False-positive finding: Iodine (I-131) avid simple renal cysts on post therapy scan in papillary thyroid carcinoma patients - Imaging follow-up – A case series

We present five patients with papillary thyroid carcinoma who underwent total thyroidectomy and received Iodine (I)-131 as ablation therapy. Post-treatment radioiodine scintigraphy with Single Photon Emission Computed Tomography of the lumbar region revealed foci of unusual I-131 uptake in renal cysts. Three of the five patients underwent I-123 whole-body scans (WBS) 6 months later; none of them, including those who underwent sclerotherapy for renal cysts, showed any I-123 uptake. The remaining two patients exhibited no changes in the size or characteristics of the renal cysts on follow-up scans using Fluorine-18 Positron Emission Tomography–Computed Tomography and abdominal Computed Tomography. In one patient diagnosed with multiple simple cysts along with distant metastases to the lung and bone, follow-up scans after the second radioiodine therapy showed partial improvement in other metastatic lesions, whereas multiple renal cysts remained unchanged. Despite the high incidence of renal cysts, post-therapeutic radioiodine uptake in renal cysts is relatively rare. To the best of our knowledge, there is no reports available on the findings during I-123 WBS follow-up or the second post-radioiodine therapy scan follow-up. Even if uptake is observed on the post-radioiodine therapy scan, it may not be present on I-123 WBS follow-up, necessitating careful interpretation.