Effect of jaw width in jaw tracking mode on the radiotherapy dose of partial arc VMAT in patients undergoing left breast-conserving surgery

To analyze the effect of jaw width in jaw tracking mode on the dose of radiotherapy partial arc VMAT (P-VMAT) for patients undergoing left breast-conserving surgery and to explore the best jaw width as the initial inverse optimization parameter. Twenty patients who underwent left breast-conserving surgery were randomly selected. Six groups of P-VMAT plans were designed (named Plan0, Plan0.3, Plan0.6, Plan0.9, Plan-0.3, and Plan-0.6). The width of the jaw of each plan was changed in 0.3 cm steps along the X direction (from - 0.6 to 0.9 cm) according to the beginning of the half beam (Plan0). The PTV coverage, conformity index (CI), homogeneity index (HI), monitor units (MU) and organs at risk (OARs) dose were evaluated by repeated measurement data analysis of variance between plan0 and the other plans. Additionally, the correlations between CI, HI, MU and OARs to change in jaw width were analyzed using Spearman's bivariate correlation analysis. The PTV dose distributions of Plan-0.3 and Plan-0.6, which have smaller jaw widths than those of Plan0, did not meet the clinical requirements. CI, HI and MU were correlated with jaw width (r = 0.554, -0.501, -0.641, p < 0.05, respectively). The V5, V10, V20, V40, Dmean and Dmax of the heart were correlated with jaw width (r = 0.288, 0.284, 0.191, -0.27, 0.186, -0.245, p < 0.05, respectively). The V2.5, V5, V10, V20, V40 and Dmean of the left lung (Lung-L) were correlated with jaw width (0.298, 0.421, 0.516, 0.391, -0.241, 0.356, p < 0.05, respectively). Among all the plans to ensure PTV target coverage, Plan0 had the lowest clinical indicators for the heart and Lung-L (p < 0.05, respectively). The internal boundary of the jaw set as 0 cm (Plan0) represents the optimal jaw width for the initial optimization of the plan design. This method is the simplest and most effective for radiotherapy treatment planning for breast-conserving surgery for breast cancer as well as allows ideal dose distribution.