An Analysis of the Impact of Different Levels of Inspiratory Volume Using Active Breathing Control on the Intrafraction Motion and Dose Coverage of Target Volumes in Patients Undergoing Thoracic Radiotherapy

Abstract

To determine whether the level of inspiratory volume affect the extent to which active breathing control (ABC) reduces intrafraction motion and affect the dose coverage of target volumes in patients receiving external beam radiation therapy (EBRT) to the thoracic region. This is a prospective study involving 20 patients undergoing thoracic radiotherapy using ABC for respiratory motion management and volumetric modulated arc therapy (VMAT) as the treatment technique. They will be randomized to one of two groups. The first will be the control group of 80% inspiratory volume and the other test group will be 70%. At least one set of repeated CBCTs will be done weekly. All images including CBCTs and Planning CTs will be sent to commercially available software for analysis of intrafraction motion using Dice Similarity Coefficient (DSC) and determination of systematic and random errors. The target dose conformality will be assessed using conformation number (CN). Intention-to-treat analysis will be employed for statistical purpose. The DSC for the 70% and 80% inspiratory volume group was 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 superior-inferior (SI) direction was the highest among the systematic and random translational errors. The rotational systematic and random errors appear to be slightly less for the 80% group compared to the 70% group. The average percentage change in CN for the 70% and 80% group was 10.91% and 8.14%, respectively. Their difference was significant using Mann Whitney-U test (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 using Spearman’ s correlation. Last but not least, there was no significant difference for the change in CN when the change in volume is greater than 0.05L or less than or equal to 0.05L for either group of patients. There is more evidence suggesting the target volumes from the 80% inspiratory volume group have less intrafraction motion compared to the 70% inspiratory volume group. The findings from the calculation of 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. The 0.1L over the reference volume used at the author’s department is feasible in terms of not leading to a significant increase in the change in CN.