A quantitative dosimetric evaluation of inter-fraction setup variability on high dose volumes when following weekly imaging protocols for hypofractionated whole breast irradiation

ABSTRACT Introduction Due to the larger fractionated dose in hypofractionation, high dose regions have the potential to cause more adverse effects compared to conventional fractionation. It has been found that desquamation decreases when the maximum dose is minimized to 105% or 107% of the prescription dose. The Breast RTOG 1005 protocol recommends breast patients be imaged at a minimum before the first treatment, before the fifth treatment, and every 5 fractions subsequently. This study aims to determine how inter-fraction setup variability affects the homogeneity of a whole breast plan with respect to the 105% and 107% high dose regions when following the weekly imaging protocols. Methods This is a retrospective study where ten early-stage, right-sided breast cancer patients were selected. Each patient was treated to a dose of 4005 cGy in 15 fractions to the right breast with no boost. These patients were treated between the dates of January 2022 and January 2023. The data from their on-treatment shifts were collected and plans were generated simulating how those shifts would affect the isodose distribution if not corrected by image guidance. Results Statistical analysis was performed to compare the difference in means between the original plan and the new plan which encapsulated the overall effect of each patient’s daily shifts. The results of this study showed no statistical significance in the difference of means of the 105% and 107% high dose regions. Wilcoxon Signed Ranks Test and Sign Tests were performed. The significance level was set at 0.05 Conclusion The purpose of this study was to determine how inter-fraction setup variability affects the homogeneity of a hypofractionated whole breast plan with respect to the 105% and 107% high dose regions when following the RTOG 1005 weekly imaging protocols. The results of this study showed no statistical significance in the difference of means of the 105% and 107% high dose regions. This study concludes that the weekly imaging protocol is adequate for evaluating the high dose regions for early-stage hypofractionated whole-breast irradiation