Technical note: Characterization of novel iterative reconstructed cone beam CT images for dose tracking and adaptive radiotherapy on L‐shape linacs

Abstract

Background Cone‐beam computed tomography (CBCT) allows for patient setup and positioning, and potentially dose verification or adaptive replanning prior to each treatment delivery. Poor CBCT image quality due to scatter artifacts and patient motion has been a major limiting factor. A new image reconstruction algorithm was recently clinically implemented for improving image quality through iterative reconstruction (iCBCT). Purpose This study aims to characterize iCBCT image quality, establish image value (HU)‐to‐relative electron density (RED) calibration curves for dose calculation, and assess the dosimetric accuracy for different anatomical sites. Material and methods Both conventional CBCT and iCBCT scans were acquired from a Varian TrueBeam On‐Board Imager system. A Catphan 604 phantom was scanned to compare image quality between the traditional Feldkamp–Davis–Kress (FDK) and novel iterative reconstruction techniques. Computerized Imaging Reference Systems (CIRS) electron density phantom was used to construct site‐specific HU‐RED curves corresponding to various scan settings. The CIRS Dynamic Thorax phantom, Rando pelvis phantom, and BrainLab head phantom were used for assessing dosimetric accuracy calculated on iCBCT images, compared to that on traditional FDK‐based CBCT images. All phantoms were scanned on a computed tomography (CT) to obtain baseline HU values for comparison. Results Test results obtained from Catphan showed statistically significant improvement with iCBCT, compared to FDK CBCT. Average HU differences from the baseline CT values were improved to within ±30 HU for iCBCT, compared to FDK CBCT for phantom studies. Dose calculated on iCBCT for both phantoms and patient cases directly using baseline HU‐RED calibration from CT showed 0.5%–2.0% accuracy from the baseline dose calculated on CT, which is comparable to doses calculated using site‐specific HU‐RED calibration curves. Conclusion iCBCT provides improved image quality, improved HU accuracy compared to CT baseline, and has potential to provide online dose verification as part of the adaptive radiotherapy workflow directly using the baseline HU‐RED calibration curve from CT.

Image quality evaluation of a new high-performance ring-gantry cone-beam computed tomography imager

“…The performance of IR has been investigated in various studies and compared to FDK reconstruction (Cai et al 2019, Gardner et al 2019, Jarema and Aland 2019, Lim et al 2022, and IR has even been demonstrated to decrease metal artifacts (Washio et al 2020). It has also been shown that megavolt imaging had preference over kV imaging in terms of reducing metal artifact (Harris et al 2023).…”

“…Furthermore, the new CBCT imager enabled the use of CBCT eFoV, where many previous on-board CBCT imagers have a limited FoV size and therefore not ideal for dose calculation purposes (e.g. for large patients or off-axis positioning of the patient) (Siewerdsen and Jaffray 2001, Jin et al 2010, Lim et al 2022. This study focused on CT number and geometry accuracy in the region outside the sFoV for HyperSight CBCT and CT.…”

Evaluation of a cone-beam computed tomography system calibrated for accurate radiotherapy dose calculation

“…Therefore, there has been a growing interest in utilizing CBCT imaging for plan adaptation instead of repeat computed tomography (CT) [5] , [6] , [7] , [8] . However, the lower image quality caused by photon scatter, limited field-of-view (FOV), and inconsistency in CT numbers for conventional CBCT compared to fan-beam CT can lead to lower dose calculation accuracy [2] , [9] , [10] , [11] , [12] , [13] , [14] , [15] .…”

“…In current linear accelerator (linac) based CBCT systems, image reconstruction is often carried out using filtered back projection with the Feldkamp-Davis-Kress (FDK) algorithm [10] . However, this approach has been found to handle image noise and artifacts insufficiently, and could lead to larger variations in CT numbers compared to fan-beam CT images [10] , [16] .…”

Clinical Practice Evolvement for Post-Operative Prostate Cancer Radiotherapy—Part 1: Consistent Organs at Risk Management with Advanced Image Guidance

“…However, traditional CBCT images based on the Feldkamp-Davis-Kress (FDK) filter-back projection algorithm can have poor soft tissue contrast due to scattering contamination [17,18]. Iterative CBCT (iCBCT), a new commercial reconstruction technique, has been evaluated by our group and demonstrated improved image quality for IGRT for various disease sites [19]. With enhanced IGRT, we aim to quantify the benefit of bladder filling and the use of an ERB for post-prostatectomy RT.…”