Incidental finding of colon adenocarcinoma on I-131 whole body scan in a patient with recurrent papillary thyroid carcinoma.

‎Iodine-131 (I-131) whole body scan is a procedure routinely done after radioactive iodine therapy. It helps detect presence of functioning thyroid tissue remnants in the neck, as well as distant metastases. However, not all that take up I-131 are of thyroidal origin. Our patient is a 74-year-old woman with recurrent papillary thyroid carcinoma, who underwent a repeat radioactive iodine therapy (RAIT). Her post-RAIT whole body scan showed an unusual iodine-avid focus in the transverse colon which was later confirmed to be another primary malignancy of colonic origin through colonoscopy with biopsy, as well as post-surgical histopathology. This case report distinctively demonstrates uptake of I-131 in extra-thyroidal malignancy. It also emphasizes the importance of evaluating carefully any irregular-looking focal iodine uptake in areas with seemingly physiologic activity on a routine I-131 whole body-scan.