Intrafraction 4D‐cone beam CT acquired during volumetric arc radiotherapy delivery: kV parameter optimization and 4D motion accuracy for lung stereotactic body radiotherapy (SBRT) patients

Abstract

PurposeElekta XVI 5.0 allows for four‐dimensional cone beam computed tomography (4D CBCT) image acquisition during treatment delivery to monitor intrafraction motion. These images can have poorer image quality due to undersampling of kV projections and treatment beam MV scatter effects. We determine if a universal intrafraction preset can be used for stereotactic body radiotherapy (SBRT) lung patients and validate the accuracy of target motion characterized by XVI intrafraction 4D CBCT.MethodsThe most critical parameter within the intrafraction preset is the nominal AcquisitionInterval, which controls kV imaging acquisition frequency. An optimal value was determined by maximizing the kV frame number acquired up to 1000 frames, typical of pretreatment 4D CBCT. CIRS motion phantom intrafraction phase images for 16 SBRT beams were obtained. Mean target position, time‐weighted standard deviation, and amplitude for these images as well as target motion for three SBRT lung patients were compared to respective pretreatment 4D CBCTs. Evaluation of intrafraction 4D CBCT reconstruction revealed inclusion of MV only images acquired to remove MV scatter effects. A workaround to remove these images was developed.ResultsAcquisitionInterval of 0.1°/frame was optimal. The number of kV frames acquired was 567–1116 and showed strong linear correlation with beam monitor unit (MUs). Phantom target motion accuracy was excellent with average differences in target position, standard deviation and amplitude range of ≤0.5 mm. Target tracking for SBRT patients also showed good agreement. Evaluation of phase sorting wave forms showed that inclusion of MV only images significantly impacts intrafraction image reconstruction for patients and use of workaround is necessary.ConclusionsA universal intrafraction imaging preset can be used safely for SBRT lung patients. The number of kV projections with MV delivery parameters varies; however images with fewer kV projections still provided accurate target position information. Impact of the reconstruction workaround was significant and is mandated for all 4D CBCT intrafraction imaging performed at our institution.

Image quality of 4D in-treatment CBCT acquired during lung SBRT using FFF beam: a phantom study

“…For smaller lung targets, the target localization may be more affected by the MV treatment scatter artifacts. Although it was demonstrated that the lung target in the XSight tracking phantom was localized with sub-millimeter accuracy, similar to the results in a recent study [20], target localization accuracy may be lower in patient cases.…”

“…Consequently, the number of projections acquired for the in-treatment CBCT could vary depending on the VMAT plans delivered with the CBCT acquisition. Instead of defining the gantry speed, an angular interval, so called the acquisition interval in XVI, was defined for acquiring the in-treatment CBCT; the acquisition interval is an acquisition parameter that defines the minimum gantry rotation, upon which kV projection acquisition is triggered (0.1° for this study as suggested in a previous study [ 20 ]). As summarized in Table 1 , more projections were acquired for the three-dimensional (3D) in-treatment CBCT than the 3D pretreatment CBCT: 495 vs. 333 projections.…”

“…The quality of the 3D-reconstructed Catphan CBCT images acquired with various tube currents (20,25,32,40, and 50 mA), acquisition angles (200° and 360°), and treatment beam energies (6 and 10 MV FFF) was quantitatively analyzed and the results are summarized in Fig. 4.…”

A modern review of the uncertainties in volumetric imaging of respiratory‐induced target motion in lung radiotherapy

“…[301][302][303][304][305] Despite the aforementioned challenges, researchers have done a tremendous amount of work to address these issues, and 4D-CBCT has been shown to be of important clinical utility. Multiple studies have reported superior target visualization and increased confidence in ITV localization with 4D-CBCT technology, thereby recommending it as a more accurate image-guidance technique for moving targets as compared to 3D-CBCT, [306][307][308][309][310][311] even improving the interobserver variability. 312 Four dimensional CBCT was shown to result in dosimetric advantages via the ability to reduce margins and thus irradiate less normal tissue, 313 as well as facilitating accurate verification of target position for both inter-and intra-fractional motion for lung SBRT with VMAT delivery.…”

On the evaluation of mobile target trajectory between four‐dimensional computer tomography and four‐dimensional cone‐beam computer tomography

“…Determining where an image is binned in the respiratory cycle is based on the position of high-contrast anatomical landmarks such as the diaphragm in each image. [8][9][10] After the images are binned, a 3D CBCT is reconstructed for each phase of the respiratory cycle, where the typical number of phases is 10. The 4D CBCT allows for accurate visualization of the tumor in each phase of the respiratory cycle by reducing motion artifacts associated with traditional 3D CBCT imaging acquisitions.…”