SUN-348 Incidentally Noted Benign Duodenal Polyp Discovered On Post I-131 Treatment Scanning For Papillary Thyroid Cancer

Abstract Disclosure: R. Wang: None. M. Djekidel: None. C. Taneja: None. Background: Post I-131 whole-body scans are widely used to identify local, regional or distant metastatic areas of iodine-avidity and map proper therapy targeting in patients with differentiated thyroid cancer. However, incidental findings of nonthyroidal iodine-avid lesions have been reported which may require additional workup and possible intervention. We present a rare case of radioiodine uptake in a benign duodenal polyp on post I-131 treatment scans for papillary thyroid cancer (PTC). Case Presentation: A 21-year-old female with a pediatric history of Wilms tumor (diagnosed at age 1) treated with surgery, radiation therapy to chest, chemotherapy and stem cell treatment was found to have an incidental thyroid nodule on cervical spine MRI after a motor vehicle accident. Thyroid ultrasound showed bilateral nodules including a left-sided 3 cm nodule with highly suspicious features such as taller than wide shape, lobulated borders, punctate echogenic foci and extrathyroidal extension (ACR TI-RADS 5). FNA revealed PTC (Bethesda VI), and the patient subsequently underwent total thyroidectomy. Surgical pathology revealed unifocal left tall-cell variant PTC without angiolymphatic invasion or extrathyroidal extension, and 1/1 metastatic level 6 lymph node 0.3 cm without extranodal extension. Thyroglobulin (Tg) at 5 weeks post-surgery was 12.4 ng/mL and increased to 68 ng/mL post-administration of recombinant TSH. She was treated with 95.5 mCi of I-131, and post-treatment scans showed iodine uptake in bilateral thyroid bed and central neck compartment. Additionally noted was a focus of iodine uptake in the upper abdomen corresponding to the duodenum on SPECT-CT, which was thought to be either physiologic uptake or representing an iodine-avid gastroduodenal/duodenohepatic lymph node. The patient underwent upper GI endoscopy and removal of a duodenal polyp corresponding to the findings noted on post I-131 treatment scans. Pathology showed a benign hamartomatous duodenal polyp. She has been monitored subsequently and has low-level declining Tg levels (2.2 ng/mL) without evidence of structural disease at 30 months post-surgery. Conclusion: Advanced molecular imaging with SPECT-CT post I-131 treatment for differentiated thyroid cancer is critical in characterizing extrathyroidal iodine uptake, and accurate work-up and diagnosis of these findings is crucial to rule out metastatic disease and guide patient care. This case highlights an incidental finding of iodine uptake in a benign duodenal polyp unrelated to thyroid cancer. The imaging findings in combination with elevated Tg levels posed a diagnostic challenge and necessitated pursuing histologic diagnosis to rule out metastatic thyroid cancer. Further research may be needed to fully characterize sodium-iodide symporter (NIS) expression in the GI tract and elsewhere which could potentially explain such findings. Presentation: Sunday, July 13, 2025