Incidentaloma on Staging CT Revealed to Be Breast to Renal Tumour-to-Tumour Metastasis

Tumour-to-tumour metastases (TTM) are a rare phenomenon in which a primary tumour has metastasised within another distant primary tumour. We present the case of a 63-year-old female who presented with right-sided breast cancer. An incidental left-sided renal mass was detected on staging CT of the thorax, abdomen, and pelvis (CT-TAP). The patient had no evidence of metastases below the diaphragm. Histology following a radical left nephrectomy revealed metastatic breast cancer within a renal cell carcinoma (RCC). The patient underwent chemotherapy and surgery for right-sided breast cancer. Follow-up imaging demonstrated the metastatic spread of the breast cancer. This is an unusual case of TTM from breast to an initially occult RCC primary.