BACKGROUND Struma ovarii is a rare specific ovarian tumor. It is a highly differentiated monodermal teratoma with a malignant transformation rate as low as 5%. Thus, malignant transformation and metastasis are extremely rare. The clinical manifestations of this disease are not typical and are easily misdiagnosed. CASE SUMMARY A 55-year-old female patient had a history of pain in the right hepatic region for approximately 1 year. Computed tomography and magnetic resonance imaging showed a solid cystic mass in the right adnexal region and a solid mass in the right retroperitoneum. The patient underwent surgical resection, and the combined morphological and immunohistochemical results led to the final diagnosis of right struma ovarii with papillary carcinoma and right retroperitoneal lymph node metastasis. CONCLUSION Malignant struma ovarii with distant metastasis is extremely rare, and the clinical manifestations of this disease are nonspecific. Accurate preoperative diagnoses are difficult to obtain, and pathological examination is the gold standard for diagnosing this disease.

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