Small cell carcinoma with rhabdomyosarcomatous differentiation of the urinary bladder: an integrated clinicopathological and molecular characterization of five cases.

This study delineates the clinicopathological and molecular features of five rare cases of small cell carcinoma (SmCC) of the urinary bladder with rhabdomyosarcomatous differentiation, distinguishing them from alveolar rhabdomyosarcoma (ARMS). All patients were adult males (median age: 67 years) presenting with hematuria and solitary exophytic bladder masses (mean size: 2.9 cm). Histologically, tumors exhibited predominant SmCC morphology with focal anaplastic features. Focal area showing features suggesting rhabdomyosarcomatous differentiation included scattered cells with eosinophilic cytoplasm and eccentric nuclei resembling rhabdoid or immature skeletal muscle (2/5), and pseudoglandular/alveolar growth patterns mimicking ARMS (2/5). All tumors had coexisting urothelial carcinoma components (accounting for 5-15% of tumor volume). Immunohistochemistry revealed epithelial (AE1/AE3, CAM5.2), neuroendocrine (synaptophysin and/or chromogranin A, INSM1, CD56), and myogenic (desmin, MyoD1, myogenin) marker co-expression within SmCC. Molecular profiling demonstrated universal TP53/RB1 inactivation (5/5), recurrent TERT promoter mutations (4/5), and high tumor mutational burden (mean: 27.4 muts/Mb), while fluorescence in situ hybridization demonstrated the absence of ARMS-defining rearrangements involving PAX3, PAX7, and FOXO1 in all cases. Among four patients with available follow-up (5-19 months), one died of disease at 5 months, one developed multiple bone metastases by 5 months, while two showed no tumor recurrence or metastatic progression (both were treated with radical cystectomy followed by adjuvant etoposide-cisplatin chemotherapy). This study provides the first integrated clinicopathological and molecular characterization of SmCC with rhabdomyosarcomatous differentiation in the bladder. The presence of SmCC-specific alterations (TP53/RB1/TERT) and absence of ARMS-defining fusions support classification as a unique SmCC subtype with divergent rhabdomyosarcomatous differentiation.