Sample size and accrual
toxicity
The primary goal of this study is to determine if 2 fractions of MRI-LINAC SBRT does not increase GI or GU toxicity over 5 fractions of radiotherapy at 2 years post treatment. The primary endpoints are change scores in the GI and GU domains per EPIC questionnaire. The change scores will be based on the 2-year score minus the pretreatment (baseline or before radiation therapy) score. The hypothesis for this endpoint is that the EPIC mean change score is no worse in the 2 fraction arm than it is in the 5 fraction arm for either type of 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 deviations of the change scores are assumed to be 13.2 for the GI symptoms and 10.5 for the GU symptoms based on estimates generated in the RTOG 0415 trial [A sample size of 122 with 61 in each arm will ensure 80% power for GI endpoint and 83% power for GU endpoint to detect non-inferiority using a one-sided two-sample t-test at the significance level of 0.05. Adjusting for a projected 10% EPIC/non-compliance rate, 136 patients (68 per arm) will be randomized.The projected accrual rate is 10 patients per month. Based on this information, it is projected that the study will complete accrual in about 18 months. The primary endpoint analysis will occur approximately 4 years from study activation.