Setup error and motion during deep inspiration breath-hold breast radiotherapy measured with continuous portal imaging

Background. The position and residual motion of the chest wall of breast cancer patients during treatment in deep inspiration breath-hold (DIBH) were investigated. Material and methods. The study included 58 left-sided breast cancer patients treated with DIBH three-dimensional (3D) conformal radiotherapy in 15 or 25 fractions. The DIBH levels were monitored using an external marker block placed on the chest, either shifted 5 cm to the right at the level of the xiphoid process (Group 1, 27 consecutive patients) or placed medially on the inferior part of the sternum (Group 2, 31 consecutive patients). At every third treatment fraction, continuous portal images were acquired. The time-resolved chest wall position during treatment was compared with the planned position to determine the inter-fraction setup errors and the intra-fraction motion of the chest wall. Results. The DIBH compliance was 95% during both recruitment periods. A tendency of smaller inter-fraction setup errors and intra-fraction motion was observed for group 2 (medial marker block position). However, apart from a significantly reduced inter-field random shift (sigma = 1.7 mm vs. sigma = 0.9 mm, p = 0.005), no statistically significant differences between the groups were found. In a combined analysis, the group mean inter-fraction setup error was M = - 0.1 mm, with random and systematic errors of sigma = 1.7 mm and sigma = 1.4 mm. The group mean inter-field shift was M = 0.0 (sigma = 1.3 mm and sigma = 1.1 mm) and the group mean standard deviation of the intra-field motion was 0.5 mm. The absolute setup error had a maximum of 16.3 mm, exceeding 5 mm in 2.2% of the imaged fields. Conclusion. Compared to free breathing treatments, the primary benefit of the DIBH technique was the separation of the heart from the target rather than more accurate targeting. Despite a small gating window, occasional large errors in the chest wall position were observed for some patients, illustrating limitations of the external marker block as surrogate in a broad patient population.