Background: Metastatic tumors in the breast from extramammary primary malignancies are uncommon, and the kidney has been the site of a primary in very rare cases.
Case: Fine needle aspiration cytology and immunohistochemical findings in a 70-year-old woman are described in whom a left-sided breast mass from a metastatic clear cell (conventional) carcinoma of the kidney was diagnosed due to unusual cellular findings. A few years earlier, the patient had a right-sided primary breast carcinoma for which a lumpectomy with clear margins had been done. Also, since on comparison the cell pattern of the breast primary did not match the morphology of the clear cell tumor on the left side, further immunohistochemical staining was undertaken on sections of cell blocks and smears from the aspirate and confirmed positivity for oil-red-O, CD10, renal cell carcinoma (RCC) marker and AE1/AE3 and negativity for ER, PR, CK7, HER2/neu and mucus, which favored the diagnosis of RCC. Furthermore, the ER, PR, CK7, HER2/neu were positive in sections of the primary breast carcinoma, while the staining for CD10, RCC marker and AE1/AE3 were negative. On further inquiry following cytodiagnosis, it was found that a few years earlier the patient had had a left nephrectomy for a clear cell carcinoma.
Conclusion: The findings emphasize a cautious approach in interpreting cytologic findings in aspirates with unusual cell features that do not resemble those of a primary breast carcinoma and warrant further workup using selective immunohistochemical stains, which can be useful in resolving the diagnostic dilemma of distinguishing a primary from a metastatic carcinoma.