Image quality & dosimetric property of an investigational Imaging Beam Line MV-CBCT

Department of Radiological Sciences, St. Jude Children's Research Hospital, 262 Danny Thomas Place, Mail Stop 220, Memphis, TN 38105, USA.
Journal of Applied Clinical Medical Physics (Impact Factor: 1.17). 02/2009; 10(3):3023. DOI: 10.1120/jacmp.v10i3.3023
Source: PubMed


To measure and compare the contrast to noise ratio (CNR) as a function of dose for the CBCTs produced by the mega-voltage (MV) imaging beam line (IBL) and the treatment beam line (TBL), and to compare the dose to target and various critical structures of pediatric patients for the IBL CBCT versus standard TBL orthogonal port films. Two Siemens Oncor linear accelerators were modified at our institution such that the MV-CBCT would operate under an investigational IBL rather than the standard 6MV TBL. Prior to the modification, several CBCTs of an electron density phantom were acquired with the TBL at various dose values. After the modification, another set of CBCTs of the electron density phantom were acquired for various doses using the IBL. The Contrast to Noise Ratio (CNR) for each tissue equivalent insert was calculated. In addition, a dosimetric study of pediatric patients was conducted comparing the 1 cGy IBL CBCT and conventional TBL orthogonal pair port films. The CNR for eight tissue equivalent inserts at five different dose settings for each type of CBCT was measured. The CNR of the muscle insert was 0.8 for a 5 cGy TBL CBCT, 1.1 for a 1.5 cGy IBL CBCT and 2.8 for a conventional CT. The CNR of the trabecular bone insert was 2.9 for a 5 cGy TBL CBCT, 5.5 for a 1.5 cGy IBL CBCT and 14.8 for a conventional CT. The IBL CBCT delivered approximately one-fourth the dose to the target and critical structures of the patients as compared to the TBL orthogonal pair port films. The IBL CBCT improves image quality while simultaneously reducing the dose to the patient as compared to the TBL CBCT. A 1 cGy IBL CBCT, which is used for boney anatomy localization, delivers one-fourth the dose as compared to conventional ortho-pair films.

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    • "Megavoltage cone beam CT adds considerable amount of dose to the treatment volume and surrounding tissue, hence its inclusion in the treatment plan was suggested early on and multiple papers report the dose to phantom and patient from MV CBCT imaging using different dose settings [23,24,28,54,73e76]. Similar work was performed for the imaging dose from Siemens IBL system, and compared to that of the original 6 MV imaging beam (TBL) [45] [77]. Inclusion of kilovoltage imaging beam in the treatment plan, however, requires beam data collection and modeling. "
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    ABSTRACT: Imaging dose in radiation therapy has traditionally been ignored due to its low magnitude and frequency in comparison to therapeutic dose used to treat patients. The advent of modern, volumetric, imaging modalities, often as an integral part of linear accelerators, has facilitated the implementation of image-guided radiation therapy (IGRT), which is often accomplished by daily imaging of patients. Daily imaging results in additional dose delivered to patient that warrants new attention be given to imaging dose. This review summarizes the imaging dose delivered to patients as the result of cone beam computed tomography (CBCT) imaging performed in radiation therapy using current methods and equipment. This review also summarizes methods to calculate the imaging dose, including the use of Monte Carlo (MC) and treatment planning systems (TPS). Peripheral dose from CBCT imaging, dose reduction methods, the use of effective dose in describing imaging dose, and the measurement of CT dose index (CTDI) in CBCT systems are also reviewed. Copyright © 2015 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
    Physica Medica 07/2015; DOI:10.1016/j.ejmp.2015.06.003 · 2.40 Impact Factor
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    • "To further improve image quality and reduce the imaging dose, several authors have modified the linac to include a low atomic number (Z) target to increase the proportion of low-energy photons in the beam (Faddegon et al 2008, Flampouri et al 2002, Flynn et al 2009, Pouliot et al 2005, Robar et al 2009, Sawkey et al 2010, Faddegon et al 2010, Beltran 2009). All have reported significant improvements in planar and CBCT imaging. "
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    ABSTRACT: Computed tomography images have been acquired using an experimental (low atomic number (Z) insert) megavoltage cone-beam imaging system. These images have been compared with standard megavoltage and kilovoltage imaging systems. The experimental system requires a simple modification to the 4 MeV electron beam from an Elekta Precise linac. Low-energy photons are produced in the standard medium-Z electron window and a low-Z carbon electron absorber located after the window. The carbon electron absorber produces photons as well as ensuring that all remaining electrons from the source are removed. A detector sensitive to diagnostic x-ray energies is also employed. Quantitative assessment of cone-beam computed tomography (CBCT) contrast shows that the low-Z imaging system is an order of magnitude or more superior to a standard 6 MV imaging system. CBCT data with the same contrast-to-noise ratio as a kilovoltage imaging system (0.15 cGy) can be obtained in doses of 11 and 244 cGy for the experimental and standard 6 MV systems, respectively. Whilst these doses are high for everyday imaging, qualitative images indicate that kilovoltage like images suitable for patient positioning can be acquired in radiation doses of 1-8 cGy with the experimental low-Z system.
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    ABSTRACT: To determine the properties of a megavoltage cone-beam CT system using the unflattened beam from a sintered diamond target at 4 and 6 MV. A sintered diamond target was used in place of a graphite target as part of an imaging beam line (an unflattened beam from a graphite target) installed on a linear accelerator. The diamond target, with a greater density than the graphite target, permitted imaging at the lower beam energy (4 MV) required with the graphite target and the higher beam energy (6 MV) conventionally used with the tungsten/stainless steel target and stainless steel flattening filter. Images of phantoms and patients were acquired using the different beam lines and compared. The beam spectra and dose distributions were determined using Monte Carlo simulation. The diamond target allowed use of the same beam energy as for treatment, simplifying commissioning and quality assurance. Images acquired with the diamond target at 4 MV were similar to those obtained with the graphite target at 4 MV. The slight reduction in low energy photons due to the higher-Z sintering material in the diamond target had minimal effect on image quality. Images acquired at 6 MV with the diamond target showed a small decrease in contrast-to-noise ratio, resulting from a decrease in the fraction of photons in the beam in the energy range to which the detector is most sensitive. The diamond target provides images of a similar quality to the graphite target. Diamond allows use of the higher beam energy conventionally used for treatment, provides a higher dose rate for the same beam current, and potentially simplifies installation and maintenance of the beam line.
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