Clinical vignette:
83-year-old man presented for evaluation of non-toxic multinodular goiter. Medical history was significant for renal cell carcinoma (RCC) metastatic to the hip treated with chemotherapy. A surveillance PET-CT demonstrated an FDG-avid right-sided thyroid nodule with SUV of 4.58 (Fig 1). Thyroid ultrasound revealed a 2.2cm hypoechoic nodule in the right mid-lobe with faint microcalcifications (Fig 2). The patient denied dysphagia, neck radiation, or family history of thyroid cancer. Thyroid function tests were normal. FNA showed cohesive groups of cells with large nuclei, irregular nuclear contours, granular chromatin with “champagne bubble cytoplasm” (Fig 3). Subsequent thyroidectomy specimen was positive staining for RCC marker and PAX8 by immunohistochemistry, confirming metastatic RCC.
Thyroid nodules are often incidentally found on imaging studies with an estimated 25% incidence on neck CT scan (1). The risk of malignancy in thyroid nodules identified on PET-CT is reported 27.8-74.0%, and nodules with higher SUVs have a higher likelihood of malignancy (1). The overall incidence of metastatic disease to the thyroid gland is approximately 2% in autopsy series (2). The most common primary malignancy in cases of thyroid metastases is RCC followed in descending order by lung, gastrointestinal, and breast malignancies. In our patient, both PET-avidity and presence of microcalcifications in the thyroid nodule suggested a high risk of malignancy.
This case highlights the importance of high clinical suspicion in patients with PET-avid thyroid nodules, especially those with a history of malignancy. Such patients should undergo FNA to evaluate for malignancy given the high pre-test probability.