Metastasis of breast cancer to renal cancer: report of a rare case.

Tumor-to-tumor metastasis (TTM) is a rare phenomenon. We present a case of an invasive ductal carcinoma (IDC) of the breast metastasizing to a clear cell renal cell carcinoma (RCC). Breast cancer (BC) metastasis to the RCC is rarely reported, especially in resected kidney tumor. In several cases reported, IDC was the exclusively histologic type of BC metastasized to RCC. It seems that the different molecular type of IDC doesn't affect the metastatic tendencies to RCC. TTM was an indicator of diffuse disease. For any patient with a history of breast cancer, especially with multi-organs metastasis, resection of kidney tumor should be carefully considered.