44 Chapter 4 ratio test. For the OS analyses a forward stepwise multivariable cox regression was used based on the log-likelihood. To test the internal validity, the entire variable selection for both the toxicities and survival was repeated in 1000 bootstrap samples (i.e. with replacement). The selected model optimism was evaluated by calculating the difference between the performance of the models in each bootstrap and in the original sample, according to the TRIPOD statement.[19] Both the area, and the adjusted area under de ROC curves are presented in order to quantify the predictive power of the analyses. Finally, the effect of the toxicities on OS was analysed using Kaplan Meier analyses. Table 1 Tumour and treatment characteristics Tumour and treatment characteristics Stage 1 73(34.7%) Stage 2 57(26.3%) Stage 3 78(36.2%) Stage 4 8(3.7%) N0 102(47.2%) N1/N2/N3 114(52.8%) Tumour location Cervical 47(21.8%) Mid 114(52,8%) Distal/GE junction 55(23.2%) Pathology SCC 212(98%) Other 4(2%) Prescribed dose 60 Gy(50.4-66.0) Elective irradiation 135(62.5%) Chemotherapy 205(94.9%)
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