Malignant struma ovarii with late recurrence harbouring high microsatellite instability

Struma ovarii (SO) is a rare ovarian teratoma containing abundant mature thyroid tissue. Malignant transformation is even less common and distant metastasis is documented in about 5%-10%. The time from diagnosis of primary SO to metastatic disease varies. As malignant SO is rare, there are no uniform diagnostic criteria or treatment guidelines. Management is usually extrapolated from that of thyroid malignancy. We report a patient who relapsed 12 years from the initial diagnosis and metastasised to the lungs 5 years after the first recurrence. Our patient was treated with total thyroidectomy followed by radioactive iodine, and retreated on progression in the lungs. The tumour harboured high microsatellite instability and treatment with programmed cell death protein 1 inhibitor was initiated. This case shows the long latency of SO with the rare phenomenon of metastasis. It also highlights the importance of molecular testing for rare cancers such as this.