Hounsfield Unit characterization and dose calculation on a C-arm linac with novel on-board cone-beam computed tomography feature and advanced reconstruction algorithms

Abstract

Purpose: Current cone-beam computed tomography (CBCT) on-board c-arm linear accelerators (linacs) lack CT number precision sufficient for dose calculation due to increased scatter from the cone geometry. This investigation evaluated CT number and dose calculation accuracy in-phantom on a novel on-board CBCT unit with potential for improved dose calculation accuracy.

Methods: Eight head and eight body configurations of an electron density phantom using 16 materials were acquired with a clinical CT-simulator(CT-sim) and novel on-board CBCT imager. CBCT projection data was reconstructed using conventional Feldkamp-Davis-Kress(FDK) and patient scatter-corrected iterative Acuros CTS with metal artifact reduction (Acuros-CTS-iCBCT-MAR) to create a robust CT number to physical density curve. CT-sim and CBCT images of anthropomorphic head and thorax phantoms were acquired, and three treatment plans were generated per phantom. All CBCT images were registered to the CT-sim of the phantoms, and the dose was recalculated on the CBCT images. 3D gamma analysis was performed (10% dose threshold, local, 1% dose difference, 1 or 2 mm distance to agreement), and dose-volume histogram (DVH) metrics were reported for target coverage and organ sparing.

Results: CT numbers for materials ≤1.08 g/cc showed high agreement between CT-sim and CBCT acquisitions. CT number precision improved with Acuros-CTS-iCBCT-MAR compared to FDK in all materials. High agreement between CT-sim and CBCT reconstructed with Acuros-CTS-iCBCT-MAR was observed in 3D gamma analysis showing 93.8% voxels passing in the worst case for a spine stereotactic body radiotherapy (SBRT) plan at 1%/1 mm. Maximum deviation in target coverage was -3.3% PTV-D98% in the lung SBRT plan among the novel reconstructed CBCT images. Plan comparison using FDK reconstructions yielded similar or worse agreement in 3D gamma analysis, with 69.7% voxels passing in the worst case for a spine SBRT treatment plan. Maximum target coverage deviation was -11.7% PTV-D98% among the FDK-reconstructed CBCT.

Conclusions: Novel CBCT solutions on c-arm linacs show promise for enabling direct-to-unit or offline adaptive dose calculation, potentially increasing versatility and efficiency of patient-centered care.

Keywords: Hounsfield Unit characterization; dose calculation; fast cone‐beam CT; inhomogeneity correction.

FIGURE 1

Mean difference in CT number averaged across all phantom and rod configurations per CBCT protocol substracted from FBCT average with nearest corresponding kVp. Error bars repersent combined standard deviation across all scans calculated per protocol and material. CT, computed tomography; CBCT, cone beam computed tomography; FBCT, fan beam computed tomography; kVp, kiloVolt peak.

FIGURE 2

Distribution of calculated gamma value from 1%/1 mm local criteria in SRS and SBRT or 1%/2 mm for IMRT and 10% dose thresholding in the axial slice containing maximum dose across body sites, protocol (T = Thorax, P = Pelvis, L = Pelvis Large, H = Head, and H^*= Head with custom parameters) and reconstruction methods. IMRT, intensity‐modulated radiotherapy; SBRT, stereotactic body radiotherapy; SRS, stereotactic radiosurgery.