Interesting False Positive Radioiodine Uptake on I-131 Whole Body Scintigraphy with Different Mechanisms in Two Patients Diagnosed Differentiated Thyroid Carcinoma: A Review of Literature

Abstract

Introduction: Radioiodine total body scan is used to detect recurrent differentiated thyroid cancer in neck and metastatic lesions. We recently encounter cases of false positive in two veteran: one suffered from shrapnel wound 45 years ago in the back; while another veteran had focal contamination underneath his Dutch beard. Cases Report: The first case was a 69-years-old Vietnam War male veteran with paraplegia and a history of papillary thyroid carcinoma, he was s/p total thyroidectomy 5/2013 (T3, N1). He received 143 mCi following the surgery. Ultrasound examination revealed recurrence, which was treated with left neck dissection and followed with a dose of 254 mCi of I-131 on 8/2014. A postRAI treatment scan revealed a focal uptake in the posterior component of T10/T11 vertebra, which, in retrospect, was present in the prior post-ablation scan in 7/2013 without any interval change. A chest CT in 9/2014 revealed a sub-centimeter metallic density in the same site; likely shrapnel deposited 45 years ago. Thus, focal radioiodine uptake likely relates to inflammatory/ benign etiology. Suppressed Tg was 3.9 in 8/2015, decreased from 5.3 ng/mL in 2/2015. The second case was a 41-years-old male veteran s/p total thyroidectomy with central neck dissection in 10/2018 and found one positive LN on left. Final path report was papillary thyroid cancer - staging T1a, N1. The patient received 75.9 mCi of I-131 for remnant ablation in 12/2018 and followed with a total body scan, which showed a focal activity subjacent to left submandibular gland. The activity persisted in a repeat scan after the patient claiming had thoroughly washed his Dutch-style beard. The SPECT/CT revealed the focal activity was not in the left submandibular area and might represent a contamination in the adjacent mustache. This was confirmed by a repeat planar image performed after the beard was shaved off. Discussion: Focal radioiodine uptake is a sensitive marker for detection of recurrence of differentiated thyroid cancer, s/p total thyroidectomy and RAI ablation. However, radioiodine ptake is not specific for thyroid tissue. It can also be seen in healthy tissue, including thymus, breast, liver, and gastrointestinal tract, or in benign diseases, such as cysts and inflammation, or in a variety of benign and malignant non-thyroidal tumors, which could be mistaken for thyroid cancer. These case studies provide examples of potential false-positive uptake of radioiodine in the whole-body scan and illustrate how such unexpected findings can be appropriately evaluated. Chronic trauma may recruit leukocytes that known to induce iodide organification by means of a myeloperoxidase. Therefore, retention of radioiodine in leukocytes of posttraumatic tissues may also explain various reports of false-positive uptake in sites of inflammation. Secretion of mucin containing iodide salts has also been suggested as another possible mechanism of iodine accumulation associated with chronic inflammatory conditions. Since radioiodine, normally concentrates in saliva, in-patient with beard should always watch for hidden contamination. Conclusion: Wounds caused by shrapnel fragments could be a common problem for veterans returning from overseas. Recognizing that false positive results could occur in RAI total body scan is clinically important.