Long-Term Disease Control With Lenvatinib in Metastatic Malignant Struma Ovarii: A Case Report

Struma ovarii is a rare monodermal teratoma composed predominantly of thyroid tissue. Although most cases are benign, some are diagnosed as malignant on the basis of histological features, local invasion, or metastatic disease. We report a patient with malignant struma ovarii who has been maintained on long-term lenvatinib (LVB) therapy for postoperative metastatic disease. A woman in her 40s was incidentally found to have a pelvic mass after a fall. Resection of the ovarian tumor led to a diagnosis of struma ovarii, with small foci of thyroid-like tissue and predominantly areas resembling poorly differentiated thyroid carcinoma. Genomic profiling revealed an NRAS mutation. Because of residual disease, total thyroidectomy followed by radioactive iodine therapy was performed, but no uptake was observed. Subsequently, multiple hepatic metastases and pelvic dissemination developed, and conventional chemotherapy including bevacizumab was ineffective. LVB induced marked regression of hepatic and pelvic lesions. However, reductions in dose due to anorexia, fatigue, and proteinuria were accompanied by increased serum thyroglobulin (Tg) levels and progression of hepatic metastases. Switching to sorafenib led to further progression, but reintroduction of LVB reduced Tg levels. Treatment was interrupted for six months due to myocardial infarction and generalized peritonitis, during which Tg levels rose sharply and hepatic metastases regrew. After resuming LVB with dose reduction and planned drug holidays, disease stabilization was achieved, and the therapy has been maintained. The patient has survived seven years since recurrence, including six years on LVB. The tumor behaved similarly to poorly differentiated thyroid carcinoma, with Tg levels reflecting disease activity and LVB demonstrating the potential for long-term tumor control.