A triphasic pattern is evident, consisting of fibrous fascicles (pink), myxoid mesenchyme (dark blue) and fat. Well-defined bundles of dense,uniform, fibrous tissue interweave and project into fat with admixed regions of mesenchyme.
Primitive mesenchyme is arranged in nests, concentric whorls, or bands. The areas appear more cellular and consist of immature small stellate cells in a slightly myxoid appearing matrix.
Somewhat hyalinized fibrous tissue, mature adipocytes, and mesechmymal spindle cells in a myxoid (bluish) matrix are interwoven in this area.
At low power, the three components are easier to separate. Note the ovoid nests of myxoid spindle cells dispersed in mature fat.
Grossly, these lesions are poorly circumscribed, firm fibrofatty tumors (less than 5 cm) located in the deep dermis or subcutaneous tissue.
There are three key features forming an organoid triphasic pattern. When found together, they are pathognomic for fibrous hamartoma of infancy. (1) fascicles of myofibroblasts (2) nests of primitive-appearing mesenchyme in a myxoid stroma (3) mature adipocytes. These three components may vary in relative proportions among different tumors. There may also be areas of collagenous tissue containing vessels and inflammatory cells (Fletcher, Folpe).
Fibrous hamaratoma of infancy presents as a slowly growing subcutaneous mobile mass that predominantly arises in male infants less than 2 years of age. It can appear in a wide variety of locations, but the axilla, upper arm and shoulder areas are most common. 15-25% are present at birth.
Cured by local excision. In one study, 12% recurred locally, in an average time of 5.4 months (Dickey). There is no tendency for spontaneous regression.
Dickey GE, Sotelo-Avila C. Fibrous hamartoma of infancy: current review. Pediatr Dev Pathol. 1999 May-Jun;2(3):236-43.
Fletcher CDM, ed. Diagnostic Histopathology of Tumors. 3rd Ed. Philadelphia, PA: Elsevier; 2007: 1543.
Folpe AL, Inwards CY. Bone and Soft Tissue Pathology: Foundations in Diagnostic Pathology Philadelphia, PA: Elsevier; 2010: 50-1.