Cellular neurothekeoma with atypical features in a child

Cellular neurothekeoma (CNT) is an uncommon benign skin neoplasm and a variant of neurothekeoma (NT). Histopathological examination of cellular neurothekeoma often reveals mild cellular atypia and occasional mitoses. CNTs with atypical features (including large size, deep penetration, marked cytologic pleomorphism, high mitotic rate, diffusely infiltrative borders, vascular invasion) are very rare. The rare presence of atypical features may sometimes make CNT difficult to distinguish from soft tissue sarcoma. We describe a case of a 9-year-old girl with a progressively enlarging left thigh tumor (1.5 × 1.3 × 1 cm) over 6 months. After total resection of the tumor, examination revealed histological findings consistent with cellular neurothekeoma but with prominent atypical features, including deep penetration into the subcutaneous adipose tissue, marked cytological pleomorphism, high mitotic rate (5/10HPF), and diffusely infiltrative border. Immunohistochemically, these cells were positive for α-smooth muscle actin, but negative for cytokeratin and S-100. Her clinical course was uneventful without recurrence after one and a half years of follow-up.