Clinical and pathological characteristics of metastatic tumors to the urinary bladder

Metastatic carcinoma to the urinary bladder is a rare and under-recognized condition in surgical pathology. In this study, we identified 8 cases of true bladder metastasis at our institution in the past 20 years, excluding secondary tumors via direct extension or serosal implantation. The most common tumor type is malignant melanoma (3/8), followed by clear cell renal cell carcinoma (2/8), adenocarcinoma of the gastrointestinal tract (2/8), and breast invasive lobular carcinoma (1/8). There are 6 cases of endoscopically exophytic metastasis and 2 cases of diffuse metastasis, commensurate with 6 patients with hematuria and 2 patients with urinary obstruction as respective clinical symptoms. Exophytic bladder metastasis usually presents with similar clinical features as urothelial carcinoma, while diffuse metastasis often masquerades as a urinary tract infection. In the latter circumstance, a markedly thickened bladder wall discerned via imaging study is the best indication for the bladder biopsy to circumvent the misdiagnosis. Histologically, the metastatic tumors can also mimic conventional urothelial carcinoma or its histological variants, and thus pose a potential diagnostic challenge to pathologists. The lack of an in situ component in primary bladder cancer may hint at bladder metastasis. Accurate diagnosis of bladder metastasis requires heightened alertness to this rare condition in addition to a multidisciplinary approach.