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Abstract

Angiomatoid fibrous histiocytoma (AFH) is a neoplasm of uncertain histogenesis, which most commonly arises in the subcutaneous tissue of the extremities of children and young adults. The sclerosing variant of AFH is a rare subtype characterized by the presence of fibrous tissue, mineralized deposits, and a hardening or sclerotic appearance. While AFH typically presents as a slow-growing, painless soft tissue mass in young adults, the sclerosing variant can mimic other bone tumors, particularly osteosarcoma. Here, we present two cases of sclerosing AFH with varied histomorphology, strong and diffuse reactivity for TLE1 and utility of NGS in diagnosis. A 39-year-old male with past medical history of primary cutaneous marginal zone B-cell lymphoma presented with L4-L5 soft tissue mass. Another case was that of a 50-year-old female with a recent diagnosis of mammary carcinoma with osteoclast-like giant cells who presented with a soft tissue mass of the right thigh. Histology from both showed spindle cells and histiocytoid cells, along with blood-filled cystic spaces and a surrounding rim of lymphoplasmacytic infiltrate. There was increased fibrosis and calcification within the tumor matrix raising the possibility of osteosarcoma. Tumor cells were strongly and diffusely positive for TLE1, multifocally positive for desmin, and negative for SMA, myogenin, AE1/3, SATB2, CD163 and CD45. The p53 was wild type and the INI1 stain was retained. Ki67 proliferative index was <1%. The solid tumor gene mutation panel was negative. RNA-sequencing and FISH for EWSR1-ATF1 gene rearrangement were positive, confirming the diagnosis of AFH. Our study underscores the different histologic phenotypes of AFH, reactivity for TLE1 immunohistochemical stain and NGS.