Beam's-eye-view imaging during non-coplanar lung SBRT

Purpose: Beam's-eye-view (BEV) imaging with an electronic portal imaging device (EPID) can be performed during lung stereotactic body radiation therapy (SBRT) to monitor the tumor location in real-time. Image quality for each patient and treatment field depends on several factors including the patient anatomy and the gantry and couch angles. The authors investigated the angular dependence of automatic tumor localization during non-coplanar lung SBRT delivery. Methods: All images were acquired at a frame rate of 12 Hz with an amorphous silicon EPID. A previously validated markerless lung tumor localization algorithm was employed with manual localization as the reference. From ten SBRT patients, 12 987 image frames of 123 image sequences acquired at 48 different gantry-couch rotations were analyzed. delta was defined by the position difference of the automatic and manual localization. Results: Regardless of the couch angle, the best tracking performance was found in image sequences with a gantry angle within 20 degrees of 250 degrees (delta = 1.40 mm). Image sequences acquired with gantry angles of 150 degrees, 210 degrees, and 350 degrees also led to good tracking performances with delta = 1.77-2.00 mm. Overall, the couch angle was not correlated with the tracking results. Among all the gantry-couch combinations, image sequences acquired at (theta = 30 degrees, phi = 330 degrees.), (theta = 210 degrees, phi= 10 degrees), and (theta = 250 degrees, phi = 30 degrees) led to the best tracking results with delta = 1.19-1.82 mm. The worst performing combinations were (theta = 90 degrees and 230 degrees, phi= 10 degrees.) and (theta= 270 degrees, phi = 30 degrees) with delta > 3.5 mm. However, 35% (17/48) of the gantry-couch rotations demonstrated substantial variability in tracking performances between patients. For example, the field angle (theta= 70 degrees, phi= 10 degrees) was acquired for five patients. While the tracking errors were = 1.98 mm for three patients, poor performance was found for the other two patients with delta = 2.18 mm, leading to average tracking error of 2.70 mm. Only one image sequence was acquired for all other gantry-couch rotations (delta = 1.18-10.29 mm). Conclusions: Non-coplanar beams with gantry-couch rotation of (theta = 30 degrees, phi= 330 degrees), (theta = 210 degrees, phi = 10 degrees), and (theta = 250 degrees, phi = 30 degrees) have the highest accuracy for BEV lung tumor localization. Additionally, gantry angles of 150 degrees, 210 degrees, 250 degrees, and 350 degrees also offer good tracking performance. The beam geometries (theta = 90 degrees and 230 degrees, phi = 10 degrees) and (theta= 270 degrees, phi= 30 degrees) are associated with substantial automatic localization errors. Overall, lung tumor visibility and tracking performance were patient dependent for a substantial number of the gantry-couch angle combinations studied. (C) 2015 American Association of Physicists in Medicine.