High and low radioiodine doses to indicate a simple renal cyst.

To the Editor: High doses of iodine-131 (131I) are commonly used in patients with differentiated thyroid cancer (DTC) after total or subtotal thyroidectomy, in order to ablate the remaining cancer or normal thyroid tissue [1]. Whole body scan performed after about 5 days can visualize possible metastatic lesions [2] and may also show the stomach, salivary glands, the breasts expressing sodium iodide symporter (NIS) [3-6], the liver due to metabolism of radioiodinated thyroglobulin and the urinary tract due to urinary excretion of the administered 131I [3-5]. A 65 years old female underwent a near total thyroidectomy for a 0.9cm DTC of the papillary type on the right lobe. Histopathology revealed extrathyroid extension of the cancer with multifocal metastases in both lobes, and in one pretracheal lymph node. After surgery, the patient underwent 131I ablation with 5.55GBq of 131I and whole body scan was obtained after 5 days. The scan demonstrated usual 131I accumulation in the thyroid remnant, the salivary glands and the liver. Increased 131I uptake at the area of the right kidney was also noted (Fig. 1). The patient had a 5 years history of an asymptomatic large simple cyst in the right kidney. Computed tomography (CT) scan of the abdomen showed only the known cyst (7x6cm) involving the middle portion of the right kidney (Fig. 2). Serum laboratory tests performed at the time of ablation revealed thyrotropin: >81μIU/mL (reference range: 0.3-4.0μIU/mL), 3.2ng/mL thyroglobulin (reference range: 0-50.0μIU/mL), and 115.5U/mL antithyroglobulin antibody (reference range 0-35.0U/mL). The patient was followed-up with thyroid hormone replacement and serum thyroglobulin measurements every year. The thyroglobulin levels for the follow-up period remained undetectable (detection limit 0.2ng/mL). Three years after ablation, the patient underwent diagnostic whole body scan using 185MBq of 123I. Thyroid function tests 4 weeks after thyroid hormone withdrawal were: thyrotropin >81μIU/mL, thyroglobulin was undetectable, antithyroglobulin antibody was 74.0U/mL, and the scan demonstrated no tracer uptake in the thyroid bed. No tracer uptake was apparent at the right kidney area where the cyst had been previously seen (Fig. 3). The origin of simple renal cysts is not clear, but enlargement of a renal tubule and the eventual loss of its connection to the parent tubule has been suggested. The epithelium of the renal cyst may still be excreting as the renal tubular epithelium and might express NIS [8, 9]. Thus, activity of NIS of renal tissue of the renal cyst might be the mechanism for the radioiodine uptake of the renal cyst on the post-ablation 131I whole body scan. Renal pelvis and ureter are not usually visualized due to the rapid transit time of the radioiodine through the kidneys [10]. Uncomplicated renal cysts do not usually accumulate radioiodine, although several cases have been identified with this tracer indicating either communication between the cyst and the renal collecting system or diffusion of radioiodine from renal sinus lymphatics, and/or NIS activity of the renal cyst tissue [7, 8, 10]. Renal metastases of DTC in the renal cyst, although very rare, have been reported [11] and may also show radioiodine uptake [12]. In the current case, the possibility of renal cyst metastases from the DTC was clinically ruled out by the typiCorrespondence