Enlarged longitudinal dose profiles in cone‐beam CT and the need for modified dosimetry

Abstract

In order to examine phantom length necessary to assess radiation dose delivered to patients in cone‐beam CT with an enlarged beamwidth, we measured dose profiles in cylindrical phantoms of sufficient length using a prototype 256‐slice CT‐scanner developed at our institute. Dose profiles parallel to the rotation axis were measured at the central and peripheral positions in PMMA (polymethylmethacrylate) phantoms of 160 or diameter and length. For practical application, we joined unit cylinders ( long) together to provide phantoms of length. Dose profiles were measured with a pin photodiode sensor having a sensitive region of approximately and thickness. Beamwidths of the scanner were varied from 20 to . Dose profile integrals (DPI) were calculated using the measured dose profiles for various beamwidths and integration ranges. For the body phantom (‐diam phantom), 76% of the DPI was represented for a beamwidth and 60% was represented for a beamwidth if dose profiles were integrated over a range, while more than 90% of the DPI was represented for beamwidths between 20 and if integration was carried out over a range. The phantom length and integration range for dosimetry of cone‐beam CT needed to be more than to represent more than 90% of the DPI for the body phantom with the beamwidth of more than . Although we reached this conclusion using the prototype 256‐slice CT‐scanner, it may be applied to other multislice CT‐scanners as well.