Sample size and accrual
toxicity
The primary objective of this study is to determine if 5 fractions of MRgRT does not increase GI or GU toxicity over 20 fractions of MRgRT. The primary endpoints are change scores in the bowel (GI) or urinary (GU) domains of the EPIC. The change scores will reflect the 2-year score minus the pre-treatment (baseline) score. It is hypothesized that the EPIC mean change score will be no worse in the 5-fraction arm than it is in the 20-fraction arm for both GI and GU toxicity. The sample size is calculated based on a non-inferiority design. The non-inferiority margins are set to be a change score of 6 points for the GI symptoms and 5 points for the GU symptoms. The standard deviation of the change scores are assumed to be 13.2 for the GI symptoms and 10.5 for the GU symptoms based on estimates in the RTOG 0415 trial [