Lack of hierarchical survival prognosis in AJCC staging for colon cancer: Implications for future summary stage classification.

Abstract

256 Background: Current staging for colon cancer utilizing the American Joint Committee on Cancer TNM framework stratifies disease into groups according to how advanced the disease is and provides important standardized, individual prognostic information. Ideally, staging systems should exhibit hierarchical logic, with increasing stage reflecting worse prognosis. We sought to evaluate hierarchy within the 8th edition AJCC staging system for colon cancer. Methods: All patients with primary colon (including rectosigmoid) cancer diagnosed 2010-2017 were identified from the National Cancer Database (NCDB). Patients were only included if they carried a histologic diagnosis of invasive primary (non-recurrent) cancer, underwent radical surgery, and had complete pathologic staging data (T, N, M category data). Patients with missing follow-up data were excluded. Kaplan-Meier curves were used to assess overall survival by AJCC 8th edition pathologic summary stage group to determine whether staging was hierarchical for colon cancer. Multivariable modeling and log likelihood ratio test was then used to identify relative contributions of T and N variables to overall survival. Results: Among 270,584 colon cancer patients, pathologic staging was noted to be hierarchical by T category alone and by N category alone. However, summary staging was non-hierarchical in both instances. Multivariable modeling confirmed the AJCC non-hierarchy issue (colon HRs: 1, 1.54, 2.62, 2.60, 1.02, 2.15, and 4.38 for stage 1 as reference, 2a, 2b, 2c, 3a, 3b, 3c respectively). Multivariable modeling demonstrated that high T category (T4a, T4b) conferred the greatest risk of mortality based on hazard ratio (T4a HR 2.76, T4b HR 3.04), while high T category as well as high N category (N2a, N2b) contributed substantially to the survival model based on z-score. Further modeling assessing the predictive power of T and N category with respect to overall survival demonstrated that T category more strongly predicted overall survival than N category in colon cancer (likelihood chi-square test statistic pT:7009 vs pN:3586). Conclusions: Hierarchy in survival based on AJCC summary stage was not observed for patients with stage II-III colon cancer resulting in a paradoxical better observed survival with some stage III cancer subgroups compared to stage II cancer subgroups. High T category appears to more significantly impact survival than N category for patients with N0-N1 disease, while high N category appears more important for patients with T1-3 disease. Future revisions of the AJCC staging system should account for the differential impact of T and N category identified herein to more accurately confer prognostic information.