Immunohistochemistry-Based Molecular Profiling of Muscle-Invasive Bladder Cancer: Analysis of 100 Consecutive Cases with Morphological Correlation.

Background/Objectives: This study aimed to profile the molecular variants of muscle-invasive bladder cancer (MIBC) based on immunohistochemical analysis and to make a correlation with morphological characteristics in a series of 100 consecutive patients. Methods: A retrospective single-center study was conducted on 100 consecutive cases of MIBC (2021-2024). A selected immunohistochemical (IHC) panel (including CK5/6, CK20, and p16) was applied in all cases to classify the tumors into known molecular variants (luminal papillary, luminal non-specified, luminal unstable, stroma-rich, basal/squamous, neuroendocrine-like). Results: Seven molecular subtypes are identified: basal (33%), luminal papillary (24%), luminal unstable (16%), luminal non-specified (10%), basoluminal (double-positive) (9%), neuroendocrine-like (double-negative with neuroendocrine morphology) (6%), and stroma-rich (2%). This distribution largely matches published data (Consensus Classification and The Cancer Genome Atlas (TCGA)), with minor differences (e.g., a lower share of the stroma-rich variant). A strong correlation is found between the histological subtypes of some tumors and their molecular variant (χ², p < 0.001): for example, all cases of urothelial carcinoma with squamous differentiation are basal, micropapillary tumors are entirely luminal, and small-cell carcinomas are neuroendocrine-like. Conclusions: The results demonstrate that the morphological subtype of urothelial carcinoma largely predetermines the molecular profile. Combining classic histopathology with IHC-based profiling allows for a more complete characterization of the tumor and aids prognosis and personalized treatment in MIBC.