Osteosarcoma arising in a benign dermoid cyst of the ovary

We present a rare case of an osteosarcoma arising in a benign dermoid ovarian cyst. The cyst, removed intact from the right ovary of a 52-year-old Japanese woman, was unilocular, measured 10 x 9 x 8 cm, weighed 176 g, and contained yellow-brownish greasy material. From the inner surface of the cyst there protruded a subsessile solid mass, measuring 3.1 x 2.5 x 2.0 cm, with a variegated cut surface. Histologically, the cyst was an ordinary dermoid cyst, and the solid mass consisted of osteosarcoma with focal chondroblastic differentiation. Postoperative combination chemotherapy was prescribed and the patient remains alive and well 16 months postoperatively. This case is the third documented in the world literature.