Dosimetric effect of respiratory motion on planned dose in whole-breast volumetric modulated arc therapy using moderate and ultra-hypofractionation

Background and purpose: The interplay efect of respiratory motion on the planned dose in free-breathing rightsided 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 moderately hypofractionated (40.05 Gy in 15 fractions of 2.67 Gy) and ultra-hypofractionated (26 Gy 5 fractions of 5.2 Gy) schemes. 3D-CRT plans were generated as a reference. All plans were divided into respiratory phase-specifc plans and calculated in the corresponding phase images. Fraction-specifc dose was formed by deforming and summing the phase-specifc doses in the planning image for each fraction. The fraction-specifc dose distributions were deformed and superimposed onto the planning image, forming the course-specifc respiratory motion perturbed dose distribution. Planned and respiratory motion perturbed doses were compared 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 diferences were located in the areas of steep dose gradients parallel to respiratory motion. The largest diference 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. One patient was excluded from the analysis due to large respiratory amplitude. Conclusion: Respiratory motion of less than 5 mm in magnitude did not result in clinically signifcant changes in the planned free-breathing WBI dose. The 5 mm margins were sufcient to account for the respiratory motion in terms of CTV dose homogeneity and coverage for VMAT techniques. Steep dose gradients near the PTV edges might decrease the CTV coverage. No clinical signifcance was found due to the choice of fractionation.peerReviewe