A rare case of tumor-to-tumor metastasis: breast cancer metastatic to a benign renal mass.

Tumor-to-tumor metastasis is a clinical entity that has been documented in medical literature as early as the 1900s. It is rare and published primarily as case reports, although its incidence seems to be on the rise as a result of improving medical care in developed countries. We report an unusual case of metatstatic breast tumor to a benign renal tumor, in a patient with a remote history of breast cancer. This case reinforces in our minds the enigmatic nature of breast cancer and its wide spectrum of clinical behavior.