Accuracy of Deformable Image Registration for Reirradiation of Lung Cancer Following Stereotactic Body Radiotherapy

Abstract

Stereotactic body radiotherapy (SBRT) for lung cancer typically causes radiographic lung changes that can affect the registration accuracy at the time of reirradiation. In this study, we investigate the accuracy of deformable image registration (DIR) in the presence of post SBRT lung changes using anatomical landmarks. We hypothesize that DIR accuracy is worse in regions near radiographic changes than in normal lung tissue. Planning, 3-, 6-, and 12-month follow-up CT images were deformably registered for 10 patients exhibiting radiographic lung changes following SBRT. Several multi-pass b-spline based DIRs were performed using a commercially available algorithm by varying the target intensity window and the structures with overridden image intensities. The structures included: the planning target volume (PTV) and ground glass (GG), and consolidated (C) radiographic changes, while the intensity windows investigated included: full image contrast, lung window (-1024HU to 200 HU), and mediastinum window (-135HU to 215HU). Landmark points were manually located in both the treated lung (mean 26.5 points, range 17-30 per scan within 5cm of the PTV) and contralateral normal lung (mean 29.7 points, range 29-30). Accuracy was evaluated by calculating the average 3D landmark registration error. The Pearson correlation coefficient was used to investigate the influence of the PTV, GG, and C lung volumes and normal lung registration error on the treated region registration error. For all time points considered together, the most accurate registration near treated tissue involved masking both the PTV and GG lung changes and using full image contrast. The average landmark error near the treated region with this registration was 7.7 +/- 4.9 mm and 5.2 +/- 3.5mm in the normal lung. The landmark error for the individual time points, 3-, 6-, and 12-month respectively, were 3.7mm, 9.8mm, and 9.8mm near the treated region and 4.8mm, 5.7 and 5mm in the normal lung tissue. Landmark errors at 3-months were strongly correlated with the volume of the GG (rˆ2 = 0.84) and C (rˆ2 = 0.84) lung changes. The 6- and 12-month landmark errors were moderately correlated with GG (rˆ2 = 0.29, rˆ2 = 0.43) and C (rˆ2 = 0.31, rˆ2 = 0.47) volumes. There was moderate correlation between landmark errors near the treated region and normal tissue that increases over time from rˆ2 = 0.28 at 3 months to rˆ2 = 0.69 at 12 months. No strong correlation was found with the PTV volume. Landmark registration errors in the treated region can be a few millimeters larger than in normal tissue and appear to increase with time, before stabilizing. There is also an increasing correlation over time between the normal tissue registration errors and those in the treated region. DIR uncertainty in the treated and normal lung needs to be considered during retreatment composite planning.