Mammary Carcinoma Metastasizing to Renal Cell Carcinoma: A Rare Example of Tumor-to-Tumor Metastasis.

IntroductionSimultaneous occurrence of multiple primary cancers is rare, and even rarer is the phenomenon of one cancer metastasizing to another. Here, we present a mammary carcinoma and renal cell carcinoma (RCC) diagnosed concurrently while presenting as a tumor-to-tumor metastasis.Case ReportA 79-year-old woman with right pleural effusion underwent thoracocentesis. Cytological and immunohistochemical examination revealed malignant cells suggestive of adenocarcinoma, mammary origin. Computed tomography of abdomen and chest revealed a 1.6 cm right breast nodule, 4.4 cm right axillary mass, and right renal lesion measuring 3.0 × 1.6 cm. Core biopsy of the renal lesion showed 2 populations of cells. One population exhibited moderate cytoplasm and enlarged nuclei displaying anisonucleosis and hyperchromasia, arranged singly as well as in sheets and clusters. These cells stained positive for GATA3, estrogen receptor, mammaglobin, and keratin 7. Intermixed were the second population of cells with clear cytoplasm and low nuclear grade, forming sheets, replacing renal tissue. These cells were positive for CA9, CD10, and PAX8 and negative for P40. Based on these findings, a diagnosis of clear cell RCC harboring metastatic breast carcinoma was established.ConclusionRCC and breast carcinoma are known to increase the occurrence of each other. When breast and renal lesions are identified concurrently, the possibility of breast metastasis should be strongly considered. The concurrent appearance of RCC harboring metastatic breast cancer is rare. This contributes valuable insights into the complex interplay between different cancer types. Understanding these rare phenomena can aid in refining diagnostic approaches and treatment strategies.