Cone beam computed tomography in 6- and 60-second acquisitions: implications for adaptive radiotherapy when respiratory motion is present.

Purpose. To investigate the effects of respiratory motion during fast (∼6 s) and slow (∼60 s) cone beam computed tomography (CBCT) acquisition modes, with a focus on implications for adaptive radiotherapy (ART).Methods. CBCT images are compared with 4D fan beam CT acquisitions, considering average ('AVE') and maximum ('MIP') intensity projections. Data are acquired using a respiratory motion phantom representing a human thorax with a lung tumour. A range of sup-inf motion amplitudes (3 to 11 mm) and periods (3 to 5 s) are considered. HU perturbations, target contouring implications, and dosimetric effects are considered.Results. Fast mode CBCT motion artefacts are more severe for larger amplitudes and longer periods. Motion artefacts are minimal in slow mode. The standard deviation of HU differences (CBCT minus AVE) in regions-of-interest encompassing the tumour are within 44 HU for slow mode, increasing up to 75 HU for fast mode. Target volumes contoured using HU thresholding on slow mode CBCTs are smaller than those on the AVE/MIP by up to 7%/29%. HU thresholding was not applied to fast mode CBCTs because motion artefacts were judged to be too severe. Gamma pass rates for dose distributions calculated on fast or slow mode CBCTs compared to the AVE are≥99% (criteria: 1%, 1 mm, 10% dose threshold). Dose differences (fast mode CBCT minus AVE) are larger for larger amplitudes and longer periods, and tend toward negative values. Dose differences (slow mode CBCT minus AVE) are generally smaller and more consistent across all amplitudes and periods considered.Conclusions. Dosimetric perturbations resulting from motion artefacts are not severe for the amplitudes and periods considered. However, motion artefacts (especially in fast mode) have implications for image registration, target contouring, and treatment plan optimization for ART.