Effects of multiple breath hold reproducibility on treatment localization and dosimetric accuracy in radiotherapy of left-sided breast cancer with voluntary deep inspiration breath hold technique

The purpose of this study was to investigate the effects of breath hold reproducibility on positional and dosimetric errors in radiotherapy of patients with left-sided breast cancer (LSBC) treated with voluntary deep inspiration breath hold (vDIBH) technique. Clinical data from 2 groups of patients with LSBC were retrospectively investigated: (1) those irradiated for the whole breast only (WB group, n = 20) using typically from 3 to 5 breath holds per treatment session and (2) those irradiated simultaneously also for supraclavicular lymph nodes (WB + SLN group, n = 27) using from 7 to 9 breath holds per fraction. Setup and field images (n = 1365) from tangential breast fields, and anterior and posterior lymph node fields were analyzed to obtain total, inter-, and intrafractional residual positional errors of the chest wall and clavicle. The dosimetric effect of intrafractional positional errors was investigated at the abutment level of breast and lymph node fields. The total systematic setup error in the longitudinal (superior-inferior [SI]) direction was 1.4 and 1.9 mm (1 standard deviation, p = 0.049) for the WB and WB + SLN groups, respectively, whereas in the anterior/lateral direction, the error was 1.2 mm for both groups. In the SI direction, the systematic intrafractional error was also larger in the WB + SLN group (1.9 vs 1.1 mm, p = 0.003). The latter positional errors correlated moderately (ρ = 0.51) with the number of breath holds. Mean intrafractional errors of at least 2 mm were observed for 38% of the patients in the WB + SLN group. These errors resulted in a dosimetric error from 8.3% to 10.1% (1 cc). The total localization errors and needed setup margins were wider for the WB + SLN group, due to increased amount of breath holds in treatment session. Mean intrafraction movements ≥ 2 mm were shown to occur with this patient group in the SI direction, requiring intrafractional positional monitoring and corrective actions in daily practice.