Case Report: Primary gallbladder undifferentiated small round cell sarcomas treated with gemcitabine, paclitaxel, bevacizumab and pembrolizumab achieved remarkable tumor regression.

Undifferentiated small round cell sarcoma (USRCS) are a rare, aggressive group of tumors associated with rapid progression, metastasis, and poor prognosis. Preoperative diagnosis remains elusive, particularly for extraosseous variants, and effective treatments are lacking. This case reports the use of pembrolizumab, bevacizumab, and conventional chemotherapy (gemcitabine plus paclitaxel) as a first-line treatment for a patient with primary USRCS in the gallbladder neck; the patient achieved remarkable partial remission for more than 9 months. This case represents the second documented instance of gallbladder USRCS and the first managed with this specific combination regimen. These findings suggest that integrating immunotherapy and targeted agents with conventional chemotherapy may offer a promising therapeutic strategy for USRCS.