Metastatic Renal Cell Neoplasm Within a Papillary Thyroid Carcinoma as Incidental Finding in an Asymptomatic Patient: a Case Report

Tumor-to-tumor metastasis is a rare phenomenon affecting the thyroid gland. Otherwise, it is not unusual for a neoplasm to be detected incidentally, without clinical symptoms. But what is exceptionally rare is that a malignant neoplasm debuts as an incidental finding in the form of a tumor-to-tumor metastasis. We contribute with the case of a previously healthy 41-year-old woman who presented an asymptomatic 39-mm-sized nodule on the left thyroid lobe in a routine health checkup. Ultrasound-guided fine needle aspiration cytology confirmed a papillary thyroid carcinoma. Because of the size, a total thyroidectomy was performed. In the surgical specimen, in addition to a classic papillary carcinoma with an extensive follicular pattern (pT2 pN0), a second component was identified, consisting of a clear cell population with immunostaining profile of renal origin. Subsequent studies found a 110-mm-sized left renal mass. A nephrectomy was performed and the pathological exam confirmed a renal cell carcinoma, pT2 pNX pM1, with metastasis also to the adrenal gland. To our knowledge, this is the first case described of this clinical presentation involving those two neoplasms. We present herein such an unusual clinical case along with a brief clinical and pathological review on tumor-to-tumor metastasis, with special emphasis on renal-to-thyroid ones.