Abstract

Low grade fibromyxoid sarcoma is a rare, slow-growing soft tissue tumor typically found in the deep soft tissue, most often on the lower extremities, as well as occasionally involving intrathoracic and intraabdominal cavities. These tumors are characterized by MUC4 positivity, and fusions in FUS-CREB3L2 are found in the vast majority of tumors. The tumors rarely present above fascial tissues. Cases of superficial presentation are predominantly described in the subcutis of young patients, with only a few reported cases of dermally based low-grade fibromyxoid sarcoma found in the literature. We present a case of a 74-year-old woman who was seen for an exophytic, ulcerated papule on the second toe, which clinically was concerning for an acquired digital fibrokeratoma. A shave biopsy was obtained, which demonstrated an exophytic and ulcerated dermal nodule, which was composed of a storiform pattern of spindle-shaped, elongated cells in a fibromyxoid stroma. Cytologic pleomorphsm was subtle. Immunohistochemical staining was negative for desmin, SMA, S100, CD34, and beta-catenin. MUC4 was positive in the tumor cells, and fusion gene assay demonstrated a FUS::CREB3L2 fusion, confirming the diagnosis of low-grade fibromyxoid sarcoma. The patient has been referred to surgical oncology for definitive treatment of the tumor. Our case demonstrates a very unusual presentation of this entity, due to the dermal location and patient age. Dermatopathologists should be aware of low-grade fibromyxoid sarcoma, as these tumors can rarely present in subcutaneous and even dermal tissues.