Background. Extraskeletal osteosarcomas are rare malignancies that account for about 1% of all soft tissue sarcomas. Few large series have been reported. Methods. Clinical records and histologic slides of all patients with extraskeletal osteosarcomas treated at the Mayo Clinic between 1915 and 1988 were reviewed. Results. The study group consisted of 40 patients, most of whom presented in the sixth and seventh decades of life (mean age, 50.7 years). There was a male predominance (male‐to‐female ratio, 1.9:1). The lower limbs most commonly were involved (68%), usually the thigh and buttock regions. Ninety‐three percent of tumors presented as an enlarging soft tissue mass, with a history of trauma in nine patients. In nine patients, the lesions were initially interpreted histologically as benign, most commonly as myositis ossificans. Histologically, all were high grade osteosarcomas. Multiple local recurrences (45%) are a feature of this tumor. All recurrences occurred within 3 years. Distant metastasis (65%) is also common and is usually to the lungs (81%). Radical resections appear to be the best option for local control, with resection of the pulmonary metastasis occasionally producing a cure. By univariate analysis of Kaplan‐Meier survival curves, the patients with predominantly chondroblastic tumors fared better than those with predominantly osteoblastic tumors (P=0.03). Analysis of survival differences of the three main subtypes together (osteoblastic, chondroblastic, and fibroblastic) was not significant. A small‐sized initial lesion did not equate with better survival. Seventy‐three percent died of the disease, with a mean follow‐up of 5.9 years. Conclusion. Extraskeletal osteosarcoma is a high grade malignant tumor associated with a 5‐year survival rate of 37% (95% confidence interval, 28%–59%). Local recurrences and distant metastasis are common and usually occur by 3 years after excision. Patients with the chondroblastic subtype survive longer than those with the osteoblastic subtype. Cancer 1995; 76:2253–9.

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