Abstract

Fibro-osseous pseudotumor of the digit (FOPT) is a very rare entity characterized by a proliferation of fibroblasts, myofibroblasts and osteoblasts with heterotopic ossification in the skin and subcutaneous tissue. FOPT typically presents with localized, painful and erythematous soft tissue swelling in the proximal phalanx and can show rapid growth raising clinical suspicion for a malignant neoplasm, such as extraskeletal osteosarcoma. Bizarre parosteal osteochondromatous proliferation and myositis ossificans are closely related and in the differential diagnosis. Similar to myositis ossificans and nodular fasciitis, USP6 gene rearrangements have been reported, although the exact pathogenic mechanism remains unclear. Herein, we present a case of a 40-year-old female with a 2-year history of a right thumb mass. It was previously presumed to be a wart, or possible digital mucous cyst, which did not respond to conservative treatment. X-ray imaging showed no bone destruction or foreign body material. Excision of the mass was decidedly performed. Histopathologic examination demonstrated a circumscribed bland spindle cell proliferation within the superficial dermis with associated ossification and surrounding epidermal collarette with central ulceration. Rare mitotic figures were present without necrosis nor significant cytologic atypia. Immunohistochemical studies revealed SATB2 positivity within the majority of the lesional cells, while cytokeratin AE1/AE3, SOX-10 and HHV8 were negative. CD31 and ERG highlighted background vasculature. A diagnosis of FOPT was rendered. The patient remained free of local recurrence at 1-month follow-up. It is our aim to increase awareness of this rare benign entity, its clinical and histologic mimickers, and expected management.