The first report of bladder metastasis from breast cancer was by Haid et al.  in 1980, but the first autopsy report was published in 1956 . To date, there have been about 50 reported cases of metastatic breast cancer to the bladder . Bladder metastasis and retroperitoneal metastasis are considered to occur more frequently with invasive lobular breast carcinoma. The metastatic pathway could be hematogenous, lymphogenous, or direct retroperitoneal invasion.
The most common symptoms of bladder carcinoma are frequent urination and gross hematuria. Other clinical features include difficulty in urination, ureteral obstruction or urinary incontinence, a pelvic mass, bilateral hydronephrosis, and ultimately renal failure. Bladder carcinoma can also present as an incidental finding in imaging studies. Some cases are found simultaneously with the primary tumor or after a long course of more than 30 years . On the other hand, some cases of breast cancer are diagnosed after discovering a bladder metastasis. In most cases, the cancer has already become widespread at the time of diagnosis, and it is rare for only bladder metastasis to be detected, as in our case . Screening with magnetic resonance imaging and positron emission tomography–computed tomography is useful for the diagnosis of metastasis to other sites. In the case of renal dysfunction with suspected obstructive nephropathy, an examination of the upper urinary tract is also necessary. To confirm the diagnosis, observation of the bladder mucosa by cystoscopy and histological examination of a specimen obtained by biopsy or transurethral resection are necessary. Cystoscopic findings include obvious tumors, nonspecific inflammation, and a thickened bladder wall covered with normal mucosa. The presence of changes in the bladder mucosa is useful in distinguishing a primary bladder tumor from a metastatic bladder tumor. A submucosal tumor is suggestive of a secondary bladder tumor, but ulcerative lesions can be seen in some cases. In the present case, a diagnosis was difficult based on morphology alone, and an immunohistochemical analysis was necessary. Common screening markers for suspected breast tumors include the expression of cytokeratin, CK-7, CK-18, CK-19, CK-20, GCDFP-15, and ER/PgR. The treatment of metastatic breast cancer involves chemotherapy and hormonal therapy. Local resection is often performed for diagnostic purposes and to improve local symptoms. In our case, the immunohistochemical characteristics of the metastatic lesion were similar to those of the primary tumor. This was an important factor leading to a definitive diagnosis, in addition to the pathological results obtained using hematoxylin and eosin staining. However, not uncommonly, the ER/PgR status of the metastatic lesion can differ from that of the primary tumor, with one study reporting an inconsistency rate as high as 24% . HER2-positive breast cancer tends to be more aggressive than other breast cancers and less responsive to hormonal therapy. If the patient has hydronephrosis because of a urinary obstruction, constructing a nephrostomy before the start of chemotherapy should help to improve renal function. The prognosis of secondary bladder carcinoma is very poor, and most patients die within 1 year. However, some cases with a survival of 5 years or more after diagnosis have been reported . Bladder metastasis from breast cancer is often advanced at the time of diagnosis. It is recommended that accurate diagnosis be pursued and systemic therapy be started promptly. In our case, we confirmed the presence of a bladder metastasis from a primary breast carcinoma by confirming the presence of CD7 positivity, CD20 negativity, ER positivity, GCDFP-15 positivity, and HER2 positivity, in addition to the result of a pathological examination.