Abstract 3891: Morphologic spectrum of bladder cancer variants and incidental prostate pathology in cystoprostatectomy specimens.

Abstract High-grade bladder cancer exhibits substantial morphologic heterogeneity, and several variant histologies show distinct biological behavior and therapeutic responsiveness. To define the full pathologic spectrum in cystoprostatectomy specimens, we analyzed 44 cases treated between 2010 and 2023. The cohort (mean age 67.1 ± 12.2 years) predominantly presented with advanced disease; pT3a was the most common stage (25%), and one-third showed invasion of≥pT3. High-grade urothelial carcinoma was the primary subtype (14 cases). True variants were frequent and included squamous differentiation (6), sarcomatoid (2), micropapillary (1), nested (1), giant-cell/pleomorphic (1), and small-cell carcinoma (1). Papillary carcinoma and CIS were reclassified into the high-grade urothelial category. Margin positivity occurred in 6 patients (13.6%), especially among aggressive variants such as squamous, sarcomatoid, and micropapillary carcinoma. Lymph-node metastasis was identified in 3 patients (6.8%), all with high-grade invasive disease. Incidental prostate adenocarcinoma was common, spanning Grade Groups 1-4 and often multifocal. These tumors were low-volume, organ-confined, and rarely showed adverse features such as extraprostatic extension, seminal vesicle invasion, or positive margins. Cribriform/glomeruloid patterns were uncommon. Additional findings included HGPIN, chronic inflammation, and treatment-related changes. Actual bladder cancer invasion into prostatic stroma (pT4a) was rare (one case). Significantly, incidental prostate cancer did not correlate with bladder cancer histology or stage, reflecting their biological independence. Variant histologies have direct therapeutic implications: sarcomatoid, micropapillary, and small-cell carcinoma are associated with aggressive behavior, higher positive-margin rates, and may warrant intensified systemic therapy or enrollment in variant-specific clinical trials. Accurate identification prevents under-treatment of high-risk patients. In contrast, the incidental prostate cancers were clinically silent and indolent; recognizing them avoids unnecessary PSA surveillance, misinterpretation of imaging findings, and prevents staging errors, especially in distinguishing incidental cancer from actual prostatic stromal invasion. Overall, integrated bladder-prostate pathologic assessment improves staging accuracy, guides risk-adapted management, and supports multidisciplinary decision-making. These findings underscore the biological and clinical significance of variant histology in bladder cancer, while confirming that incidental prostate cancer in cystoprostatectomy specimens is typically low-risk and does not influence bladder cancer outcomes. Citation Format: Shannon Carskadon, Aryan Joshi, Nilesh S. Gupta, Nallasivam Palanisamy. Morphologic spectrum of bladder cancer variants and incidental prostate pathology in cystoprostatectomy specimens [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 3891.