Room: Stars at Night Ballroom 4
Purpose: The natural abdominal compression induced by the change in the patient positioning from supine to prone alters the characteristics of the respiratory patterns. We sought to characterize the differences in breathing trends between prone and supine positioning using established metrics, as well as two new metrics introduced to better describe the suitability of a given breathing pattern for gating.
Methods: The following quantities were evaluated for breathing patterns from 20 patients (1-4 breathing traces per patient for both prone and supine): constancy of wave period, ratios of inhale-to-exhale periods, and mean relative changes in amplitude. A new measure of the baseline drift was computed based on a linear regression of breathing cycle inhale/exhale peaks. The effective duty cycle for amplitude-based gating as a result of varying gating limits was computed and related to the motion variability within each gating period in a proposed metric called the gating efficiency index.
Results: The choice of supine or prone positioning had a negligible impact on the duty cycle. Other metrics showed greater separation, including the variation in period, variation in ratio of inhale to exhale and change in amplitude per wave. Baseline drift was smaller for prone breathing and showed less variation between patients. Baseline drift tended to be in opposite directions for prone versus supine. The choice of narrow windows near initial exhale peaks in amplitude-based-gating strikes an optimal balance of longer duty cycles with less variation within each gating window.
Conclusion: Breathing is more consistent in prone than supine, and more favorable to gating due to less intrafraction variation in amplitude or period and less variation in the baseline drift. Narrower amplitude windows decrease the motion variability within the window at a faster rate than the duty cycle decreases.