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.
four instead of the normal two chromosomes for some but not all chromosomes), single chromosome gains for several chromosomes, and loss of heterozygosity in the p arms of chromosomes 1 and 11 in the first case and in the second case a loss of the p arm in chromosome 10, lose of chromosome 14, and lose of a portion of the q arm of chromosome 22q. These two cases along with recurrent EGFR gene abnormalities strongly support the notation that FHI is as neoplastic tumor. Diagnosis
The diagnosis of FHI is dependent on its presentation as a subcutaneous tumor that often occurs in individuals at birth or ages <1–2 years old; its highly characteristic histopathology consisting of fibroblast/myofibroblast, adipose tissue, and myxoid zones; and its content of tumor cells which have one of the EGFR gene mutations described in the previous section. The combination of these factors clearly distinguishes FHI from three recently defined tumors (i.e. fibroblastic connective tissue nevus, medallion-like dermal dendrocyte hamartoma, and plaque-like CD34-positive dermal fibroma) as well as various pediatric spindle-shaped cell neoplasms. Tumor imaging methods such as magnetic resonance imaging may also be helpful in suggesting that a tumor is an FHI. Treatment and prognosis
FHI tumors, if left untreated, have been documented to grow for as long as ~5 years following their diagnosis. The treatment of choice for these tumors is complete surgical excision with clear margins (i.e. with removal of all tumor tissue).