Room: Exhibi Hall | Forum 6
Purpose: To investigate the image quality of the intrafraction 4D cone-beam computed tomography (CBCT) obtained during lung stereotactic body radiation therapy (SBRT) using flattening-filter free (FFF) X-ray beams. The impact of the X-ray energy and tube current on the CBCT image quality was evaluated.
Methods: The intrafraction 4D CBCT image quality was evaluated by comparing the localized coordinates of a solid lung target in a phantom obtained with the intrafraction 4D CBCT to those with the pretreatment 4D CBCT. The CBCT images were acquired on the XSight lung tracking phantom (CIRS, USA), in which the lung target motion in the superior-inferior (SI) direction was simulated, mimicking a respiratory motion with 3 cm excursion and 5 s cycle. A gantry-mounted kV imager (XVI R5.0, Elekta Limited, Stockholm, Sweden) was used for the CBCT acquisitions. For the intrafraction 4D CBCT, two treatment plans with flattening filter-free (FFF) X-ray beams (6 and 10 MV) were created for a patient and were delivered during the CBCT acquisition. Moreover, five different tube currents were tested: 20, 25, 32, 40, and 50 mA. The moving target was manually localized for each of the ten respiratory phases using MIM (MIM Software, Inc.) and the target localization error was calculated as the difference in the localized SI coordinates between the pretreatment and intrafraction 4D CBCT images.
Results: The mean and standard deviation of the target localization errors were 0.8 Â± 0.1 mm and 0.9 Â± 0.1 mm across the tube currents except 50 mA for 6 and 10 MV FFF beams, respectively; the target localization was not feasible for several phases for 50 mA tube current.
Conclusion: The intrafraction 4D CBCT with an appropriate tube current (20, 25, 32, and 40 mA) can be clinically used to monitor the phase-dependent variation of the lung target position.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by a research agreement with Elekta Limited.