Radioiodine uptake in a renal cyst mimicking a metastasis in a patient affected by differentiated thyroid cancer: case report and review of the literature.

We report a 55-year-old man affected by differentiated thyroid carcinoma with lymph nodes metastases. After conventional levothyroxine withdrawal, the patient underwent thyroid remnant ablation by oral administration of radioiodine-131 ablative activity (3700 MBq). 7 days later, post-therapy whole body scan (pWBS) demonstrated thyroid remnant and showed a large area of abnormal radioiodine uptake located in the posterior left upper abdomen. Ultrasonography of the abdominal region revealed a cyst in left kidney corresponding to the focal radioiodine uptake. On the same day, 131I static images of the posterior upper abdomen obtained before and after administration of furosemide, showed that radioiodine uptake in the lesion was markedly lower after diuretic administration, indicating that radioiodine accumulation in the renal cyst is linked to an active secretory process by the renal tubule. Due to its non-negligible frequency, renal cyst should be taken into account in DTC patients as a potential pitfall at 131I pWBS.