Neuroendocrine metastasis to the thyroid from unknown primary and extrathyroidal disease response to peptide receptor radionuclide therapy

Neuroendocrine tumor (NET) metastasis to the thyroid is rare, and its presentation as the first manifestation of primary malignancy elsewhere is even more uncommon. We present a case of a 41-year-old female who underwent biopsy of enlarging thyroid nodules with findings suspicious for medullary thyroid cancer (MTC). Subsequent thyroidectomy demonstrated NET of unknown primary in the left lower lobe. Immediate workup with (68)Ga-DOTATATE-PET/CT revealed abnormal somatostatin receptor (SR) expressing lesions in the liver, right cervical nodes, thoracic paravertebral soft tissue, precoccygeal soft tissue, and right acetabulum concerning for sites of neuroendocrine malignancy. Due to disease progression while on octreotide injections, a decision was made at the multidisciplinary NET board for the patient to receive peptide receptor radionuclide therapy (PRRT) which includes 4 cycles of (77)Lu-DOTATATE (Lutathera). The patient had no side effects nor toxicities during the 8 months of PRRT and achieved a partial treatment response in the early post-treatment scan at 6 weeks. This case illustrates the importance of distinguishing NET metastasis to the thyroid from MTC to ensure appropriate workup and treatment as well as predict the response of neuroendocrine malignancies to PRRT based on the visualized overexpression of SR in the SR-PET scans, despite the organ of origin.