Tumor-to-Tumor Metastasis of Renal Cell Carcinoma to a Follicular Variant of Papillary Thyroid Carcinoma: A Case Report and Literature Review

Abstract

Renal cell carcinoma (RCC) is the most common renal malignancy. It has a variable clinical course with metastasis to unusual sites occurring months to years after the initial diagnosis. However, metastasis can also be the first presentation of RCC. Although relatively uncommon, the thyroid gland is the most common location for RCC metastasis in the head and neck region. Tumor-to-tumor metastasis is an exceedingly rare occurrence. Only 10 cases were reported of RCC metastasis to primary thyroid neoplasms. We present a case of clear cell RCC metastasizing to a follicular of variant papillary thyroid carcinoma (FVPTC) 14 years after the initial diagnosis of RCC. A review of similar reported cases revealed that the most common primary thyroid recipient of tumor-to-tumor metastasis of RCC was FVPTC. The rich lymphovascular network in FVPTC compared to other thyroid tumors, which may promote the deposition of metastatic tumor cells, might explain this predilection. Careful review of the clinical and radiological findings and checking for any history of malignancy when examining thyroid nodules is important for guiding further studies. Performing a targeted panel of immunohistochemical stains for any suspicious areas is also essential for the diagnosis of such unusual cases.

Keywords: clear cell renal carcinoma; follicular variant of papillary thyroid carcinoma; immunohistochemistry; renal cell carcinoma (rcc); thyroid neoplasms; tumor-to-tumor metastasis.

Figure 1. Largest thyroid nodule shows an encapsulated lesion composed of small follicles (black arrows), with a second population of clear cells (red arrows), hematoxylin and eosin stained section (H&E; x2).

Figure 2. The lining cells exhibit nuclear enlargement, clearing, overlapping, and nuclear grooves, hematoxylin and eosin stained section (H&E; x20).

Figure 3. A second population of clear cells is seen adjacent to the follicles (red arrows), hematoxylin and eosin stained section (H&E; x4).

Figure 4. Clear cells are arranged in nests and sheets with small round nuclei, inconspicuous nucleoli, and a rich vascular background, hematoxylin and eosin stained section (H&E; x10).

Figure 5. Both components were positive for PAX-8 (x10).

Figure 6. The follicular component was immunoreactive for TTF-1 (x10).

Figure 7. The follicular component shows immunoreactivity for thyroglobulin (x10).

Figure 8. Membranous staining for CD10 in the clear cell component (x10).