Papillary Thyroid Carcinoma Arising Within a Mature Cystic Ovarian Teratoma With Synchronous Primary Thyroid Malignancy: A Case Report.

Malignant transformation of mature cystic teratomas (MCT) of the ovary is uncommon, with squamous cell carcinoma being the most frequent histotype. Papillary thyroid carcinoma (PTC) arising from ectopic thyroid tissue within an ovarian teratoma is an exceptionally rare entity, with only a limited number of cases documented in the international literature. We report the case of a 34-year-old woman with an incidentally discovered left para-ovarian cystic mass detected during routine gynecological surveillance following a prior hysterectomy for cervical intraepithelial neoplasia. The patient underwent laparoscopic left salpingo-oophorectomy; intraoperative frozen-section biopsy was negative for malignancy. However, definitive histopathological examination identified a papillary thyroid microcarcinoma, follicular variant, arising within a MCT of the left adnexa, without ovarian surface involvement or lymph node metastasis (pT1). This finding prompted formal thyroid evaluation, which revealed a synchronous primary PTC of the thyroid gland, encapsulated follicular variant, measuring 2.3 × 2 cm (pT2), requiring staged bilateral thyroid surgery and planned adjuvant radioactive iodine ablation. This case illustrates the diagnostic challenge of occult thyroid carcinoma within ovarian teratomas, the limitations of intraoperative frozen-section analysis, and the critical importance of a multidisciplinary approach integrating gynecological surgery, endocrinology, and nuclear medicine. The diagnosis of teratoma-derived PTC should prompt the consideration of a formal thyroid evaluation. Thyroid ultrasonography is recommended, whereas serum thyroglobulin should be interpreted only as an adjunctive marker within the overall clinical context and should not be used in isolation for clinical decision-making.