204. ALTERATIONS IN LOCAL OESOPHAGEAL MICROBIOTA AND OUTCOMES AFTER OESOPHAGECTOMY FOR OESOPHAGO-GASTRIC CANCER: A PILOT STUDY

Abstract Oesophageal cancer is a global health burden, with an estimated 600,000 new cases and 550,000 deaths per year. It is the 9th most common cancer yet is ranked fifth for cancer mortality. The current gold standard curative strategy includes oesophagectomy, which is a morbid procedure. In particular, anastomotic leaks are a feared complication postoperatively that result in significant morbidity. Studies of patients undergoing colorectal cancer surgery show that alterations in the local microbiota can contribute to complications including anastomotic leak. Furthermore, some pathogenic bacterial species have been implicated in shorter recurrence-free survival periods after curative oncologic surgery. Unlike bacterial culture, which requires viable bacteria to be preserved from the time of sampling through to processing, 16S RNA sequencing can be performed on both fresh and fixed tissues. However, there are few studies using either tissue source and to date, none that compare results from the same patient. The aims of this pilot study were to: 1. Assess the feasibility of testing archived histological specimens (i.e. formalin fixed) that in turn facilitates retrospective analysis of patients with known clinical outcomes (i.e. anastomotic leaks) 2. Identify any difference (variance, reduce or over-abundance) between the microbiota of patients who suffered anastomotic leak and those who did not 3. Generate preliminary data to inform sample sizes, statistical methods, workflow and cost of a future prospective trial Using an established Upper GI Cancer Surgery Database, we performed propensity score matching to identify 15 patients who had a clinically evident anastomotic leak to be matched with 15 patients who did not have an anastomotic leak. Known predictors of anastomotic leak (e.g. neoadjuvant radiotherapy, smoking status, COPD, chronic kidney disease, diabetes mellitus, chronic liver disease) have been controlled for. Fresh frozen and formalin-fixed samples for the same patients were available. The formalin-fixed tissue was comprised of the "anastomotic doughnuts" created at the time of the circular stapled anastomosis. This tissue was chosen because of its immediate proximity to the anastomosis. All samples were analysed using 16S RNA sequencing to obtain a full microbial profile. Sequencing data was analysed by scientific investigators blinded to the clinical outcome (i.e. anastomotic leak vs. no leak) using a pre-existing workflow and bioinformati