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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.<\/jats:p>","DOI":"10.3389\/fimmu.2026.1719285","type":"journal-article","created":{"date-parts":[[2026,3,17]],"date-time":"2026-03-17T14:11:47Z","timestamp":1773756707000},"update-policy":"https:\/\/doi.org\/10.3389\/crossmark-policy","source":"Crossref","is-referenced-by-count":0,"title":["Case Report: Primary gallbladder undifferentiated small round cell sarcomas treated with gemcitabine, paclitaxel, bevacizumab and pembrolizumab achieved remarkable tumor regression"],"prefix":"10.3389","volume":"17","author":[{"given":"Dan","family":"Liu","sequence":"first","affiliation":[{"name":"Department of Radiology, Sichuan Provincial People\u2019s Hospital, University of Electronic Science and Technology","place":["Chengdu, Sichuan, China"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Xiaoge","family":"Liu","sequence":"additional","affiliation":[{"name":"Department of Ultrasound, Ya\u2019an People\u2019s Hospital","place":["Ya an, Sichuan, China"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Mou","family":"Li","sequence":"additional","affiliation":[{"name":"Department of Radiology, Sichuan Provincial People\u2019s Hospital, University of Electronic Science and Technology","place":["Chengdu, Sichuan, China"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Xin","family":"Li","sequence":"additional","affiliation":[{"name":"Department of Radiology, Sichuan Provincial People\u2019s Hospital, University of Electronic Science and Technology","place":["Chengdu, Sichuan, China"]}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1965","published-online":{"date-parts":[[2026,3,17]]},"reference":[{"key":"B1","doi-asserted-by":"publisher","DOI":"10.14670\/HH-11-792","article-title":"Ewing sarcoma and the new emerging Ewing-like sarcomas: (CIC and BCOR rearranged-sarcomas). 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