Accuracy of a new paired imaging technique for position correction in whole breast radiotherapy

Image-guided position verification in breast radiotherapy is accurately performed with kilovoltage cone beam CT (kV-CBCT). The technique is, however, time-consuming and there is a risk for patient collision. Online position verification performed with orthogonal-angled mixed modality paired imaging is less time-consuming at the expense of inferior accuracy compared to kV-CBCT. We therefore investigated whether a new tangential-angled single modality paired imaging technique can reduce the residual error (RE) of orthogonal-angled mixed modality paired imaging. The latter was applied to 20 breast cancer patients. Tangential-angled single modality paired imaging was investigated in 20 breast and 20 breast cancer patients with locoregional lymph node irradiation. The central lung distance (CLD) residual error and the longitudinal residual error were determined during the first 5 treatment fractions. Off-line matching of the tangential breast field images, acquired after online position correction, was used. The mean, systematic, and random REs of each patient group were calculated. The systematic REs were checked for significant differences using the F-test. Tangential-angled single modality paired imaging significantly reduced the systematic CLD residual error of orthogonal-angled mixed modality paired imaging for the breast cancer patients, from 2.3 mm to 1.0 mm, and also significantly decreased the systematic longitudinal RE from 2.4 mm to 1.3 mm. PTV margins, which account for the residual error (PTVRE), were also calculated. The PTVRE margin needed to account for the RE of orthogonal-angled mixed modality paired imaging (i.e., 8 mm) was halved by tangential-angled single modality paired imaging. The differences between the systematic REs of tangential-angled single modality paired imaging of the breast cancer patients and the breast cancer patients with locoregional lymph node irradiation were not significant, yielding comparable PTVRE margins. In this study, we showed that tangential-angled single modality paired imaging is superior to orthogonal-angled mixed modality paired imaging to correct the position errors in whole breast radiotherapy.