Well-differentiated extraskeletal osteosarcoma: report of 2 cases, 1 with dedifferentiation.

Extraskeletal osteosarcoma (ESOS) is a rare soft tissue sarcoma, typically characterized by high-grade histological features and a grave prognosis. However, 4 cases of well-differentiated ESOS with a better prognosis have been documented in the literature within the last 40 years. We report 2 additional cases, 1 with multicentric presentation and dedifferentiation, and we emphasize the histological features that are useful in distinguishing this lesion from other soft tissue tumors. Well-differentiated ESOS seems to represent a rare but distinct low-grade variant of ESOS. The limited published experience suggests that although the biologic behavior of this tumor is better than that of classical ESOS, there are cases with progression to a higher grade, leading eventually to final demise.