SU‐E‐I‐26: A Monte Carlo Based Study on X‐Ray Characteristics of Two Commercial Multi‐Detector CT Simulators

Abstract

Purpose: As multi‐detector computed tomography (MDCT) scan is routinely performed for treatment planning in radiation oncology, understanding the characteristics of MDCT x‐ray beam is essential to optimize scan protocols as well as to estimate patient dose. The purpose of this study was to characterize the x‐ray beams of two commercial MDCT simulators with Monte Carlo (MC) simulations. Methods: X‐ray tube systems of two wide bore MDCT scanners (GE LightSpeed RT, LS and Philips Brilliance Big Bore, BB) were modeled in the BEAMnrc/EGSnrc systems. All the tube components were modeled: tungsten target, inherent filter, bowtie filter, and beam collimators. Three benchmark processes were performed to validate the MC models. (1) X‐ray spectra comparison between MC and manufacturersˈ data, (2) Half‐value layers (HVLs) between MC and measurements using aluminum sheets and multi‐functional radiation detectors, (3) Lateral and axial beam profiles between MC and radiochromic film measurements To understand the scatter effect of the MDCT beams, scatter‐to‐primary energy fluence ratios (SPRs) were derived and the total SPR for each CT system was calculated with CTDI head and body phantoms.. Results: MC generated x‐ray spectra agreed well with the manufacturer‐specified data over the entire energy range. Effective photon beam energies for 120kVp beam were found to be (MC, vendor‐specified): (60.8, 60.2keV) for LS scatter, and (65.4, 64.9keV) for BB scanner. MC‐computed HVLs were found to be within 1% of the ion chamber measurements on average. The lateral and axial profiles between film and MC showed overall good agreement within 10% on average. The total SPR ranged from 3.5 to 4.9% for head phantom and from 7.1 to 11.1% for body phantom. Conclusion: Complete characterizations of two MDCT scanners have been performed using the BEAMnrc/EGSnrc MC system. This information can be utilized to optimize scan protocols as well as patient‐specific dosimetry for the MDCT systems.