Fibrous hamartoma of infancy: a clinicopathologic analysis of 60 cases.

Fibrous hamartoma of infancy is a benign soft tissue tumor with a characteristic triphasic organoid histologic appearance. It typically occurs within the first 2 years of life. The usual anatomic locations include the upper extremities, axilla, and upper back. Diagnostic challenges occur when this tumor arises in older children, outside of the usual anatomic sites, or when unusual histologic features are encountered. This study reports 60 cases of fibrous hamartoma of infancy from institutional and consultation files. All had a triphasic organoid histologic pattern, but half also displayed an unusual pseudoangiomatous histologic pattern. The male to female ratio was 2.0 (40 boys, 20 girls), with a mean age of 1.5 years (range, 16 d to 8 y) at diagnosis. Tumor size ranged from 0.5 to 9 cm, with a mean of 3.7 cm. Sites included the trunk (40 cases), extremities (17 cases), and head and neck (3 cases). All cases had triphasic elements of mature fibrous tissue, mature adipose tissue, and immature mesenchymal tissue in varying proportions, with the additional pseudoangiomatous pattern in 32 cases. Immunohistochemical analysis demonstrated reactivity for smooth muscle actin and CD34 in the mature fibrous tissue, S100 protein in the mature adipose tissue, and variable CD34 reactivity in immature mesenchymal and pseudoangiomatous foci. Ki-67 proliferative activity was noted in the immature mesenchymal and pseudoangiomatous foci, and Bcl-2 reactivity was restricted to mesenchymal and pseudoangiomatous foci. Follow-up information in 12 cases revealed no evidence of recurrence in 10 patients and local recurrence in 2 patients, each at 3.5 years after primary excision. This study demonstrates an expanded age range (up to 8 y) and anatomic distribution (30 cases outside of the classic locations of the upper extremities, axilla, and upper back) of fibrous hamartoma of infancy. The pseudoangiomatous morphologic variation can lead to challenges in diagnosis and may reflect a maturational phenomenon from the immature mesenchymal component.