Unusual Sites of Metastatic and Benign I 131 Uptake in Patients with Differentiated Thyroid Carcinoma

INTRODUCTION: Differentiated thyroid carcinoma (DTC) is the most common pathological type of thyroid carcinoma, which includes papillary and follicular subtypes. DTC is usually indolent, characterized by good prognosis, and long-term survival. Total thyroidectomy is the mainstay of treatment in DTC which is followed by diagnostic whole body 131I (WBI) scan. Like other primary malignancies of the head and neck, DTC follows a consistent pattern of spread in the cervical LNs. The central compartment, level VI and VII, is the first sentinel node followed by spread to the lateral compartments levels II–V, followed by the contralateral side. Inspite of nodal involvement, DTC usually have a favourable outcome. Presence of extrapulmonary distant metastases could predict a poor prognosis for high-dose (131)I therapy. However, distant metastasis occurs often as a grave event and mortality rates vary depending on metastatic sites. AIM AND OBJECTIVES: A range of rare (131)I concentrating DTC deposits in sella, orbit, choroid, skeletal muscles, liver, skin, costochondral soft tissue, pancreas and kidney, and a few benign (131)I concentrating sites are being depicted. MATERIALS AND METHODS: Metastatic sites from DTC can be easily identified by performing a whole body (131)I (WBI) scan along with a stimulated thyroglobulin (Tg) estimation (TSH >30 uIU/ml). Apart from thyroid and thyroid-related diseases, certain benign non-thyroidal pathologies can concentrate radioiodine ((131)I). From 13,000 of our patients who underwent radioiodine scan for thyroid cancer, we have selected a few cases of (131)I concentrating benign and malignant lesions for illustration. RESULTS: Out of 13000 DTC patients who underwent whole body (131)I scintigraphy in our department from Jan 2007 till Mar 2018, 25 patients revealed benign sites of (131)I uptake. 61 % patients had residual thyroid tissue with or without associated nodal involvement. Remaining patients had distant metastases. Rare sites of functioning thyroid metastases and benign sites of I 131 uptake have been selected for illustration. CONCLUSION: Apart from the WBI (two-dimensional, planar) images, single-photon emission computed tomography-computed tomography (SPECT-CT) has been incremental in localizing benign lesions which greatly depends on their location. This pictorial review highlights the need to create an awareness to detect metastatic deposits of DTC at unexpected sites. Otherwise patients will need further investigation to rule out unsuspected sites of functioning distant metastases.