Statistical analysis
REGRESSION
The cumulative mortality rate was estimated using the Kaplan–Meier method. Differences between the pre-CCRT PET–CT and non-pre-CCRT PET–CT groups were determined using the log-rank test. After adjustment for confounders, the Cox proportional regression model was used to model the time from the index date to all-cause mortality among the cases and controls. HRs were calculated in multivariate analysis with adjustment for sex, age, AJCC clinical stage, differentiation, CCI score, diagnosis year, and hospital volume. All analyses were performed using SAS (version 9.4; SAS, Cary, NC, USA). Two-tailed 