Background: Neoadjuvant radiation therapy for locally advanced rectal adenocarcinoma decreases lymph node yield. This study investigated the association between survival and number of lymph nodes evaluated in patients with pathologically negative nodes after neoadjuvant therapy. Methods: Patients with locally advanced rectal adenocarcinoma who underwent neoadjuvant therapy and had pathologically negative lymph nodes were included from the Surveillance, Epidemiology, and End Results (SEER) database over a 7-year interval (January 2004 to December 2010). Systematic dichotomization for optimal cut-off point identification was performed using statistical modelling. Results: A total of 3995 patients met the inclusion criteria. The majority had T3 (66·7 per cent) and moderately differentiated (71·5 per cent) tumours. The median number of lymph nodes retrieved was 12 (i.q.r. 7–16). An optimal cut-off of nine lymph nodes was identified. Increasing age (P < 0·001), increasing T category (T4 versus T1, P < 0·001; T3 versus T1, P = 0·010), response to neoadjuvant therapy (P < 0·001) and number of nodes evaluated (P < 0·001) were significant factors for overall survival in univariable analysis. After adjustment in the multivariable model, the group with nine or more nodes examined had significantly better overall survival (hazard ratio (HR) 0·76, 95 per cent c.i. 0·65 to 0·88, P < 0·001; 5-year survival 83·2 versus 78·0 per cent) and cancer-specific survival (HR 0·76, 0·64 to 0·92, P = 0·004; 5-year survival 87·9 versus 85·1 per cent) than the group with one to eight nodes examined. Conclusion: Overall and cancer-specific survival were worse where fewer than nine lymph nodes were identified after neoadjuvant therapy for locally advanced rectal cancer.
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