Abstract ID: 85 Investigating the physics of a CBCT projection shading correction based on a prior CT

Abstract

The adoption of cone beam computed tomography (CBCT) image guidance in proton therapy has spurred research on CBCT image correction for dose calculation. Initially, methods based on deformable image registration gained attention [1] , however projection correction approaches based on prior CT information [2] have been shown to perform well for several body sites [3] . The shading correction algorithm used in [2] , [3] relies on image processing of the subtraction of digitally reconstructed radiographs (DRR) of a DIR-registered prior CT, and CBCT projections. In this study we have performed Monte Carlo (MC) simulation of CBCT imaging to disentangle the different sources of projection degradation, and to evaluate the physicality of the shading correction. The GATE MC toolkit’s fixed-forced-detection actor was employed, along with source and detector models optimized for an Elekta XVI CBCT system, to simulate CBCT imaging of an electron density phantom. The shading correction algorithm was applied to the measured projections and the so-called scatter component (SCA) was compared to the MC simulated scatter. Measured and simulated CBCT projections (scatter + primary) agreed well, with the largest discrepancy found for a bone insert (3% of log transformed projections). Important disagreement was observed with the MC scatter signal when the contribution from beam hardening was kept in the SCA. Undoing the beam hardening correction from the SCA using functions derived from MC primary projections and DRRs greatly improved agreement of scatter signals from MC and SCA (3% on average), with the largest discrepancy found for the bone insert (12%). Residual discrepancies were shown to stem from the intrinsic limitations of the SCA. Proton therapy dose calculations on corrected CBCT will be presented in addition to the results above.