Retroperitoneal metastasis of breast cancer misdiagnosed as retroperitoneal fibrosis

A 66-year-old woman, who had breast cancer (invasive ductal carcinoma) 3 years before presenting, attended the nephrology clinic of Kochi Medical School hospital with acute kidney injury in June, 2021. Laboratory tests showed elevated serum creatinine, blood urea nitrogen, carcinoembryonic antigen, and cancer antigen 15-3, with an unremarkable urinalysis. [18F]fluorodeoxyglucose-PET-CT revealed a retroperitoneal mass and associated bilateral hydronephrosis (figure, A; appendix p 1). Enhanced CT showed that the retroperitoneal mass surrounded the ureters (appendix p 1).