"Pauci-Hemosiderotic" Fibrolipomatous Tumor: A Mimicker of Various Lipomatous Lesions.

Hemosiderotic fibrolipomatous tumor is a rare soft tissue tumor that preferentially affects the dorsum of foot, shows recurrent t(1;10) translocation targeting TGFBR3 and OGA ( MGEA5 ) genes, and has a high recurrence potential. Hemosiderin deposits, mature adipocytes, and interspersed spindle cells are the 3 cardinal morphologic features of this tumor. We describe a "pauci-hemosiderotic" example involving the left wrist of a 45-year-old female, posing a diagnostic pitfall. The tumor comprised mature adipose tissue traversed by variably thick fibrous septa containing short fascicles of spindle cells. Prominent small- to medium-sized blood vessels were present, often with perivascular fibrosis or aggregates of foamy histiocytes, sometimes associated with red cell extravasation. Hemosiderin was not conspicuous, but fine deposits could be found focally on careful search and with the aid of Perls stain. The diagnosis was further confirmed by diffuse expression of CD34 and presence of OGA translocation by fluorescence in situ hybridization. Pathologists should be aware that hemosiderin deposition can be scanty and focal in hemosiderotic fibrolipomatous, but the rich vasculature with a "damaged" appearance is a useful diagnostic clue.