Follicular Variant of Papillary Thyroid Cancer with Bilateral Renal Metastases Discovered Incidentally During Work-Up of Primary Endometrial Cancer: A Rare Occurrence

BACKGROUND Follicular variant of papillary thyroid cancer (FV-PTC) is the second most common subtype of papillary thyroid cancer (PTC) after classic PTC. FV-PTC is characterized by nuclear features consistent with classic PTC but has a follicular architecture that lacks classic papillary morphology. Thyroid cancer rarely metastasizes to the kidney. Only 6 cases of FV-PTC metastasizing to the kidney have been reported in the English literature. We are reporting a case of FV-PTC with bilateral renal metastases discovered incidentally during work-up of primary endometrial cancer. CASE REPORT A 70-year-old woman presented with post-menopausal bleeding secondary to endometrial cancer. Staging work-up showed multiple bilateral lung nodules, bilateral soft tissue kidney masses, and multinodular goiter. The pathological and immnohistochemical profile of the lung biopsy was consistent with primary well-differentiated lung adenocarcinoma. Follow-up computerized tomography scan showed stable lung nodules and enlarging renal masses, which was suggestive of bilateral renal cancer. While the histologic features of the renal biopsy were not typical, the immunohistochemical staining of renal biopsy was positive for Paired box 8, thyroid transcription factor-1, thyroglobulin, and cytokeratin 7, suggesting the thyroid as the primary cancer site. The final histopathology on surgical specimen of total thyroidectomy revealed follicular variant of papillary thyroid cancer. CONCLUSIONS The presence of pulmonary nodules and kidney masses does not always suggest the lung or the kidney as primary tumor sites. The clinician should be aware of the possibility of metastasis and look for the primary source, which in the present case was FV-PTC. Immunohistochemistry plays an important role in determining the primary site of origin. In case of multiple-organ metastases, each metastatic lesion should be biopsied as soon as possible for definitive diagnosis and appropriate treatment.