Investigating 5DCT Motion Model-Derived Hysteresis As a Biomarker for Mechanically Affected Lung
Abstract
Purpose
To test the feasibility of using lung hysteresis, or the difference in inhalation and exhalation motion trajectories, as a biomarker for mechanically affected lung.
Methods
A random cohort of lung cancer patients who underwent pre-treatment 5DCT imaging were selected for analysis. Tidal volume (alpha) and airflow (beta) parameters were extracted from 5DCT model parameters. Hysteresis was calculated as the (beta*airflow)/(alpha*tidal volume) vector magnitude ratio. The PTV was defined by the physician applying a 5-7mm margin on the IGTV (Internal Gross Target Volume) contoured from the 5DCT model. The mean voxel-by-voxel hysteresis of the PTV was compared with the hysteresis of the normal lung tissue at 7 regions of interest to evaluate hysteresis at incremental distance from tumors: a 5mm and 10mm PTV expansion; and 10mm thick isocentric rings at 5mm, 10mm, 15mm, 20mm, and 30mm. Descriptive statistics (mean, median, standard deviation) and Kendall’s tau correlation were used to compare regional hysteresis.
Results
20 single site lung cancer patients were investigated (n=7 RLL; n=8 RUL; n=5 LUL). Overall, the mean hysteresis increased with increasing distance from the PTV ([mean, standard deviation]: PTV [0.371, 0.208]; PTV05 [0.381, 0.207]; PTV10 [0.385, 0.206]; PTVD05 [0.391, 0.201]; PTVD10 [0.403, 0.195]; PTVD15 [0.424, 0.204]; PTVD20 [0.435, 0.211]; PTVD30 [0.43, 0.181]). In addition, the correlation between the PTV regional hysteresis and normal lung tissue regional hysteresis decreased with increasing distance from the PTV (Kendall’s τ-statistic, p<0.001: PTV05 = 0.968; PTV10 = 0.916; PTVD05 = 0.884; PTVD10 = 0.853; PTVD15 = 0.747; PTVD20 = 0.716; PTVD30 = 0.547). The regions with the lowest correlation were the PTV05 and PTVD30 (τ=0.537, p<0.001).
Conclusion
Hysteresis systematically decreased with increasing distance from the tumor, thus demonstrating that motion hysteresis correlates to a known disease state. Future work will investigate motion hysteresis as a biomarker for regional diseases like COPD.