemorrhage, lymphocytic cuffing, chondroid stroma, and cellular vacuolization. Cellular neurothekeoma has a wider range of features than is commonly recognized. The presence of nests of epithelioid tumor cells with characteristic cytologic features, no matter how focal, is a clue to the diagnosis.