Microscopic histopathological analyses of hematoxylin and eosin stained FHI tumor tissues consistently reveal three very different component zones in virtually all cases: 1) haphazardly arranged, intersecting bundles of bland fibroblast-like/myofibroblast-like cells; 2) mature adipose (i.e. fat) tissue; and 3) highly vascular, myxoid (i.e. connective tissue appearing more blue or purple than normal connective) nodules of primitive-appearing spindled-shaped to star-shaped cells. Chronic inflammatory cell (e.g. lymphocytes, macrophages, and plasma cells) aggregates are commonly present in these lesion. The relative proportion of fibroblast/myofibroblast, adipose tissue, and myxoid zones commonly vary from case to case.Earlier Immunohistochemical analyses of FHI tumor tissues in a small number of cases have given varying results. A more recent and larger study reported that 75% of tested cases showed variable expression of smooth muscle actin proteins by the cells located primarily in fibroblast/myofibroblast zones and occasionally by the cells located in myxoid zones; the S100 protein was expressed by the fat cells of the adipose tissue zone in all cases; CD34 protein was expressed by the cells in myxoid and fibroblast/myofibroblast zones in all cases; and cells expressing the desmin protein were not detected in any case. A smaller, more recent study also found that 8 of 13 FHI cases had tumor cells in the fibroblast/myofibrobalast zone that expressed smooth-muscle actin protein, 6 of 8 cases had tumor cells in the myxoid zone that expressed the CD34 protein, and 7 of 7 cases had tumor cells in the adipose tissue zone express the S100 protein.
Fibrous hamartoma of infancy (FHI) is a rare, typically painless, benign tumor that develops in the subcutaneous tissues of the axilla (i.e. armpit), arms, external genitalia, or, less commonly, various other areas. It is diagnosed in children who are usually less than 2 years old or, in up to 20% of cases, develops in utero and is diagnosed in an infant at birth.The cells involved in FHI include bland fibroblasts/myofibroblasts, mature fat cells, and primitive-appearing spindle-shaped and/or star-shaped cells. The tumor was first described by R.D. Reye in 1956 who termed the disorder "subdermal fibromatous tumor of infancy" and regarded it as a reactive lesion. In a 1964 study of 30 patients, F.M. Enzinger renamed the tumor hamartoma of infancy; he regarded it to be a hamartoma (i.e. a local malformation of various normal cell types due to a systemic genetic condition) rather than a reactive lesion. However, subsequent studies have found gene mutations in FHI tumor cells and conclude that it is a true neoplasm, i.e. a growth of cells which is uncoordinated with that of the normal surrounding tissue and persists in growing abnormally even if the original trigger is removed. In 2020, the World Health Organization classified FHI in the category of benign fibroblastic and myofibroblastic tumors.Fibrous hamartoma of infancy is generally a benign tumor but may be locally aggressive, locally infiltration, and in uncommon cases produce symptoms such as tenderness. Surgical excision is the treatment of choice for FHI tumors. Presentation
The largest study to date examined 197 cases of FHI.