Abstract

Extraskeletal osteosarcoma (EOS) is a high-grade soft-tissue tumor characterized by the production of malignant osteoid, without attachment to the underlying bone/periosteum. Rarely, EOS presents as a cutaneous tumor. The clinical behavior of primary cutaneous EOS (PC-EOS) remains incompletely characterized. Herein we present the largest case series of PC-EOS reported to date. 16 PC-EOS cases from the archives/consultation files were retrieved (male:female = 1:1, age = 31-96 years, mean age: 66 years). The tumors measured 1-10 cm (mean: 3.2 cm) – and were located on the lower extremity (7), head (6), upper extremity (2), and trunk (1). They consisted of pleomorphic, spindled-to-epithelioid cells, with fascicular, nodular, or sheet-like growth patterns and foci of malignant osteoid. Immunohistochemistry did not reveal specific lines of differentiation, and there was no evidence of other tumor types. A literature review was conducted to identify all well-characterized cases of PC-EOS. A combined analysis of present and past cases was performed to determine overall trends in clinical characteristics and outcomes. The mean follow-up period was 23.9 months, during which 67.5% of patients experienced progression-free survival and 18% of patients died of disease. Rates of local recurrence and metastasis were 10% and 25%, respectively – approximately double to past estimates. This data suggests that the prognosis of PC-EOS is less favorable than previously thought. The differential diagnosis includes benign entities (e.g., myositis ossificans) and malignant neoplasms with heterologous osteosarcomatous differentiation (e.g., carcinosarcoma, transdifferentiated melanoma). Wide excision remains the standard of care, and the role of chemotherapy and radiation remains inconclusive. Recognition of this rare entity can facilitate prompt diagnosis and appropriate treatment.

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