Microsatellite instability evaluation of unresectable pancreatic cancer using endoscopic ultrasound-guided fine-needle aspiration specimens

Background: The efficacy of pembrolizumab for solid tumors with a high frequency of microsatellite instability (MSI-H) has been reported. For the treatment of pancreatic cancer, it is important to collect appropriate specimens for MSI evaluation; however, the factors that affect the ability to evaluate MSI have not been clarified. Methods: We prospectively collected MSI evaluation results for patients diagnosed with pancreatic cancer by endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA). Results: From September 2019 to April 2021, MSI was found in 73 patients, and could be evaluated in 72 (98.6%). MSI could not be evaluated for one patient as it took 584 days from EUS-FNA to MSI evaluation, and there was DNA degradation in the specimen. MSI-H was found in one patient. Conclusion: The success rate for MSI evaluation using EUS-FNA specimens was high, and no factors significantly affecting the ability to evaluate MSI could be identified. However, long-term storage of specimens could result in the inability to evaluate MSI because of DNA degradation.