Undifferentiated carcinoma with osteoclast-like multinucleated giant cells arising in an ovarian mature cystic teratoma.

Malignant transformation is an unusual complication of mature cystic teratomas. Squamous cell carcinomas and adenocarcinomas are predominant among these malignant tumors. We present a 70-year-old Japanese woman with a giant ovarian tumor (30 x 23 x 9 cm) and multiple peritoneal dissemination. The huge tumor was filled with necrotic and fatty tissue and coagula accompanied with hair balls histologically diagnosed as undifferentiated carcinoma with multinucleated giant cells arising in an ovarian mature cystic teratoma. The residual disseminated tumors grew very rapidly and occupied the whole abdominal cavity again 3 weeks after the surgery. The patient died 5 weeks after the operation.