An assessment of cone beam CT in the adaptive radiotherapy planning process for non-small-cell lung cancer patients

Abstract

Objective: To investigate the potential use of cone beam CT (CBCT) in adaptive radiotherapy (ART) planning process for non-small-cell lung cancer (NSCLC).

Methods: 17 retrospective patients with NSCLC Stage T1-T4, who had completed a course of radiotherapy with weekly CBCT imaging were selected for the study. The patients had been delineated and planned for three-dimensional (3D) conformal treatment (prescription: 55 Gy in 20 fractions) based on free-breathing four-dimensional CT data. Of these initial 17 patients, 12 had full quantitative data on gross tumour volume (GTV) position and volume throughout treatment. GTV delineation was carried out on weekly CBCT by a clinical oncologist. For each patient, mean percentage change in GTV and centre of mass (COM) displacement (based on 3D vectors) were calculated throughout treatment. Volume overlap between GTVs was calculated. Correlation of the COM displacement and planning GTV (pGTV) was assessed. A linear mixed model with patients as random effects was fitted to the data to assess potential benefit from using ART for these patients.

Results: Comparison of CBCT-based GTV acquired prior to Fraction 1 (cbctGTV1) to pGTV showed mean 20 ± 19% volume increase using a related sample Wilcoxon signed rank test p = 0.04. Correlation was identified between volume reductions and dose delivered (beta = -0.003, p < 0.001)-a highly statistically significant association. Compared with cbctGTV1, the mean ratios ± standard deviation were cbctGTV2, 0.93 ± 0.08; cbctGTV3, 0.84 ± 0.12; and cbctGTV4, 0.75 ± 0.14. The dice similarity coefficient was 0.81 ± 0.14, 0.78 ± 0.17, 0.73 ± 0.19, respectively. The COM was consistent throughout treatment (mean 0.35 ± 0.24 cm). A fitted model predicts that a mean change of 30% volume relative to cbctGTV1 occurs at a dose of approximately 50 Gy.

Conclusion: Using a 30% reduction in volume, ART would not be of benefit for all radiotherapy-alone-treated patients with NSCLC assessed in this study. For individual patients and patients with atelectasis, CBCT imaging was able to identify volume change.

Advances in knowledge: For patients treated with 55 Gy in 20 fractions, target volume changes throughout treatment have been demonstrated using CBCT and can be used to highlight patients who may benefit from ART.

Figure 1.

Planning gross tumour volume (pGTV) plotted against cone beam CT-based gross tumour volume 1 (cbctGTV1). r² linear = 0.932.

Figure 2.

Ratio of weekly cone beam CT-based gross tumour volume (cbctGTV2–4) to baseline cone beam CT-based gross tumour volume 1 (cbctGTV1) showing range/interquartile range and mean.

Figure 3.

Mean estimated change relative to baseline (centre line) from the fitted model together with upper and lower 95% confidence interval (top and bottom line, respectively). Baseline is cone beam CT-based gross tumour volume 1 (cbctGTV1) volume.

Figure 4.

Centre of mass (COM) displacement between each of cone beam CT-based gross tumour volume at 2, 3 and 4 (cbctGTV2–4) with baseline cone beam CT-based gross tumour volume 1 (cbctGTV1). Displacement expressed as three-dimensional vector (cm).

Figure 5.

Mean ratio volume change on each data set with planning gross tumour volume (pGTV) 100%.