Post-therapy iodine-131 localization in unsuspected large renal cyst: Possible mechanisms

Sensitive and specific, whole-body ¹³¹I scintigraphy remains an important technique for diagnosing metastases from differentiated papillary or follicular thyroid carcinoma. False-positive ¹³¹I localization is well recognized and can occur in a variety of conditions. We present a case of intense ¹³¹I localization in a previously unsuspected large renal cyst; the lesion was not visualized on routine preablation diagnostic ¹³¹I scintigraphy but was obvious on posttherapeutic whole-body imaging, underscoring the value of posttherapy imaging in detecting abnormalities not apparent on diagnostic studies. Radioiodine within the urinary bladder or, at times, the renal collecting system is expected, because ¹³¹I excretion is primarily by glomerular filtration. In the case presented here, ¹³¹I activity within the renal cyst supports the concept that iodide is subject to an active secretory process by the renal tubule.