## Histological Classification of Bladder Cancer **Key Point:** Urothelial carcinoma (also called transitional cell carcinoma) is the most common histological type of bladder cancer, accounting for 90–95% of all cases [cite:Harrison 21e Ch 89]. ### Epidemiology Urothelial carcinoma arises from the transitional epithelium (urothelium) that lines the bladder. It can present as either non-muscle-invasive disease (NMIBC, ~75% at diagnosis) or muscle-invasive disease (MIBC, ~25% at diagnosis). ### Other Histological Types (Rare) | Histological Type | Frequency | Key Features | |---|---|---| | Squamous cell carcinoma | 3–5% | Associated with chronic irritation, schistosomiasis (endemic areas), neurogenic bladder | | Adenocarcinoma | 1–2% | Often from urachal remnant or intestinal metaplasia; poor prognosis | | Small cell carcinoma | <1% | Neuroendocrine origin; highly aggressive | | Mixed/Other | <1% | Sarcoma, lymphoma (rare) | **High-Yield:** Urothelial carcinoma is the only common type; all others are rare and often have worse prognosis or specific risk factors (schistosomiasis, chronic irritation, urachal anomaly). **Clinical Pearl:** The term "transitional cell carcinoma" is older nomenclature; modern pathology uses "urothelial carcinoma" to reflect the cell of origin (urothelium).
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