In Western population and clinical trial based validation, AOL had reliable prediction of OS for colon cancer patients . This study was performed to test whether AOL prediction of survival is applicable for the Korean colon cancer patients, whose prognosis is similar to other Asian patients [9, 13, 14].
In the current study, AOL had acceptable prediction for OS for all patients and almost every subgroup. Especially, AOL more accurately predicted OS for patients with stage II than stage III. Gil et al.  reported that AOL had acceptable reliability for patients with stage III disease and tended to overestimate survival for patients with stage II disease who received 5-FU from the population-based data.
What are the reasons for inverse validation between Western and Korean population? One possible explanation is ethnic difference. Asian colon cancer patients tend to experience better survival than Western patients in the same stage [9, 11, 12, 15]. Our data showed that the observed OS was better than that predicted by AOL, although there was no statistical difference. Especially, AOL overestimated survival by 8.3% in patients younger than 50 years. Therefore, AOL tended to overestimate survival for Western patients with stage II disease treated with 5-FU and exactly predicted survival for Korean cancer patients with stage II.
The other possible reason of different validation is different subgroup between two populations. The median age of Korean population was 8-year younger than the Western population. Interestingly, the median age of the clinical trial cohort of Western patients was 64 years old, which is younger than population cohort of Western patients. In both Gil et al. and current study, the default comorbidity assumption of “minor health problems” was used, since we could not retrieve reliable comorbidity data from each cohort. Accordingly, Western population cohort included more old age patients and high risk comorbidity patients than the trial based cohort. AOL overestimated survival in the western population cohort and similarly estimated in the clinical trial cohort. In contrast to this, Korean population cohort included more young age and low risk comorbidity patients than the Western population, which AOL accurately predicted survival for patients in stage II and III.
The last possible reason for the difference in predicted and observed outcomes can also be explained by the limitation of the AOL estimation regarding risk reduction by chemotherapy. Estimates of prognosis are mainly based on the SEER estimates of outcome for colon cancer patients in the general population. The efficacy of therapy is estimated based on the proportional risk reductions that which were obtained from meta-analyses of the effectiveness of adjuvant therapy and from the data published or presented from individual randomized clinical trials . Therefore, AOL tended to overestimate survival of Western colon cancer patients, and more accurately predicted in Korean patients whose survival are better than the Western patients.
Many studies have reported the number of evaluated lymph nodes (ELN) was positively associated with survival of colon cancer patients with not only stage II, but stage III . However, some studies reported that the number of ELN was positively correlated with survival in stage II, but did not affect the long-term outcome in stage III [13, 17]. In the current study, no significant association observed between ELN and survival. Kaplan-Meier analyses demonstrated 5-year overall survival rates for the number of ELN 1–3, 4–10, and > 10 of 90%, 73%, and 80%, respectively (Additional file 1: Table S1). A possible reason why the patients with less than 4 of ELN had the most favorable survival might be that these patients had favorable variables than the other patients, such as higher portion of younger age, low T stage, less number of positive node, and lower histologic grade (Additional file 2: Figure S1). In addition, the number of patients with less than 4 of ELN was only 30 (2.1%). Except for these patients, the number of ELN had positive trend of good survival in our study.
This study had several limitations. First, tumor grade was not available for some of our cases (9.4%), and the information of comorbidity could not be checked because the YTR is a retrospective database. Second, our study examined a time period when the standard chemotherapy was a 5-FU-based chemotherapy, the currently used oxaliplatin-based chemotherapy in patients with stage III and high risk stage II, although predicted survival used by AOL recommendation for the benefit of 5-FU based chemotherapy. Third, we had insufficient clinicopathological parameters such as preoperative carcinoembryonic antigen level , lymphovascular involvement , microsatellite instability , and other molecular markers .
In conclusion, we found that the AOL prediction system, which is based on Western patients, is suitable for Korean colon cancer patients with not only stage II but also stage III. Therefore, AOL which is a easily accessed tool, provides important information for the physician in terms of survival for Korean and Asian colon cancer patients, whose disease patterns and survival are similar [9, 13, 14].