Utility of four-dimensional computed tomography for analysis of intrafractional and interfractional variation in lung volumes

PURPOSE: To assess the viability of four-dimensional (4D) computed tomography (CT) in describing intrafractional and interfractional changes in lung volumes and to determine which breathing phase, if any, produces the most highly reproducible lung volumes among fractions. METHODS AND MATERIALS: Weekly 4D CT scans were acquired for 13 patients with non-small-cell lung cancer during a course of radiotherapy. Contours delineating the right lung, left lung, and total lung were obtained by adapting library models of the anatomic structures to the CT images and propagating them to all 10 respiratory phases represented in the 4D CT image data set. Lung volumes were calculated using software tools in a commercial radiation treatment-planning system and analyzed for interfractional volume reproducibility using t tests and for phase reproducibility using a phase-dependent uncertainty curve across all patients. Probability (p) values of <0.05 were considered to indicate significant differences in all comparisons. RESULTS: The average mean coefficient of variation of tidal volume across all patients was 25.0%. The average standard deviation of tidal volumes was 5.7% relative to the lung volume at end-expiration. Total volumes measured at the 30% phase were 15% more consistent than those measured at end-inspiration (p = 0.03). CONCLUSIONS: Four-dimensional CT assesses lung volume with acceptable precision; but the technique was unable to accurately predict interfractional changes in lung volume because wide variations in intra- and interfractional breathing cause high uncertainties in 4D CT data acquisition. The most reproducible breathing phase seems to be at the 30-40% phase (just before end-expiration).