Predictors of Heart and Lung Dose in Left-sided Breast Cancer Treated with VMAT Relative to 3D-CRT：A Retrospective Study

Abstract

Background Before radiotherapy for breast cancer patients, the choice of three-dimensional conformal radiation therapy (3D-CRT) and volumetric modulated arc therapy (VMAT) should be made. This study investigates the performance of two structural metrics in aiding the choice of 3D-CRT and VMAT plans in women undergoing left-sided whole breast radiotherapy. Materials and methods 119 patients previously treated with left-sided breast radiotherapy (61 3D-CRT treatments and 58 VMAT treatments) from a single institution were retrospectively studied. Two structural metrics, which are cardiac intersetion (CI) index and pulmonary intersection (PI) index, were defined and the relationship between these metrics and dose of organs at risk (OARs) were evaluated. Two-tailed Student’s t-test was performed to compare patient characteristics between 3D-CRT and VMAT. Linear regressions were calculated to investigate the association between structural metrics and absorbed dose of heart and left lung, including MHD, V5, V30 of heart, and MLLD, V5, V10, V20, V30, V40 of left lung. Results The CI index was strongly correlated with the mean dose of heart (MHD) in 3D-CRT group and VMAT group, the linear regression formulas were MHD = 4826.59 ×CI Index + 310.48 (R = 0.857, F = 163.77, P = 0.000) in 3D-CRT plans and MHD = 1789.29×CI Index + 437.50 (R = 0.45, F = 14.23, P = 0.000) in VMAT plans, the intercept of these formulas was CI index = 4.2% and MHD = 512.33 cGy. The PI index demonstrated a strongly positive correlation with mean dose of left lung (MLLD) in 3D-CRT group and VMAT group as well, the linear regression formulas were MLLD = 2879.54×PI Index + 999.79 (R = 0.697, F = 55.86, P = 0.000) in 3D-CRT plans and MLLD = 1411.79×PI Index + 1091.88 (R = 0.676, F = 47.11, P = 0.000) in VMAT plans, the intercept of these formulas was PI index = 6.3% and MLLD = 1180.46 cGy. Conclusions CI index and PI index could serve as a practical tool of choosing either 3D-CRT or VMAT before the plan was generated. We recommend that VMAT plan is preferable when CI index is greater than 4.2% and PI index is greater than 14.6%, while 3D-CRT plan is the first choice in the opposite.