Impact of total lymph node count on staging and survival after neoadjuvant chemoradiation therapy for rectal cancer

736 Background: Neoadjuvant chemoradiation therapy (CRT) results in fewer retrieved lymph nodes at the time of surgery for rectal cancer. The extent of optimal regional nodal dissection is based on guidelines developed before neoadjuvant CRT was commonly used. The purpose of this study is to assess the impact of the number of dissected and positive lymph nodes on overall survival (OS) for rectal cancer patients treated with neoadjuvant CRT. Methods: Treatment data were obtained by structured query on all patients with rectal adenocarcinoma (2000-2013) in the National Oncology Data Alliance, a proprietary database of merged tumor registries. Eligible patients were treated with neoadjuvant CRT followed by surgery and had complete data on the number of positive and dissected lymph nodes and dates of treatment. The relationships between number of lymph nodes examined and OS were separately analyzed in patients with 0, exactly 1, or any number of positive nodes. Results: The median number of lymph nodes examined was 11 (interquartile range 6-16). In 4,581 evaluable patients, there was a significant improvement in OS with the examination of more lymph nodes. Number of positive lymph nodes, number of lymph nodes dissected, age, gender, grade, marital status, and race were significant predictors of OS on multivariate analysis. On subset analysis, patients with 0, exactly 1, and any number of positive nodes were found to have better OS with increasing number of lymph nodes dissected up to eight. Increasing overall mortality was observed in patients with 0, 1, 2-4, 5-7, and ≥8 positive lymph nodes. The Kaplan-Meier curves showed a clear statistically significant difference in OS in patients divided into these five nodal groupings (p