Dosimetric Effect of Respiratory Motion On Planned Dose In Whole-Breast Volumetric Modulated Arc Therapy Using Standard And Ultra-Hypofractionation

Abstract Background and purpose: The interplay effects of respiratory motion and MLC movement on the planned dose in right-sided whole-breast irradiation (WBI) were studied by simulating hypofractionated VMAT treatment courses.Materials and methods: Ten patients with phase-triggered 4D-CT images were included in the study. VMAT plans targeting the right breast were created retrospectively with prescription doses of 26 Gy (5 fractions) and 40.05 Gy (15 fractions). 3D-CRT plans were generated as a reference. All plans were divided into respiratory phase-specific plans in fraction-by-fraction basis. The phase-specific dose distributions were deformed and superimposed onto the planning image, forming the course-specific, respiratory motion perturbed dose distribution. Distributions were compared with the original plans and changes due to respiratory motion and choice of fractionation were evaluated.Results: The respiratory motion perturbed PTV coverage (V95%) decreased by 1.7% and the homogeneity index increased by 0.02 for VMAT techniques, compared to the planned values. Highest decrease in CTV coverage was 0.7%. The largest dose differences were located in the areas of steep dose gradients parallel to respiratory motion. The largest difference in DVH parameters between fractionation schemes was 0.4% of the prescribed dose. Clinically relevant changes to the doses of organs at risk were not observed.Conclusion: The VMAT techniques were found feasible for WBI with 5 and 15-fraction treatments in terms of respiratory motion induced error. The CTV dose coverage was retained despite the decrease in PTV coverage. No clinical significance was found due to the choice of fractionation.