False-positive radioiodine whole-body scan due to a renal cyst

Abstract

Patients affected by differentiated thyroid cancer are treated surgically and by ablative radioiodine therapy. A post-therapy whole-body scan allows detection of thyroid remnants or local and distant metastases, although false-positive findings may be observed. We report a case of a 75-year-old woman with follicular thyroid cancer, who underwent ablative radioiodine treatment. On post-therapy wholebody scan, abnormal uptake in the left upper abdomen was found, although stimulated thyroglobulin level was not suggestive for distant metastases of differentiated thyroid carcinoma. Additional SPECT/CT acquisition revealed focal 131 I uptake located at the posterolateral wall of the left kidney corresponding to a round lesion 47 mm in maximal diameter. In order to verify this finding abdominal ultrasound and abdominal contrast-enhanced CT were performed, confirming multiple renal cysts in the left kidney; the largest one was the site of abnormal radioiodine accumulation. Despite the high incidence of renal cysts, especially in the elderly, radioiodine uptake in renal cysts is extremely rare. Different hypotheses on the mechanism of radioiodine uptake in the cyst were proposed, among them active secretion by sodium-iodide symporter or other transporting proteins. We conclude that abnormal radioiodine uptake in renal cysts can be an exceptional finding mimicking a metastasis.

False-positive radioiodine uptake after radioiodine treatment in differentiated thyroid cancer

“…Overall, the literature showed 145 case reports and two series regarding false-positive 131 uptake after RIT For the reasons stated above, we excluded 129 papers, considering only those in which SPECT/CT was performed. A total of 18 papers were thus included: 1 series [17] and 17 case reports [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. The series reported 23 cases of falsepositive results: 15 in the chest, six in the pelvis, one in the cranium, and one in the abdomen.…”

“…Overall, 20 cases regarded radioiodine uptake in the chest: 11 in the lung (mainly inflammatory), four on the chest bones or on soft tissue due to inflammation/trauma, one in the thymus, one mesenchymal tumor and the rest in not well-characterized sites [17][18][19][20][21]34]. A total of 15 cases regarded uptake in the abdomen/pelvis: six in the kidney (cysts), five in the ovary (teratoma, benign cysts and malignant disease), two in the uterus and the others in not well-characterized sites [22][23][24][25][26][27][28][29]. Three cases reported uptake in the head: two surgical clips, and one on one eye [30,31,33].…”

Reappraising the Diagnostic Accuracy of Post-Treatment Whole-Body Scans for Differentiated Thyroid Carcinoma

“…ptWBS is subject to diagnostic and interpretation problems like other exams [8][9][10][11][12][13] but is especially plagued by a high false-positive rate. In low-risk DTC populations, the limited detection of distant metastasis restricts its diagnostic utility [14][15][16].…”