[Mucocele of the frontal sinus simulating metastasis of follicular thyroid carcinoma in an 131I scan].

The 131I scan is the preferred test in the follow-up of differentiated thyroid cancer patients although the many unusual circumstances of radioiodine uptake that can provide false positive results must be identified. We present the case of a woman who had undergone a thyroidectomy and was being treated for follicular carcinoma with an ablative dose os radioiodine whose pre- and post-treatment scans only revealed post-surgical residual thyroid tissues. A total body scan with 131I performed at one year demonstrated the success of the ablation. However, a left supra-orbital pathological deposit was observed during a subsequent routine 131I scan. The thyroglobulin serum level was below the sensitivity level for the assay (< 1 ng/ml) and the serum antibodies against thyroglobulin were not detected. A simple x-ray and bone scintigraphy were inconclusive. The CT and MRI revealed the presence of a mucocele in the left frontal sinus which was confirmed through histological examination. The possibility of a false positive results in an 131I scan must always be kept in mind, especially in the presence of atypical uptakes and undetectable thyroglobulin serum levels. As far as we know, only one similar case has been published previously.