I-131 false-positive uptake in a thymic cyst with expression of the sodium-iodide symporter: A case report

Radioiodine whole body scan pitfalls in differentiated thyroid cancer

PURPOSE

whole body scan (WBS) performed following diagnostic or therapeutic administration of I-131 is useful in patients with differentiated thyroid carcinoma. However, it can be falsely positive in various circumstances. We aimed to report a series of pitfalls in a clinical perspective.

METHODS

A search in the database PubMed utilizing the following terms: "false radioiodine uptake" and "false positive iodine 131 scan" has been made in January 2023. Among the 346 studies screened, 230 were included in this review, with a total of 370 cases collected. Physiological uptakes were excluded. For each patient, sex, age, dose of I-131 administered, region and specific organ of uptake and cause of false uptake were evaluated.

RESULTS

370 cases of false radioiodine uptake were reported, 19.1% in the head-neck region, 34.2% in the chest, 14.8% in the abdomen, 20.8% in the pelvis, and 11.1% in the soft tissues and skeletal system. The origin of false radioiodine uptake was referred to non-tumoral diseases in 205/370 cases (55.1%), benign tumors in 108/370 cases (29.5%), malignant tumors in 25/370 cases (6.7%), and other causes in 32/370 cases (8.7%).

CONCLUSIONS

WBS is useful in the follow-up of patients with differentiated thyroid carcinoma, however it can be falsely positive in various circumstances. For this reason, it is critically important to correlate the scintigraphic result with patient's medical history, serum thyroglobulin levels, additional imaging studies and cytologic and/or histologic result.

Concurrent Benign Iodine-Avid Thymic and Liver Cysts Mimicking Metastatic Disease on 131I Whole-Body Scintigraphy

ABSTRACT

A 57-year-old woman received radioiodine therapy post total thyroidectomy for pT3aNxMx follicular thyroid carcinoma. Posttherapy 131I whole-body scan showed 131I concentration in the chest, mediastinum, and left upper thigh with stimulated thyroglobulin (Tg) of 89 μg/L. Subsequent radioiodine therapies showed persistent 131I accumulation in the anterior mediastinal soft tissue lesions and a hypodense segment VII liver lesion visualized on SPECT/CT, suggestive of iodine-avid metastatic disease despite the undetectable serum Tg (<1.0 μg/L) with no Tg antibody interference. Biopsy of the liver lesion revealed liver cyst, and consequent removal of the mediastinal lesions showed benign thymic cysts.

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

BACKGROUND AND PURPOSE

False-positive radioiodine uptake can sometimes be observed with post-radioiodine treatment (RIT) whole body scanning. Radioiodine pitfall has often been reported as being caused by benign or inflammatory disease, or, in some cases, by tumor lesions. This paper reviews the possible causes of such false-positive imaging, and suggests possible reasons for suspecting these pitfalls.

METHODS AND RESULTS

Online databases, including MEDLINE (via PubMed), Embase, ISI Web of Science, Google Scholar, and Scopus, were systematically examined, using different keyword combinations: "radioiodine false-positive imaging", "131 I false-positive imaging" and " RAI false-positive imaging". An illustrative case was described. Excluding cases in which SPECT/CT was not performed, a total of 18 papers was found: 17 case reports and one series regarding false-positive iodine-131 uptake after RIT.

CONCLUSIONS

The prevalence of radioiodine pitfall was significantly reduced through the use of SPECT/CT imaging, though its possible presence has always to be taken into account. Inflammation, passive iodine accumulation, other tumors, and, sometimes, unknown causes can all potentially generate false-positive imaging. Missing detection of false-positive imaging could result in over-staging and inappropriate RIT or it could lead to the non-detection of other cancers. We examine the reasons for these possible pitfalls.

Strongly Radioiodine-Positive Pancreatic Adenocarcinoma Mimicking Metastasis of Differentiated Thyroid Cancer

We present an interesting image of a strikingly intense radioiodine accumulation of a histologically proven pancreatic adenocarcinoma mimicking metastasis of differentiated thyroid cancer in a 63-year-old woman with recurrence of papillary thyroid carcinoma undergoing radioiodine therapy. This interesting image should draw attention to considering pancreatic adenocarcinoma in radioiodine-positive pancreatic lesions.