Tumor-to-tumor metastasis of breast carcinoma to clear cell renal cell carcinoma: a rare case with a review of the literature.

Tumor-to-tumor metastasis (TTM) is a rare clinicopathological phenomenon in which a malignant tumor metastasizes to another distinct neoplasm. Although renal cell carcinoma (RCC) is a frequent recipient tumor, metastasis from breast carcinoma to RCC is extremely rare. A 64-year-old woman with long-standing hormone receptor-positive breast carcinoma presented with a 23-mm enhancing right renal mass. Robot-assisted partial nephrectomy revealed clear cell RCC with intratumoral nests of metastatic breast carcinoma. Immunohistochemistry revealed a reciprocal staining profile: RCC cells were positive for PAX8 and CA9, whereas metastatic breast carcinoma cells expressed CK7 and GATA3 with weak ER positivity. Clear cell RCC may serve as a recipient tumor for breast carcinoma metastasis, emphasizing that TTM should be considered in the differential diagnosis of renal masses in patients with a history of breast carcinoma, even when imaging findings are consistent with conventional RCC.