Objective  To investigate the setup errors and their dosimetric impacts during volumetric modulated arc therapy (VMAT) for breast cancer after breast-conserving surgery in patients using an image guidance system.

  Methods  Twenty patients who underwent VMAT after breast-conserving surgery at Tianjin Medical University Cancer Institute & Hospital between October 2020 and December 2021 were randomly assigned into the control (n=10) and experimental groups (n=10). The error data were statistically analyzed, and the dosimetric impacts of the positional errors were compared between the two groups by recalculating the treatment plans with the measured setup errors.

  Results  In the control and experimental groups, the setup errors were (3.58±2.35) and (3.51±2.08) mm, (4.44±3.62) and (4.23±2.17) mm, and (2.85±2.36) and (2.99±1.90) mm along the left-right (LR), anterior-posterior (AP), and superior-inferior (SI) directions, respectively. In the control group, the displacement in the LR, AP, and SI directions after irradiation were (2.64±1.62) mm, (3.15±1.50) mm, and (2.49±1.70) mm, respectively. In the experimental group, the displacement in the LR, AP, and SI directions after the first and second arc deliveries were (2.07±1.65) mm and (1.85±1.22) mm, (2.29±1.93) mm and (1.78±1.26) mm, and (1.98±1.49) mm and (1.67±1.27) mm, respectively. The experimental group had more posttreatment positional errors ≤3 mm in the LR, AP and SI directions than the control group, and the difference was statistically significant (χ²=21.07, 60.76, 33.63; P<0.01). The posttreatment displacement exhibited a statistically significant difference between the two groups (t=6.36, 10.35, 5.60; P<0.05). The dosimetric parameters from the recalculated virtual treatment plans showed that the experimental group was significantly superior to the control group in terms of the proportion of gross tumor volume and clinical target volume receiving the prescription dose, the mean dose of the heart, and the dose of the lung (P<0.05).

  Conclusions  The real-time image guidance system can correct setup errors during radiotherapy. Intrafractional correction significantly reduced patient setup errors and obtained better dosimetric results.