Case 6 -

Fibrous Hamartoma of Infancy Omar Sangueza

Clinical History: The patient is a 9-month-old male with a superficial soft tissue mass involving his left arm. Diagnosis: Fibrous Hamartoma of Infancy. Fibrous hamartoma of infancy (FHI) is an extraordinarily rare tumor. Most patients with FHI are diagnosed before two years of age. Boys are more commonly affected then girls, presenting with a rapidly growing, mobile cutaneous/subcutaneous mass. The vast majority presents as solitary lesions usually involving the anterior or posterior axillary folds. Other common locations include the upper arm, trunk, inguinal region, and external genitalia. [1, 2] Involvement of the distal extremities (eg. hands and feet) is extraordinarily rare. No familial tendency has been described. Pathologically, FHI is a poorly circumscribed, superficial soft tissue lesion consisting of variable amounts of three distinct components - intersecting fibroblastic/myofibroblastic fascicles, loose, more primitive-appearing mesenchymal cells within a myxoid matrix, and adipose tissue. The fibroblastic fascicles resemble fibromatosis and often contain abundant collagen; the more primitive-appearing areas are arranged in nests and islands consisting of a more rounded but uniform cell population. Adipose tissue is interspersed between both components. [2] In older lesions, significant amounts of sclerosis may be present, replacing large portions of the lesion. Rarely, FHI may show infiltration of skeletal muscle. Significant nuclear atypia is absent; mitotic figures are scant. Immunohistochemically, FHI shows positivity for vimentin and variable reactivity for actin and desmin, mostly in the trabecular, fibromatosis-like areas. The myxoid islands of primitive appearing cells generally express only vimentin. S-100 protein is positive in the adipose tissue component. [3, 4] FHI is a benign lesion that is almost always cured by local excision. Rarely, local recurrence occurs but is curable by re-excision. Spontaneous regression has not been reported. Dickey GE, Sotello-Avila C. Fibrous hamartoma of infancy: current review. Pediatr Dev Pathol 1999;2:236-243. Enzinger FM. Fibrous hamartoma of infancy. Cancer 1965;18:241-248. Fletcher CDM, Powell G, Van Noorden S, et al. Fibrous hamartoma of infancy: a histochemical and immunohistochemical study. Histopathology 1988;12:65-74. Groisman G, Lichtig C. Fibrous hamartoma of infancy: an immunohistochemical and ultrastructural study. Hum Pathol 1991;22:914-918.