A Rare Case of Biopsy-Proven Sacral Mass Metastatic From Follicular Thyroid Cancer With a Literature Review.

Thyroid cancer with metastatic disease to the pelvis is extremely rare. The patient in our case, an 86-year-old male, presented after total thyroidectomy for follicular thyroid cancer (FTC) with symptoms of recurrent urinary tract infections and retentions, surprisingly leading to the discovery of a large sacral mass on the CT abdomen and pelvis. The biopsy showed metastatic carcinoma with morphology and immunohistochemistry to be consistent with FTC. In our case, due to symptomatology, prostate cancer was initially considered high in the differential for primary source rather than thyroid cancer. The mass was considered too large for surgery, and he was referred to a radiation oncologist for radiation therapy for the sacral mass.