I-131 uptake demonstrated in the appendix on a posttreatment scan in a patient with thyroid cancer

Causes of False-Positive Radioactive Iodine Uptake in Patients with Differentiated Thyroid Cancer

Purpose

Radioactive iodine (RAI) whole-body scan is a sensitive imaging modality routinely used in patients with differentiated thyroid cancer to detect persistent and recurrent disease. However, there can be false-positive RAI uptake that can lead to misdiagnosis and misclassification of a patient’s cancer stage. Recognizing the causes of false positivity can avoid unnecessary testing and treatment as well as emotional stress. In this review, we discuss causes and summarize various mechanisms for false-positive uptake.

Recent Findings

We report a patient with differentiated thyroid cancer who was found to have Mycobacterium avium complex infection as the cause of false-positive RAI uptake in the lungs. Using this case example, we discuss and summarize findings from the literature on etiologies of false-positive RAI uptake. We also supplement additional original images illustrating other examples of false RAI uptake.

Summary

False-positive RAI uptake may arise from different causes and RAI scans need to be interpreted in the context of the patient’s history and corresponding cross-sectional imaging findings on workup. Understanding the potential pitfalls of the RAI scan and the mechanisms underlying false uptake are vital in the care of patients with differentiated thyroid cancer.

Unusual uptake of radioiodine in the small bowel in a patient with thyroid cancer and bowel malrotation

Whole-body radioiodide scan is an essential procedure in the evaluation of radioiodine-avid metastatic foci in patients with differentiated thyroid carcinoma. Normal physiologic distribution of the radiotracer includes the stomach and colon, but the small intestine is not often seen. Gastrointestinal anatomic variants may be associated with atypical activity on I scan. We report here a case of intestinal malrotation with associated I uptake in the small intestine. Careful correlation of planar imaging with SPECT/CT revealed the likely malrotation as well as small bowel uptake superimposed over normal colonic uptake. This finding was occult on prior I scan.