Respiratory cycle characterization and optimization of amplitude-based gating parameters for prone and supine lung...

The selection of posture between supine and prone induces changes in the characteristics of respiratory patterns in lung cancer patients. We characterize these differences, as well as introduce two new metrics to describe the quality of amplitude-based gating. The stability of the following metrics were measured for 134 supine-and-prone-paired individual breathing sessions from 22 patients: amplitude, period, inhale-to-exhale period ratio, and location of end-of-exhale and end-of-inhale peaks. A new normalization characteristic of typical amplitude was introduced for comparing patients based on external surrogates. This metric was used to characterize the baseline drift and to compare the overall gating efficiency between different amplitude-based gating parameters in a new proposed metric called the gating efficiency index. While the choice of supine or prone posture had negligible impact on the overall duty cycle or gating efficiency, some metrics showed greater difference, especially with prone showing reduced variability in period, inhale-to-exhale period ratio, amplitude, and relative amplitude of end-of-exhale. Therefore, the breathing pattern resulting from prone positioning was found to be more favorable due to less intrafraction variation. The gating efficiency index was used to quantitatively show that narrow amplitude gating windows near end-of-exhale have the best balance of decreased motion variability within gating while maintaining the longest duty cycle.