Sample size calculation
The primary endpoint (decision to WW due to cCR/near-complete response) was observed in 55% and 85% at 12 weeks (in contrast to the 18 weeks used in the present trial) among patients with early cT3 and cT2 rectal cancer respectively [We will assume that the primary outcome will occur in 40% of individuals in control group and 60% in the 2-drug consolidation chemotherapy group, which corresponds to an absolute difference in proportions of 20%. It is estimated that a sample of 194 (97 per group) provides 80% statistical power to detect this difference at a significance level of 5% using the Chi-square test and assuming a two-sided significance hypothesis and considering a 1:1 allocation. The estimated dropout rate is 10% in each group, so we would have 216 individuals (108 per group). The sample size calculation was performed using SAS 9.4 (PROC POWER procedure).