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

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