MSI-High Malignant Struma Ovarii With Lung and Liver Metastases 17 Years Later

What happened, in summary

At age 50, a woman developed right abdominal pain from ovarian torsion and a 15 × 9 × 6 cm right ovarian mass. Oophorectomy in 1998 showed a cystic teratoma containing a large thyroid component, consistent with struma ovarii, and the lesion initially appeared benign. Twelve years later, progressive abdominal pain led to discovery of 2 retroperitoneal masses measuring 9 × 7 × 7 cm and 5 × 4 × 4 cm. Both were removed and contained thyroid-type tissue with atypical features. A separate left ovarian nodule was benign. She entered close endocrine follow-up and remained in clinical remission.

Five years later, further evaluation led to total thyroidectomy so radioactive iodine could be used; no cancer was found in the thyroid gland. Whole-body imaging showed uptake in the lungs and liver. She began suppressive levothyroxine and received 2 doses of radioactive iodine-131, 6 months apart. Pulmonary nodules remained stable for 2 years. When thyroglobulin rose to 463 IU/mL in February 2017, another radioactive iodine treatment was given. An experimental tyrosine kinase inhibitor was later stopped after 2 months because she could not tolerate it.

In September 2020, previously observed lung nodules progressed and new nodules appeared. Genomic testing found high microsatellite instability, with non-actionable FGFR1, NRAS, and ATM mutations. Pembrolizumab was started. By March 2021, she had received 4 doses, remained asymptomatic, and was tolerating treatment without obvious adverse effects while awaiting follow-up imaging.