Subdermal fibrous hamartoma of infancy: pathology of 40 cases and differential diagnosis.

Forty subdermal fibrous hamartomas of infancy occurred in 29 males and 11 females who were 7 months to 4 years of age at surgery. At least 4 were congenital. The lesions were situated in axilla, chest wall, and breast (17 cases); abdominal wall, inguinal region, and scrotum (8); buttock and lower limb (6); upper limb (4); neck and scalp (3); and low back (2). They were 0.7 to 10 cm, ill defined, and five underwent reexcision, which was curative in all. All had the characteristic mixture of fibrous and adipose tissue and nests of immature mesenchyme in different proportions, and nearly all showed lymphocytes and thick patent capillaries in the mesenchyme. However, the fibrous component varied considerably in amount, pattern, and cellularity, so that lesions that were typical in some areas, in others resembled collagenizing vascular granulation tissue, deep fibrous histiocytoma, or fibromatosis. Those in which adipose tissue predominated were distinguished from fibrolipoma by foci of immature mesenchyme and from lipoblastoma by their lack of a capsule and of a lobular pattern.