Program Information
MCNP6 Model of the Siemens Multitom Rax Cone-Beam CT (CBCT)
S Streitmatter*, F Noo , C Hanrahan , P Jenkins , Univ of Utah Hospitals, Salt Lake City, UT
Presentations
SU-H3-GePD-I-4 (Sunday, July 30, 2017) 4:00 PM - 4:30 PM Room: Imaging ePoster Lounge
Purpose: To model the unique aspects of the Siemens Multitom Rax™ (Robotic Advanced X-ray technology) Cone-Beam CT (CBCT) feature and assess the effects of varying source to surface distance (SSD) and < 360° trajectories on dose distributions in phantom and patient geometries.
Methods: Monte Carlo N-Particle 6.1.1b (MCNP6.1.1b) is used to model the x-ray source spectra, collimation, rotation and dosimetry aspects of Multitom Rax CBCT scans. Scan trajectory details were obtained from the manufacturer and fitted to polynomial functions; a total of seven trajectories were assessed. The source.F90 subroutine in MCNP was modified to simulate the non-circular, varying SSD arcs, x-ray spectra, and collimation. Typical table CBCT scans are taken in an 8 Hz cine mode for 20 seconds, acquiring 160 projections. kVp is determined by an AP and LAT exposures. Measurements were taken in-air and in the center of a 16 cm diameter CTDI head phantom with the Radcal® 10X6-0.6CT thimble chamber to obtain normalization factors for converting results per source particle to dose/mAs. Initially, the tcerv trajectory was assessed since it has one of the largest SID variations (up to 20 cm).
Results: Simulations of absorbed dose in the center of the 16 cm CTDI phantom for a tcerv trajectory show reasonable agreement with measurements, using in-air measurements for normalization. Initial calculations agree within 10%, however model refinements are still in progress.
Conclusion: Our MCNP6 model developed to mimic key dosimetry aspects of CBCT acquisitions on the Multitom Rax, shows good agreement between simulated and measured dose values for the 16 cm CTDI head phantom, considering all the adjustable parameters. Further development to assess absorbed and effective doses in patient-specific geometries for all trajectories and the impact of the heterogeneous dose distribution for these CBCT acquisitions are in progress.
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