Correlation between target motion and the dosimetric variance of breast and organ at risk during whole breast radiotherapy using 4DCT

PURPOSE: The purpose of this study was to explore the correlation between the respiration-induced target motion and volume variation with the dosimetric variance on breast and organ at risk (OAR) during free breathing. METHODS AND MATERIALS: After breast-conserving surgery, seventeen patients underwent respiration-synchronized 4DCT simulation scans during free breathing. Treatment planning was constructed using the end inspiration scan, then copied and applied to the other phases and the dose distribution was calculated separately to evaluate the dose-volume histograms (DVH) parameters for the planning target volume (PTV), ipsilateral lung and heart. RESULTS: During free breathing, the treated breast motion vector was 2.09 ± 0.74 mm, and the volume variation was 3.05 ± 0.94%. There was no correlation between the breast volume and target/OAR dosimetric variation (|r| = 0.39 ~ 0.48). In the anteroposterior, superoinferior and vector directions, breast movement correlated well with the mean PTV dose, conformal index, and the lung volume receiving high dose (|r| = 0.651-0.975); in the superoinferior and vector directions, breast displacement only correlated with the heart volume receiving >5 Gy (V(5)) (r = −0.795, 0.687). The lung volume and the lung volume receiving high dose correlated reasonably well (r = 0.655 ~ 0.882), and a correlation only existed between heart volume and V(5) (r = −0.701). CONCLUSION: Target movement correlated well with the target/OAR dosimetric variation in certain directions, indicating that whole breast IMRT assisted by breathing control or respiratory-adapted gated treatment promotes the accuracy of dose delivery during radiotherapy. During free breathing, the effect of breast volume variation can be ignored in whole breast IMRT.