| Summary: | Tumor-to-tumor metastasis is an uncommon but well-documented phenomenon. We present a case of a clear cell renal cell carcinoma (RCC) metastasizing to an invasive ductal carcinoma (IDC) of the breast. A 74-year-old woman with a past history of clear cell RCC status after radical nephrectomy underwent right modified radical mastectomy for an enlarging breast mass 3 years after nephrectomy. Histological examination revealed a small focus with distinct morphological features similar to clear cell RCC encased in the otherwise typical IDC. Immunohistochemical studies showed that this focus was positive for CD10 and vimentin, in contrast to the surrounding IDC, which was negative for both markers and positive for Her2/neu. Based on the histological and immunohistochemical features, the patient was diagnosed with metastasis of clear cell RCC to the breast IDC. To the best of our knowledge, this is the first reported case of a breast neoplasm as the recipient tumor in tumor-to-tumor metastasis.
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