False-positive I-131 uptake by an ovarian serous cystadenofibroma

We present the case of a 53-year-old woman after thyroidectomy with metastatic multifocal papillary carcinoma and encapsulated focal Hurthle cell carcinoma. Postoperatively, an I-131 whole-body scan revealed uptake in the low midline anterior neck. She was treated with 151.5 mCi of I-131. The 1-year follow-up I-131 whole-body scan revealed a new pelvic mass with I-131 uptake. Pelvic CT showed bilateral complex ovarian masses. She underwent surgical excision, revealing a right ovarian endometriotic cyst and a left ovarian cystadenofibroma, without thyroid tissue involvement. I-131 uptake in a cystadenofibroma is extremely rare and has been reported only once to our knowledge.