Assessment of Gross Tumor Volume Regression and Motion Changes During Radiotherapy for Non–Small-Cell Lung Cancer as Measured by Four-Dimensional Computed Tomography

Purpose: To investigate the magnitudes of the changes in mobility and volume of locally advanced non–small-cell lung cancer (NSCLC) tumors during radiotherapy, using four-dimensional computed tomography (4DCT). Methods and Materials: Five to ten 4DCT data sets were acquired weekly for each of 8 patients throughout treatment. Gross tumor volumes (GTVs) were outlined on each data set. Volumes and coordinates of the GTV centroids were calculated at the 0 (end-inspiration) and 50% (end-expiration) respiration phases. Trends in magnitudes of intrafraction and interfraction positional variations were assessed for the GTV and internal target volume (ITV) during treatment. Results: Tumor volume reduction ranged from 20% to 71% (end-inspiration) and from 15% to 70% (end-expiration). Increased tumor mobility was observed in the superior–inferior and anterior–posterior directions. However, no trends in tumor motion were observed. Motion along the superior–inferior direction was significantly greater ( p < 0.001), with mean ± SD values of 0.86 ± 0.19 cm, as compared with 0.39 ± 0.08 cm and 0.19 ± 0.05 cm in the anterior–posterior and right–left directions, respectively. A marginally significant ( p = 0.049) increase in total GTV positional variation was observed with increasing treatment weeks, and similar results were seen for the interfractional ITV mobility. Conclusions: Because of changes in tumor size and mobility, an explicit initial determination of the ITV may not be sufficient, especially where small setup margins are used. Repeat 4DCT scans might be warranted for highly mobile tumors to reduce the potential for missing the tumor.