[A CASE OF TUMOR-TO-TUMOR METASTASIS OF BREAST CANCER WITHIN RENAL CELL CARCINOMA].

A 76-year-old woman presented with a left chest ulcer. Computed tomography (CT) showed a 9-cm-sized invasive mass with ulceration in the left breast, along with regional lymph node and distant metastases, and a 4-cm-sized tumor at the upper pole of the left kidney. Needle biopsy of the left breast tumor was performed, and she was diagnosed with breast cancer, cT4cN3M1, Stage IV. She underwent endocrine therapy and chemotherapy. After undergoing treatment for 2 years, CT showed no progression in the primary and metastatic lesions except for increasing renal mass. Primary kidney cancer was suspected, and she underwent laparoscopic nephrectomy. In the surgical specimen, a solid, brown-colored nodule was observed in the upper pole of the left kidney. The pathological diagnosis was clear cell renal cell carcinoma, including a metastatic lesion of breast cancer. She was diagnosed with tumor-to-tumor metastasis of breast cancer within renal cell carcinoma. Two years after the surgery, no recurrence of renal cell carcinoma has been observed, and she is undergoing secondary endocrine therapy against breast cancer.