Extraskeletal osteosarcoma arising in an ectopic hamartomatous thymus. Report of a case and review of the literature.

A unique case of an osteosarcoma arising within a hamartoma of the thymic gland located in the left pleural cavity of an 11-year-old girl is described. The presence of abundant mature lamellar bone within this thymic hamartoma provides an explanation for the osseous histogenesis of the sarcoma. The development of an osteosarcoma from extraskeletal osseous tissue is a rare occurrence and has been documented in association with myositis ossificans and most recently in a case of dermatomyositis with metaplastic ossification.