Room: Track 1
Purpose: Motion artifacts in cone-beam CT(CBCT) cause uncertainties that prevent direct mega-voltage (MV) dose calculation for radiation therapy. The quantification of errors is challenging in absence of an artifact-free ground truth (GT). We propose a simulation-based method to quantify respiratory motion artifact-induced dose deviation.
Methods: Projections encoding physiologically plausible motion are simulated to reconstruct anatomically identical motion affected and GT volumes. A deformation vector field (DVF) between extreme 4DCT phases is obtained with DEEDS. For the first dynamic scenario, the DVF is used to simulate a normal acquisition (3RPM, 15FPS) using a synthetic sinusoidal breathing signal (20BPM) with ±50% and ±25% of the 4DCT motion amplitude. Each projection is obtained by forward-projecting the 4DCT phase deformed according to the projection specific amplitude-scaled DVF. Resulting reconstructions show typical motion streak-artifacts. Second, all deformed volumes are averaged and forward-projected to mimic an acquisition capturing the total motion in each projection. Consequently, the reconstructions contain motion-induced blurring without streak-artifacts. Third, we apply a single DVF scaled by the average breathing amplitude to obtain the GT. Dose is calculated using Acuros-AXB with a static open anterior-posterior field. We quantify the dose deviation between doses calculated on artifact and blurring-affected volumes against the GT in terms of mean absolute dose differences (MAE) and local gamma analysis with 2%/2mm threshold and 20% cutoff.
Results: The artifact-induced dose deviation has been assessed on simulations with respect to the static GT. The MAE values are 3.4E-3Gy, 1.8E-3Gy and gamma is 94.54%, 99.42% for ±50% and ±25% amplitudes, respectively. The dose distribution for ±50% and ±25% average motion shows an improved agreement of 2.9E-3Gy, 3.9E-4Gy MAE and 99.22%, 99.91% gamma.
Conclusion: The proposed method for quantification of motion artifact-induced dose deviations allows for specific improvements of CBCT acquisition and reconstruction.
Funding Support, Disclosures, and Conflict of Interest: Authors are employees of Varian Medical Systems, Switzerland.