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

Abstract

Background and purpose: Dose calculation on cone-beam computed tomography (CBCT) images has been less accurate than on computed tomography (CT) images due to lower image quality and discrepancies in CT numbers for CBCT. As increasing interest arises in offline and online re-planning, dose calculation accuracy was evaluated for a novel CBCT imager integrated into a ring gantry treatment machine.

Materials and methods: The new CBCT system allowed fast image acquisition (5.9 s) by using new hardware, including a large-size flat panel detector, and incorporated image-processing algorithms with iterative reconstruction techniques, leading to accurate CT numbers allowing dose calculation. In this study, CBCT- and CT-based dose calculations were compared based on three anthropomorphic phantoms, after CBCT-to-mass-density calibration was performed. Six plans were created on the CT scans covering various target locations and complexities, followed by CBCT to CT registrations, copying of contours, and re-calculation of the plans on the CBCT scans. Dose-volume histogram metrics for target volumes and organs-at-risk (OARs) were evaluated, and global gamma analyses were performed.

Results: Target coverage differences were consistently below 1.2 %, demonstrating the agreement between CT and re-calculated CBCT dose distributions. Differences in D_mean for OARs were below 0.5 Gy for all plans, except for three OARs, which were below 0.8 Gy (<1.1 %). All plans had a 3 %/1mm gamma pass rate > 97 %.

Conclusions: This study demonstrated comparable results between dose calculations performed on CBCT and CT acquisitions. The new CBCT system with enhanced image quality and CT number accuracy opens possibilities for off-line and on-line re-planning.

Keywords: CT number accuracy; Cone-beam computed tomography; Dose calculation; Image quality; Metal artifact reduction; Radiotherapy.

Graphical abstract

Fig. 1

Overview of the steps performed in the dose calculation evaluation for both the fan-beam CT and CBCT images of the phantoms. A rigid registration was used to copy treatment plans from CT to CBCT. Dose difference was quantified based on dose delivered to targets and organs at risks and using gamma analysis.

Fig. 2

CT-number-to-mass-density conversion curve (Hounsfield look-up table; HLUT) generated for the small (head-sized) phantom (blue line) and large (body-sized) phantom (green dashed line) for 125 kVp for the CBCT scanner (the curves for the 140 kVp spectrum can be found in Fig. S6 in the supplementary materials along with the data used to create the curves).

Fig. 3

Bar plots for dose differences (percentage of prescribed dose) in the evaluated DVH metrics between dose calculation on a fan-beam CT and CBCT (D_CBCT – D_CT) for plans 1, 2 and 4. The PTV coverage (blue bars) is quantified by D_98% (Plan 1 and 2) and D_99% (Plan 4). (Dose differences in Plan 3, 5 and 6 are shown in Fig. S8 in the supplementary materials). Abbreviations: PTV – planning target volume; PCM – pharyngeal constrictor muscle; Inf – inferior, Mid – middle, Sup – superior; Subm – Submandibular.

Fig. 4

Examples of CT and CBCT images as well as corresponding planned dose distribution on the CT image and gamma maps for all investigated plans. Orange contours show the target, if present on the given slice. The target volumes for Plan 1 and Plan 2 were the same. The slice positions were chosen based on the location of the largest dose difference. Green shading on the gamma maps indicates dose regions where gamma < 1. The gamma maps show both over- and under-dosage with a red color. Dose distribution is shown with 20 % isodose as lower threshold.