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 320 mm diameter and 900 mm length. For practical application, we joined unit cylinders (150 mm long) together to provide phantoms of 900 mm length. Dose profiles were measured with a pin photodiode sensor having a sensitive region of approximately 2.8 x 2.8 mm2 and 2.7 mm thickness. Beamwidths of the scanner were varied from 20 to 138 mm. Dose profile integrals (DPI) were calculated using the measured dose profiles for various beamwidths and integration ranges. For the body phantom (320-mm-diam phantom), 76% of the DPI was represented for a 20 mm beamwidth and 60% was represented for a 138 mm beamwidth if dose profiles were integrated over a 100 mm range, while more than 90% of the DPI was represented for beamwidths between 20 and 138 mm if integration was carried out over a 300 mm range. The phantom length and integration range for dosimetry of cone-beam CT needed to be more than 300 mm to represent more than 90% of the DPI for the body phantom with the beamwidth of more than 20 mm. Although we reached this conclusion using the prototype 256-slice CT-scanner, it may be applied to other multislice CT-scanners as well.

Difference in performance between 3D and 4D CBCT for lung imaging: a dose and image quality analysis

“…However, this length is too small to cover the wide diffusion tails of CBCT and some authors (11) suggested taking a larger dose integration length (approximately 400 mm). Accordingly, we performed measurements with $\mathrm{normalY}=100\hspace{0.17em}\text{mm}$ and 450 mm in order to quantify the difference between two extreme conditions.…”

Automated Calculation of Water‐equivalent Diameter (DW) Based on AAPM Task Group 220

“…The first limitation is that the length of phantom is too short for measuring scattered radiation 16 , 17 , 18 , 19 . The second limitation is that the diameter of the phantom does not represent the variation in diameters among patients 20 , 21 , 22 .…”

“…Regarding the first limitation, CTDI is often measured using a 10 cm ionization chamber, which is considered too short because it excludes any contribution from radiation scattered beyond the relatively short range of integration along the Z‐direction, and it tends to undervalue the cumulative dose at the center position, as reported by many investigators 16 , 17 , 18 , 19 . To overcome this limitation, the American Association of Physicists in Medicine (AAPM) issued report TG‐111 which recommended using a sufficiently long (e.g., at least 45 cm) phantom to accommodate the scattered radiation (25) .…”

Measurements of the dose delivered during CT exams using AAPM Task Group Report No. 111

“…With the new generation of CT scanners, advanced technologies with helical scanning mode or cone‐beam irradiation geometries appeared; therefore, the CTDI dose measurement methods were not suitable anymore 4 , 5 , 6 , 7 . Moreover, because of the increase of the detection system size along the z‐axis, the CT beams became larger and the use of ${\text{CTDI}}_{100}$ was not appropriate.…”