MO‐F‐WAB‐11: Investigation of CBCT‐Based Patient Positioning Accuracy in Lung SBRT: Correlation with Breathing Irregularity

Purpose: To evaluate breathing irregularity induced error in CBCT‐based patient positioning in lung SBRT and correlate the error with a measure of breathing variability. Methods: The 4D extended cardiac‐torso (XCAT) digital phantom was used to generate 10‐phase 4DCT and CBCT images using in‐house developed simulation programs. Images were generated for various respiratory profiles (one regular sinusoidal and 10 irregular) and tumor sizes (1 cm, 2 cm, 3 cm). Maximum intensity projection (MIP) and average intensity projection (AIP) images were generated from 4DCT images. Image registrations between CBCT and AIP were performed for each respiratory profile and tumor size by four clinicians (two physicians and two physicists) 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. Breathing irregularities of the respiratory profiles were measured and correlated to errors of registration. Results: Inter‐observer variation of registration was 0.15 mm, 0.34 mm, and 0.69 mm, in the medial‐lateral (ML), anterior‐posterior (AP), and superior‐inferior (SI) direction, respectively. For the regular profile, negligible errors of registration were found in all directions (median 0.6). Maximum error of registration in the ML, AP, and SI direction was 1.2 mm, 2.6 mm, and 8.4 mm, respectively. Mild correlations (R2 range: 0.37 to 0.47) were observed between error of registration error and breathing irregularity for all tumor sizes. Conclusion: Irregular breathing can induce error in CBCT‐based image registration in lung SBRT. This error increases as the breathing irregularity increases.