Phantom and clinical study of differences in cone beam computed tomographic registration when aligned to maximum and average intensity projection. 
To determine whether maximum or average intensity projection (MIP or AIP, respectively) reconstructed from 4-dimensional computed tomography (4DCT) is preferred for alignment to cone beam CT (CBCT) images in lung stereotactic body radiation therapy.  Stationary CT and 4DCT images were acquired with a target phantom at the center of motion and moving along the superior-inferior (SI) direction, respectively.  Motion profiles were asymmetrical waveforms with amplitudes of 10, 15, and 20 mm and a 4-second cycle.  Stationary CBCT and dynamic CBCT images were acquired in the same manner as stationary CT and 4DCT images.  Stationary CBCT was aligned to stationary CT, and the couch position was used as the baseline.  Dynamic CBCT was aligned to the MIP and AIP of corresponding amplitudes.  Registration error was defined as the SI deviation of the couch position from the baseline.  In 16 patients with isolated lung lesions, free-breathing CBCT (FBCBCT) was registered to AIP and MIP (64 sessions in total), and the difference in couch shifts was calculated.  In the phantom study, registration errors were within 0.1 mm for AIP and 1.5 to 1.8 mm toward the inferior direction for MIP.  In the patient study, the difference in the couch shifts (mean, range) was insignificant in the right-left (0.0 mm, ≤1.0 mm) and anterior-posterior (0.0 mm, ≤2.1 mm) directions.  In the SI direction, however, the couch position significantly shifted in the inferior direction after MIP registration compared with after AIP registration (mean, -0.6 mm; ranging 1.7 mm to the superior side and 3.5 mm to the inferior side, P=.02).  AIP is recommended as the reference image for registration to FBCBCT when target alignment is performed in the presence of asymmetrical respiratory motion, whereas MIP causes systematic target positioning error.