Impact of variation in voluntary moderate deep inspiration breath hold on the 3D dose distribution in breast cancer radiotherapy

Aim: Determination of impact of variations during voluntary moderately deep inspiration breath hold (vmDIBH) on doses to organs at risk (OAR) and target coverage in patients undergoing radiotherapy (RT) for left-sided breast cancer (BC). Methods: We recorded vmDIBH-breathing signals using a Surface Guidance system, without visual coaching, in 21 BC patients. In 13 patients, cone-beam CT (CBCT) scans were acquired with vmDIBH, immediately after treatment delivery. A 3D dose recalculation was performed on CBCT, cranio-caudally stitched with the planning CT for full dose evaluation. Mean Heart Dose (MHD), Mean Lung Dose (MLD) and V95% of the clinical target volume (CTV) were calculated. Relationships between vmDIBH level, stability and differences in Dose-Volume Histogram (DVH) parameters were analysed using linear regression and Mann-Whitney U-tests. Results: Of 150 recorded vmDIBHs, 39% of the breath-holds were within a predefined 3 mm gating window. The average deviation from this window was 0.6 mm (SD 1.9 mm), with the mean value just outside predefined boundaries. All recalculated CBCTs (n = 95) met predefined DVH criteria for MHD, MLD, and V95% of tumourbed CTV. The V95% of the whole breast CTV showed minor deviations from the planned dose (mean delta V95%: -0.8%, SD 1.6%). For patients with excursions beyond the 3 mm window, average differences were 1.43% in V95%-CTV, 0.02 Gy in MHD, and 0.05 Gy in MLD. Conclusion: Only small breathing excursions beyond 3 mm were observed during vmDIBH without visual coaching, not resulting in clinically relevant CTV Underdosage with MHD and MLD remaining within tolerance.