A Rare Case of Tumor-to-Tumor Metastasis: Renal Cell Carcinoma Metastasis to Papillary Thyroid Carcinoma.

Papillary thyroid carcinoma (PTC) is the most common thyroid malignancy. Renal cell carcinoma (RCC) metastasis to the thyroid, albeit the most common carcinomatous metastasis to the thyroid, is rare, and tumor-to-tumor metastasis of RCC to PTC is even rarer. We present a case of a 65-year-old male with a history of RCC who presented with a thyroid nodule 7 years after left radical nephrectomy. Imaging showed the thyroid nodule predating the kidney tumor. Fine-needle aspiration biopsy was performed and showed 2 distinct cell populations, 1 of which was stained with RCC markers and another that was stained positively for thyroid markers. An interpretation of atypia of undetermined significance was rendered and molecular testing was indeterminate with ThyGeNEXT not detecting mutations and ThyraMIR positive for a level 2 microRNA pattern consistent with low risk for malignancy. The patient elected for active surveillance until follow-up thyroid ultrasound showed continued growth. At this point, a right hemithyroidectomy was performed. Pathology confirmed clear cell RCC metastasis to an infiltrative follicular variant papillary thyroid carcinoma. This case highlights the possibility of tumor-to-tumor metastasis in patients with a previous history of RCC even years after nephrectomy and in the absence of other metastatic lesions.