Small cell carcinoma (SCC) occurs predominantly in the lung, with approximately 4% occurring in extrapulmonary tissues [1]. SCBC accounts for approximately 0.5% of primary bladder malignancies, which is relatively rare and has a higher grade of malignancy than urothelial carcinoma (UC) in the same period [2]. SCBC is common in older men, and the initial symptoms and imaging features lack specificity. Diagnosis relies on pathologic examination. Given the lack of specificity of early clinical symptoms and screening indicators, most patients are diagnosed with advanced disease and have a poor prognosis.

Based on the RNA expression of the transcriptional regulators ASCL1, NEUROD1, POU2F3, and YAP1, the subtypes of SCLC have been identified, namely SCLC-A, SCLC-N, SCLC-P, and SCLC-Y. POU2F3 was expressed in 7% of SCLC and was mutually exclusive with ASCL1 and NEUROD1, and ASCL1/NEUROD1 double-negative SCLC represents a distinct neuroendocrine-low subtype, which is the SCLC-P [3], [4], [5]. A study on POU2F3 in extrapulmonary small cell carcinoma (EPSCC) showed that POU2F3 positive cluster cells in the urinary system were mainly distributed in the bladder [6]. However, the clinicopathological characteristics of POU2F3 positive SCBC are still unclear. This paper aims to explore POU2F3-positive SCBC and improve the understanding of POU2F3 in SCBC by analyzing the data of 12 cases of SCBC and reviewing the literature.