Myofibroblastoma: report of a rare entity in the pediatric population.

Fibroblastic and myofibroblastic tumors constitute an important group of neoplasms in children and adolescents. These span the full spectrum of clinical behavior, ranging from benign to intermediate and malignant. We report a case of a benign mesenchymal tumor with myofibroblastic differentiation in a 9-year-old girl arising in the left groin that met the histologic features described for myofibroblastoma in adults. Two types are recognized in adults: angiomyofibroblastoma and mammary-type myofibroblastoma of soft tissue. Our case shared features of both these subtypes but was not typical of either one, and we therefore designated our case simply as "myofibroblastoma." Our case showed expression of estrogen receptor protein, which is characteristic of adult lesions, but not a deletion of 13q14, as has been reported in some adult cases. In the English-language literature, only 6 cases have been reported in patients under 21 years of age, and all but 1 were teenagers. Pediatric surgeons, oncologists, and pathologists should be aware that such a benign entity can occur in this patient population and could be confused with other lesions, including malignant ones.