Abstract Gastrointestinal stromal tumor (GIST), the most common sarcoma, metastasizes hematogenously and peritoneally. Tumor-free margins and complete cytoreduction of metastatic disease are critical for decreasing recurrence. Since most GISTs overexpress KIT (c-KIT, CD117), we hypothesized that fluorescently labeled anti-KIT antibodies can improve detection of GIST. KIT K641E+/− transgenic mice that spontaneously develop cecal GIST, and wild-type C57BL/6 mice were used in this study. Wistar rat anti-mouse monoclonal c-KIT antibody was labeled with the AlexaFluor 488. The conjugated anti-KIT-Alexa 488 antibody (50-100 μg) was delivered via tail vein at 24 hours prior to standard staging laparoscopy. A pediatric laparoscopic grasper was inserted in the left lower quadrant for bowel mobilization in order to visualize the cecum. All 4 quadrants of the peritoneal cavity were examined systematically under both fluorescence and bright light. Post laparoscopy, mice were sacrificed and their abdomens exposed for Olympus OV-100 imaging. All tumors were resected and confirmed by H&E. We divided the mice into 4 groups. KIT K641E+/− mice received anti-KIT antibody (N=4) or control antibody (N=3). Wild-type mice received the same antibodies (N=3 per group). Fluorescence laparoscopy demonstrated a high tumor signal-to-background noise ratio. Upon whole body imaging used to detect gross tumors, there were 2 false positive and 0 false negative results, leading to an accuracy of 92%. The sensitivity, specificity, positive and negative predictive values were: 100%, 87%, 85%, and 100%, respectively. This method has several translatable applications: 1) identification of peritoneal metastases; 2) following disease response; 3) non-radioactive imaging; 4) endoscopic differentiation of gastric GISTs from leiomyomas or schwannomas; 5) laparoscopic staging; and 6) assessment of margin status. Citation Format: Cristina A. Metildi, Chih-Min Tang, Sharmeela Kaushal, Stephanie Y. Leonard, Paolo Magistri, Robert M. Hoffman, Michael Bouvet, Jason K. Sicklick. In vivo fluorescence labeling of gastrointestinal stromal tumor (GIST) . [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 731. doi:10.1158/1538-7445.AM2013-731