False-Positive I-131 Whole-Body Scan in Follicular Thyroid Carcinoma Caused by Frontal Sinus Mucocele

Abstract

A 42-year-old woman had a thyroidectomy and was being treated with radioiodine for follicular carcinoma. Pre- and post-treatment scans revealed only residual thyroid tissue. During a subsequent routine I-131 scan, a left supraorbital metastasis was considered possible, but the results of skull radiographs and planar bone scanning were inconclusive. CT and MRI revealed the presence of a mucocele in the left frontal sinus, which was confirmed through histologic examination. A false-positive result in an I-131 scan is always possible, especially in the presence of atypical uptake and when the serum level of thyroglobulin is within normal limits. FIGURE 1Fig. 1: A 42-year-old woman underwent total thyroidectomy for a follicular carcinoma located in the right thyroid lobe. (A) An ablative dose of 3 GBq (80mCi) I-131 was administered 2 months later and the results of her thyroid scan were unchanged. (B) One year later, a repeated scan performed with 370 MBq (10 mCi) I-131 revealed, to our surprise, an area of intensely increased uptake in the left supraorbital region that did not disappear despite exhaustive washing, thus eliminating the possibility of skin contamination. When questioned about this, the patient indicated that during the preceding year she experienced a mild continual headache in the left frontal area and increased tearing in the left eye. Serum levels of thyroglobulin continued to be less than 1 ng/ml. Skull radiographs showed increased density in the left frontal sinus. A Tc-99m MDP bone scan revealed increased activity in the left frontal sinus.FIGURE 2Fig. 2: The findings on MRI, T1 (left) and T2 (right), were consistent with a mucocele of the left frontal sinus. Histologic examination of the tissues removed confirmed the final diagnosis of mucocele. Although generally rare, potential explanations for a false-positive result of I-131 scan are varied and include physiologic (1) and pathologic reasons (2–4). In this case, focal uptake of I-131 in the left frontal region corresponded to a mucocele in the left frontal sinus, a lesion almost always resulting from inflammation (sinusitis) with accumulation of inflammatory exudates and mucous secreted by the hypertrophied glands and mucinous cells. Similar findings have been reported by Matheja and colleagues (5).