First evaluation of in-patient dose calculation accuracy on a C-arm Linear Accelerator with advanced Cone-Beam...

Innovations in on-board Cone-Beam Computed Tomography (CBCT) show promise for accurate dose calculation due to higher CT number precision compared to prior C-arm CBCT systems. We prospectively investigated dose calculation accuracy in patients imaged using a C-arm linac equipped with an advanced CBCT imaging solution utilizing iterative reconstruction algorithms.

For image acquisition, 37 patients underwent both a CT-simulation (CT-sim) and high-quality C-arm CBCT in their treatment position. A total of 39 plans (2 patients multi-site) across four body sites (10 Head/Neck, 9 Thorax, 10 Abdomen, 10 Pelvis) were developed on the clinical CT-sim image and recalculated on the CBCT. Dose distributions were compared using 3D gamma analysis passing rates (GPR) using 1%/1mm local criteria and 10% or 50% low-dose thresholding (10%TH, 50%TH). Percent difference in target dose-volume histogram (DVH) metrics D99%, D_mean, D1%, V100%, and V95% were calculated for comparison.

The CBCT to CT-sim dose calculation comparisons overall GPR with 10%TH averaged 96.2 ± 3.6% and 50%TH GPR average was 95.0 ± 6.9%. Overall, 36 plans at 10%TH and 32 plans at 50%TH achieved > 90% GPR. Percent difference in target DVH metrics averaged 0.15 ± 1.24%, 0.52 ± 1.01%, 0.86 ± 1.29%, -0.92 ± 4.84%, and -0.31 ± 2.42% for D99%, D_mean, D1%, V100%, and V95%, respectively. Only 9 of 195 (4.6%) target DVH metrics changed by > 5%.

C-arm CBCT dose calculation was comparable to standard-of-care CT-sim in most patients, demonstrating the ability to be used for clinical evaluation and decisions. Direct dose calculations in settings of minimal anatomical motion were most robust.