A rare case of synchronous follicular thyroid carcinoma arising within a mature cystic ovarian teratoma and stage IV differentiated thyroid cancer in iodine-deficient area in Viet Nam.

Well-differentiated thyroid carcinoma rarely spreads to soft tissues. Thyroid carcinoma arising within a mature cystic teratoma is even rarer. We report an extremely rare case of synchronous follicular thyroid carcinoma arising within a mature cystic ovarian teratoma and stage IV differentiated thyroid carcinoma. A 62-year-old woman who lived in an iodine-deficient area was accidentally diagnosed with an ovarian cyst during a radiological metastatic work-up for thyroid cancer. Following laparoscopic left salpingo-oophorectomy, histopathological examination revealed a follicular thyroid carcinoma arising within a mature cystic teratoma. After that, total thyroidectomy and surgical resection of the soft tissue lesion in the supraclavicular fossa were performed, and the patient received subsequent ¹³¹I ablation therapy, but the disease progression was recorded 3 months later. We believe that iodine deficiency plays a role in the malignant transformation of thyroid tissues within a mature cystic teratoma. In elderly individuals with significant metastases, radioactive iodine therapy is ineffective.