Quantifying Lung Volume Change from Thoracic Radiation Therapy
Abstract
Purpose
To quantify changes in the lung volume due to irradiation.
Methods
Lung volumes from repeat 4D-CT scans were measured at two timepoints (simulation and twelve months post-RT) from 53 subjects treated for lung cancer. 16 subjects received standard fractionation (SFX) and 37 received SBRT. Volume change was tested statistically using linear mixed models. Three volume metrics, maximum inhale, nadir (or exhale), and tidal volumes, were collected from deep learning-based segmentations and reviewed visually. Test-retest reliability was analyzed using Intraclass Correlation Coefficient (ICC).
Results
Repeat scans showed high repeatability with maximum and nadir volumes ICC above 0.95. The SFX tidal volume post-RT was the least repeatable (ICC of 0.82). The irradiated lung for SFX patients was significantly smaller in maximum inhale and nadir volumes post-RT for all timepoints (p ≤ 0.01). The maximum lung volume averaged 2.33 +/- 0.67L pre-RT and 2.03 +/- 0.66L post-RT for the irradiated and 2.34 +/- 0.55L pre-RT to 2.45 +/- 0.50L post-RT for the contralateral lung. In SFX patients at all post-RT timepoints both these metrics where significantly smaller in the irradiated lung than the contralateral lung (p ≤ 0.01). No SBRT comparison was significant (maximum lung volume averaged 2.41 +/- 0.76L pre-RT and 2.45+/- 0.83L post-RT for the irradiated and 2.40 +/- 0.64 pre-RT 2.48 +/- 0.79L post-RT for the contralateral lung).
Conclusion
Significant reductions in lung volumes were recorded in the treated lung. In the SFX group the maximum inhale and nadir treated lung volumes declined while the tidal volume did not, possibly to maintain pulmonary function. In SBRT patients a greater sample size is needed to test for smaller changes in volume. 4D-CT based volume metrics were highly repeatable, however more variable in tidal volume likely from differences in respiratory effort.