TITLE:
Experimental Evaluation of CT Number Changes in 320-Row CBCT Volume Scan for Proton Range Calculation
AUTHORS:
Ryuta Hirai, Ryosuke Kohno, Yu Kumazaki, Tetsuo Akimoto, Hidetoshi Saitoh, Shingo Kato
KEYWORDS:
320-Row CBCT Volume Scan, CT Number, Proton Range, Relative Stopping Power
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.7 No.2,
May
4,
2018
ABSTRACT: We
investigated the longitudinal positional dependence of CT number in 320-row
Cone Beam Computed Tomography (CBCT) volume scan (320-row volume scan) using a
simple geometric phantom (SGP) and a chest simulation phantom (CSP) in order to
evaluate its effect on proton range calculation. The SGP consisted of lung
substitute material (LSM) and a cylindrical phantom (CP) made of high-density
polyethylene. The CSP was an anthropomorphic phantom similar to the human
chest. The two phantoms were scanned using 320-row volume scan in various
longitudinal positions from the central beam axis. In experiments using the
SGP, an image blur at the boundary of the two materials became gradually
evident when the LSM was placed far away from the beam central axis. The image
blur of the phantom was consistent with the gradation in CT number. The maximum
difference in CT numbers between the 64-row helical scan and 320-row volume
scan at the boundary of the two materials was consistent with approximately 50%
of the relative proton stopping power. In contrast, the CT number profile in
each longitudinal position was fairly consistent and longitudinal positional
dependence rarely occurred in the CSP experiments. Pass lengths of CT beams
through areas with widely different electron densities were shorter, and thus
did not significantly impact CT numbers. Based on findings from the CSP
experiments, we considered 320-row volume scan to be feasible for proton range
calculation in clinical settings, although the relatively large longitudinal
positional dependence of CT number should be accounted for when doing so.