Mature cystic teratoma with high proportion of solid thyroid tissue: a controversial case with unusual imaging findings.

We describe a case of a mature cystic teratoma of the ovary with high proportion of solid thyroid tissue (< 50% of the entire tumor) in a childbearing woman. The patient presented with non-specific abdominal bloating. Pelvic ultrasound and magnetic resonance imaging revealed a complex cystic-solid tumor confined to the left ovary with an anterior fat-containing locus compatible with mature cystic teratoma and a posterior predominantly solid component with low signal intensity on T2-weighted images that was histopatologically diagnosed as benign thyroid tissue. Thyroglobulin levels were in normal range. Although thyroid tissue is present in up to 20% of mature cystic teratomas, with exception of struma ovarii, it is not usually macroscopically nor radiologically identified. The differential diagnosis should include T2-hypointense adnexal lesions associated with mature cystic teratoma, malignant transformation of mature teratoma, and immature teratoma.