RESUMO
PURPOSE: To determine whether the level of inspiratory volume affect the extent to which active breathing control (ABC) reduces intrafraction motion and the dose coverage of target volumes in patients receiving external beam radiation therapy (EBRT) to the thoracic region MATERIALS/METHODS: 20 patients undergoing thoracic radiotherapy enrolled in a prospective study using ABC for respiratory motion management and volumetric-modulated arc therapy (VMAT) as the treatment technique. They were randomized to one of two groups, the control group of 80% inspiratory volume and the other test group of 70%. At least one set of repeated CBCTs was done weekly. All images including CBCTs and Planning CTs were sent to MIM softwareTM for analysis of intrafraction motion using Dice Similarity Coefficient (DSC). The target dose conformality was assessed using conformation number (CN). Intention-to-treat analysis was employed for statistical purpose. RESULTS: The DSC for the 70% and 80% inspiratory volume group were 0.93 and 0.92, respectively. For the 70% group, there was a significant negative correlation (p < 0.05) between DSC and time between two CBCTs, but not for the 80% group. The average percentage change in CN for the 70% and 80% group was 10.91% and 8.14%, respectively, and their difference was significant (p < 0.05). Furthermore, the actual change in volume had a significant positive correlation (p < 0.05) with the percentage change in CN for the 70% inspiratory volume group but not the 80% group. CONCLUSION: More evidence suggests that the target volumes from the 80% inspiratory volume group have less intrafraction motion compared to the 70% group. The findings from the percentage change in CN suggest that there could potentially be less tumor motion for higher levels of inspiratory volume and this could possibly contribute to why intrafraction motion is less for the 80% inspiratory volume group.