Post-Therapy Iodine-131 Localization in Unsuspected Large Renal Cyst: Possible Mechanisms

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