Abstract 2185: Genetic analysis of metachronous pancreatic cancers
Authors: Hirano Tomonori , Kakiuchi Nobuyuki , Takeuchi Yasuhide , Nishimura Tomomi , Masui Toshihiko , Minamiguhi Sachiko , Haga Hironori , Chiba Kenichi , Tanaka Hiroko , Shiraishi Yuichi , Miyano Satoru , Norimitsu Uza , Kodama Yuzo , Seno Hiroshi , Ogawa Seishi 
Published: July 2021

Abstract
[Introduction] Early detection of pancreatic cancer is a key to curable surgery. However, many patients are diagnosed with an advanced disease and even in those who have an early stage tumor successfully resected, the development of metachronous cancer in the residual pancreas prevents a long-term survival. Thus, it is important to control metachronous tumors to improve clinical outcomes, whose pathogenesis, however, is poorly understood. In this study, we aimed to reveal the origin of metachronous tumors using an unbiased detection of somatic mutations in primary and metachronous tumors as well as adjacent precursor lesions.
[Methods] Serially obtained formalin-fixed paraffin-embedded surgical specimens from 12 patients who had undergone curative surgery for an early-stage pancreatic cancer were subjected to laser microdissection for enrichment of tumor and precancerous components, from which DNA was extracted and analyzed for somatic mutations using whole-exome sequencing (WES) with matched normal DNA. Based on shared and private mutations across different samples, we interrogated history of clonal evolution of these lesions.
[Results] Pathology for resected primary cancer was margin-negative in all patients. The median interval between the initial and second surgery was 34.6 months (12 - 65.1 months). Paired primary and metachronous cancers were analyzed in all 12 patients. An additional 5 pancreatic intraepithelial neoplasia (PanIN) samples were also analyzed in one case. We identified a median of 86 (range: 40-145) and 20 (14-42) somatic mutations using WES in cancers and PanIN lesions, respectively. None of the patients have known pathogenic germline variants. All samples had one or more driver mutations. In each patient, all driver mutations and many passenger mutations were shared between primary and metachronous cancer samples, suggesting that those metachronous tumors are evolutionally closely related to the primary cancer, despite long years before recurrence. Negative pathology of the margins at the initial surgery suggested that those metachronous lesions originated from distant dissemination or metastasis, rather than contiguous, intraductal invasion. By contrast, none of the mutations other than hotspot KRAS mutations were shared between precursor lesions and cancer samples. Despite multiple independent clones in the precancerous lesion, all metachronous tumors originated from the primary lesions in this study.
[Conclusions] Our study suggests that even early pancreatic cancer might be disseminated within the pancreas and give rise to metachronous cancers, suggesting the importance of close monitoring of recurrence in the residual pancreas.
Citation Format: Tomonori Hirano, Nobuyuki Kakiuchi, Yasuhide Takeuchi, Tomomi Nishimura, Toshihiko Masui, Sachiko Minamiguhi, Hironori Haga, Kenichi Chiba, Hiroko Tanaka, Yuichi Shiraishi, Satoru Miyano, Uza Norimitsu, Yuzo Kodama, Hiroshi Seno, Seishi Ogawa. Genetic analysis of metachronous pancreatic cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2185.