Fibrous histiocytoma/Dermatofibroma in children (FH): the same as adults?

Abstract Fibrous histiocytoma (FH) or dermatofibroma are common cutaneous lesions mostly seen in adults and rare in the first two years of life. Two hundred sixty seven patients less than 18 years with a diagnosis of FH or dermatomyofibroma, a lesion with morphologic overlap with FH, were identified from the files of a single institution with only 13 (4.8 %) occurring in patients under 5 years of age. Ten patients had either underlying neurologic, autoimmune, or metabolic disorders or a family history of autoimmune conditions. Histologic review of H&E and immunostains from 75 FH and dermatomyofibroma in 70 patients showed: 33 classic FH, 8 classic FH characterized by a peculiar retiform morphology with thin fascicles of elongated cells forming a network reminiscent of the eruptive variant of FH, 19 deep/cellular, 5 aneurysmal, 3 lipidized (including two lesions in a patient affected by Mucopolysacharidosis IV), 3 dermatomyofibromas, and 4 isolated cases of hemosiderotic, granular cell atypical, and epithelioid FH. Immunostains for Factor XIIIa highlighted a dense network of dendritic cells in FH which was significantly reduced in the FH with retiform morphology. Smooth muscle actin staining was positive in a high percentage of FH (85.3%). The current series demonstrates that FH in children may show unique clinical and morphologic features. The retiform pattern with decreased dendritic cells found in congenital lesions and in two older patients with lesions in two locations, might have a different pathogenesis, probably related to an altered immune response in very young patients.