Abstract
Dosimetry in kilovoltage cone beam computed tomography (CBCT) is a challenge due to the limitation of physical measurements. To address this, we used a Monte Carlo (MC) method to estimate the CT dose index (CTDI) and the dose length product (DLP) for a commercial CBCT system. As Dixon and Boone (1) showed that CTDI concept can be applicable to both CBCT and conventional CT, we evaluated weighted CT dose index (CTDIw) and DLP for a commercial CBCT system. Two extended CT phantoms were created in our BEAMnrc/EGSnrc MC system. Before the simulations, the beam collimation of a Varian On‐Board Imager (OBI) system was measured with radiochromic films (model: XR‐QA). The MC model of the OBI X‐ray tube, validated in a previous study, was used to acquire the phase space files of the full‐fan and half‐fan cone beams. Then, DOSXYZnrc user code simulated a total of 20 CBCT scans for the nominal beam widths from 1 cm to 10 cm. After the simulations, CBCT dose profiles at center and peripheral locations were extracted and integrated (dose profile integral, DPI) to calculate the CTDI per each beam width. The weighted cone‐beam CTDI (CTDIw,l) was calculated from DPI values and mean CTDIw,l(CTDIw,l)¯ and DLP were derived. We also evaluated the differences of CTDIw values between MC simulations and point dose measurements using standard CT phantoms. In results, it was found that CTDIw,600¯ was 8.74±0.01 cGy for head and CTDIw,900¯ was 4.26±0.01 cGy for body scan. The DLP was found to be proportional to the beam collimation. We also found that the point dose measurements with standard CT phantoms can estimate the CTDI within 3% difference compared to the full integrated CTDI from the MC method. This study showed the usability of CTDI as a dose index and DLP as a total dose descriptor in CBCT scans.PACS number: 87.57.uq
Highlights
85 Kim et al.: CT dose index (CTDI) and dose length product (DLP) in cone-beam CT (CBCT): Monte Carlo (MC) study availability of the standardized 100 mm length pencil ion chamber, CTDI100 has been generally accepted as a standard CT dose descriptor[5] that can be expressed as follows: CTDI100 = 1 nT 50 mm D( z )dz [1]where n equals number of slices, T equals slice thickness, and D(z) equals dose profile along the axis of rotation.In recent years, the CT technology has advanced resulting in substantially wider beam width; this has enabled the acquisition of a larger imaging area with fewer numbers of rotations and shorter scan time
Full-fan CBCT scan was used for the head CT phantom, while half-fan CBCT scan was performed for the body CT phantom. (The detailed information of the CBCT scan modes can be found in Yoo et al[12]) The axial dose profiles for the nominal beam openings from 1 cm to 10 cm specified at isocenter, were obtained from the MC simulations, and CTDI and DLP were calculated using the profile data
Once we find the CTDI-like quantity for a CBCT scan, which is conceptually equivalent to multi-detector CT (MDCT) scan, DLP can be derived by this and effective dose (ED) can be estimated by multiplying a conversion factor to the DLP value
Summary
Where n equals number of slices, T equals slice thickness, and D(z) equals dose profile along the axis of rotation. Typical cone-beam CT (CBCT) systems perform a single X-ray tube rotation with a stationary table to acquire a three-dimensional (3D) image. The cone-beam creates an extended range of the axial dose profile (typically 30–100 cm) beyond the regular 100 mm pencil ion chamber, resulting in long tail portions of ionization which can not be fully collected with the conventional pencil ion chamber. Investigators previously reported the use of an extended 300 mm long pencil ion chamber,(6,8) the chamber needs to have the accuracy related to electron collection efficiency, sensitivity variation and stem leakage.[9] this requires CT phantoms with extended length
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