Unexpected presentation: metastatic adenoid cystic carcinoma (AdCC) of the breast masquerading as a large cystic-solid renal mass.

Adenoid cystic carcinoma (AdCC) of the breast is a rare histological subtype of breast cancer, which usually has a low propensity for metastasis and is associated with a good prognosis. AdCC metastasis to the kidney is rare, with only 29 cases reported in the literature. We report a case of a woman in her 60s with multiple right-sided large cystic-solid renal lesions after a recent diagnosis of marginal zone B-cell lymphoma. The patient previously had T1c N0 breast AdCC, treated with curative intent surgery and adjuvant radiotherapy 10 years prior. She underwent a radical nephrectomy, and histology was confirmed asmetastatic classic AdCC, consistent with the spread from previous AdCC breast cancer. Although she was in remission for 10 years, it is imperative to distinguish between breast AdCC metastasis and primary cystic renal cell carcinoma to be able to formulate an optimal management strategy for these tumours.