Target-Matching Accuracy in Stereotactic Body Radiation Therapy of Lung Cancer: An Investigation Based on Four-Dimensional Digital Human Phantom

Aim: To investigate the effect of irregular respiratory on target-matching accuracy in lung stereotactic-body radiation therapy (SBRT). Methods: The four-dimensional extended cardiac torso (4D-XCAT) phantom was used to generate 4D computed tomography (4DCT) and cone-beam CT (CBCT) images. Images were generated for 1 regular and 10 irregular trajectories, and for 3 tumor size groups (1, 2, and 3 cm). Image registrations between CBCT and average intensity projection (AIP) images of 4DCT were performed based on target volume matching. Error of registration was determined as the difference between manual CBCT-to-AIP registration and known registration between the two. In addition, internal target volumes (ITVs) were contoured on and compared between AIP, maximum intensity projection (MIP), and CBCT. Results: Small inter-observer variations of registration were found: 0.2, 0.3, and 0.7 mm in the medial-lateral (ML), anterior-posterior (AP), and superior-inferior (SI) direction, respectively. Small errors of registration (median ≤ 0.5 mm) were found in all three directions for the regular respiratory profile. For the irregular profiles and on average of all tumor size groups, the median ± standard deviation (SD) errors of registration were 0.5 ± 0.3 mm, 0.4 ± 0.5 mm, and 1.9 ± 1.6 mm in the ML, AP, and SI direction, respectively. Significant differences were found between MIP and CBCT-based ITV volumes for the three tumor size groups (P = 0.011, 0.010, and 0.006 for 1, 2, and 3 cm tumor size group, respectively). Conclusion: Irregular breathing can induce error in CBCT-to-AIP registration in lung SBRT. This error increases as the breathing irregularity increases.