Abstract: Megavoltage Cone Beam Computed Tomography MVCBCT is an image guided radiotherapy imaging tool used for everyday patient repositioning. Present work studies the effect on MVCBCT image quality in using a copper target in place of the original target. Monte Carlo (MC) simulations using FLUKA were carried out for the original target with flattened and unflattened 6 MV beams for different target materials and thicknesses, calculating the photon spectra incident on the phantom surface. MC simulations were also performed for the original and copper targets to calculate the local contrast (LC) in a simple phantom. Reduction is observed in the mean energy of the photon spectrum and a large increase is obtained in the low energy photons ratio when the copper and carbon targets are used in place of the original target, leading to an improvement in the quality of MVCBCT images. Further, the LC was improved by 31% when the copper target was used. The reduction in mean energy and the increase in low energy photons ratio for the carbon target was found to be higher than that for the copper target, noting that the copper target is already available in the head of most Varian LINACs for treatments requiring a higher photon energy mode (&gt; 6MV). It can be concluded that with simple modification, using a copper target with an unflattened beam will improve the MVCBCT image quality. [F. A. Abolaban, M. A. Najem, Ahmad Hussain, Majdi Alnowami, David Bradley. Improving MVCBCT image quality using a Cu target with flattening filter-free LINAC, Life Sci 

Ahmad Hussain

David Bradley

F A Abolaban

M A Najem

Majdi Alnowami

CiteSeerX

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Improving MVCBCT image quality using a Cu target with flattening filter-free LINAC 
 
F. A. Abolaban
1*
, M. A. Najem
2
, Ahmad Hussain
1
, Majdi Alnowami
1
 and David Bradley
2
 
 
1
Nuclear Engineering Department, Faculty of Engineering, King Abdulaziz University, P.O. Box 80204, Jeddah 
21589, Kingdom of Saudi Arabia 
2
Centre for Nuclear and Radiation Physics, University of Surrey, Guildford, Surrey, GU2 7XH, UK 
Email: fabolaban@kau.edu.sa  
 
Abstract: Megavoltage Cone Beam Computed Tomography MVCBCT is an image guided radiotherapy imaging 
tool used for everyday patient repositioning. Present work studies the effect on MVCBCT image quality in using a 
copper target in place of the original target. Monte Carlo (MC) simulations using FLUKA were carried out for the 
original target with flattened and unflattened 6 MV beams for different target materials and thicknesses, calculating 
the photon spectra incident on the phantom surface. MC simulations were also performed for the original and copper 
targets to calculate the local contrast (LC) in a simple phantom. Reduction is observed in the mean energy of the 
photon spectrum and a large increase is obtained in the low energy photons ratio when the copper and carbon targets 
are used in place of the original target, leading to an improvement in the quality of MVCBCT images. Further, the 
LC was improved by 31% when the copper target was used. The reduction in mean energy and the increase in low 
energy photons ratio for the carbon target was found to be higher than that for the copper target, noting that the 
copper target is already available in the head of most Varian LINACs for treatments requiring a higher photon 
energy mode (> 6MV). It can be concluded that with simple modification, using a copper target with an unflattened 
beam will improve the MVCBCT image quality. 
[F. A. Abolaban, M. A. Najem, Ahmad Hussain, Majdi Alnowami, David Bradley. Improving MVCBCT image 
quality using a Cu target with flattening filter-free LINAC, Life Sci J 2013;10(4):1624-1628]. (ISSN:1097-8135). 
http://www.lifesciencesite.com 215 
 
Keywords: MVCBCT, IGRT, Flattening Filter Free (FFF), Monte Carlo 
 
1. Introduction 
Megavoltage cone beam CT (MVCBCT) is 
one of a number of image guided modalities that are 
used to reduce the physical uncertainties occurring 
during the course of a radiotherapy treatment, related 
to patient position on the couch of a linear accelerator 
from fraction to fraction, including organ movements 
and deformation of tissues (Sharpe et al, 2007). 
Although the introduction of kilovoltage cone beam 
CT (kVCBCT) provides the opportunity to acquire 
images of much improved quality, interest in 
MVCBCT images still exists due to the simplicity of 
dose modelling in the treatment planning system 
(Gayou 2012). Several studies have been conducted 
to improve the quality of MVCBCT images by 
changing acquisition parameters such as number of 
projections, dose exposure and the scan arc length or 
by changing image reconstruction parameters such as 
the reconstruction filter, image thickness and pixel 
size (Gayou 2012). Improvements in image quality 
were obtained by changing the material and thickness 
of the target and by removal of the flattening filter 
from the LINAC head. Tsechanskiet al. (1998) 
introduced a thin target approach for portal imaging, 
simulating several targets with different materials and 
thicknesses, finding the production of sharper images 
in using a 4 MV unflattened beam with a 1.5 mm 
copper or 5 mm aluminum target and a new Kodak 
mammographic film. They concluded that using 
lower atomic number Z materials did not produce 
significant improvement. Flampouriet al. (2002) 
found the optimum imaging arrangement for a 6 MV 
beam to be a 6 mm aluminum target, improving the 
contrast by 19% for a 1.4 cm slab of bone inside 5 cm 
of water. Roberts et al. (2008) carried out a MC 
simulation and experiment to study the effect of 
using a low Z target with an X-ray Volume Imaging-
panel (XVI-panel) detector (Elekta, Stockhholm)(the 
panel being based on AmSi technology) instead of 
the conventional imaging technique. They found that 
by using this setup, the contrast for a 1.6 cm slab of 
bone inside 5.8 cm of water was improved by a factor 
of 4.62 while it reduced to a factor of 1.3 if the same 
slab of bone was placed inside 25.8 cm of water. 
Faddegonet al. (2008; 2010) developed a new image 
beam line (IBL) to be used with Siemens LINACs. 
They used a carbon target instead of electron foils 
and irradiated it with a 4.2 MeV electron beam, 
obtaining an improvement in contrast and spatial 
resolution of a factor of 3 and 2 respectively, with the 
same dose to patient or in acquiring an image with 
same quality but at lower dose. Connell and Robar 
(2009) performed several MC simulations to study 
three target materials (beryllium, aluminum and 
tungsten) with different thicknesses, irradiating them 
with 4.5 and 7.0 MeV electron energies. They found 
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that the thinner target with the higher electron energy 
produced images with better modulation transfer 
function MTF. Sawkeyet al. (2010) used a diamond 
target with 4 and 6 MV beams instead of the carbon 
target, allowing treatment beams that simplify the 
commissioning and the quality assurance. They found 
that the image quality is the same as the carbon target 
at 4 MV. In addition, Bretbachet al. (2011) developed 
a sintered pixelated array detector made of Gd2O2S 
ceramic scintillator to be used with the imaging beam 
line (IBL). They found that the contrast-to-noise ratio 
(CNR) was improved by a factor of 2.66. 
Furthermore, Parsons and Robar (2012) studied the 
effect of the presence of a 1 mm copper plate placed 
on electronic portal imaging devices (EPIDs) on the 
quality of MVCBCT images. They found that the 
copper plate reduced the number of diagnostic 
photons by 20%.  
All studies above suggest modification of 
LINACs by adding a thick low Z or thin medium Z 
target and/or the design of a new imaging line. Such 
changes would present a number of developmental 
issues, not least in developing countries in which 
LINACs in many cases lack an on-board imager and 
have not yet established an IGRT protocol. In 
addition, some of these suggestions have either not 
been accompanied by studies of the effect of 
reducing the thickness of the target on the electron 
contamination or have added a plate of Perspex to 
reduce the electron contamination in the imaging 
beam leading to absorption of a fraction of the low 
energy photons. In this work use has been suggested 
of the use of a copper target for the imaging mode 
using a 6MV beam since the former is already 
present inside the head of most Varian LINACs to be 
used with treatments requiring higher photon energy 
mode (> 6 MV). 
 
2.0 Materials and methods 
2. 1MC parameters  
The 6 MV Varian Clinac 2100C was 
simulated using the FLUKA Monte Carlo (MC) code 
(Ferrari et al, 2005); see Figure 1. 
The actual geometry of this LINAC head 
was obtained from the Varian manufacturer. Five 
different targets were simulated without a flattening 
filter (FF).  All configurations are listed in Table 1. 
 
Table1: Target configurations used in this simulation 
Targets material used in this simulation 
with FF Varian 6 MV original target   
Without FF 
Varian 6 MV  original target 
Varian copper target (~0.5 cm) 
Copper target (0.25 cm) 
Carbon (1 cm) 
Carbon (2 cm) 
The incident electron beam hitting the 
target was simulated as a pencil beam of energy 6 
MeV and a diameter 3 mm. A 40 × 40 × 40 cm
3
 
water phantom was simulated at a source surface 
distance (SSD) of 100 cm. The water phantom was 
divided into voxels in order to calculate the 
percentage depth dose as well as the entrance dose 
along the central axis (CA). The voxel size was set to 
0.5 × 0.5 × 0.25 cm
3
. The photons and electrons cut-
off energies were set to 10 keV and 511 keV 
respectively. The number of histories was set to 
1×10
9
 primary electrons to reduce the statistical 
uncertainty to an acceptable level. Each simulation 
was run on a laptop (Intel 8 Core, 2.4 GHz CPU, 8 
GB RAM) with a Linux operating system. 
2.2 MC validation 
To validate the simulation, the calculated 
percentage depth doses (PDDs) of the FF beam in the 
CA of the water phantom were compared with 
percentage depth dose (PDD) data in BJR-
supplement 25. Figure 2 shows the PDD curves of 
the simulated 6 MV FF beam and that obtained from 
BJR-supplement 25 (BJR 1996). The difference 
between them was less than 2% at all points. 
2.3 Photon spectra, electron contamination and 
entrance dose 
The photon spectra for all configurations 
were calculated in air at the isocentre. The ratio of 
low energy photons (10-150 keV) in each spectrum 
was compared with that for the conventional beam. 
The photon average energies for all setups were 
calculated. The electron contamination on the patient 
plane in each setup was obtained by calculating the 
ratio of the total number of electrons to photons at the 
isocentre. In addition, the entrance dose was 
calculated by averaging the dose within the first 1cm 
inside the water phantom on the CA.  
2.4 Image quality 
In order to calculate the improvement in 
image quality by using Varian copper target, two 
simple simulations for the conventional and the 
copper target beams were performed. Both beams 
with 20×20 cm
2
 field-size were simulated to irradiate 
two cylindrical bones with 2 cm diameter placed 
inside water phantom with 5 cm thickness at 100 cm 
from the target; see Figure 3. The images were 
obtained by scoring the photon fluence 10 cm below 
the phantom. Since changing the target and removing 
the FF will affect the dose rate, the number of 
incident electrons for the new setup was reduced to 
obtain an image with same dose as in the 
conventional setup. The LC for both images was 
calculated using the following equation (Flampouriet 
al, 2002): 
LC = 2|Ib-Iw| / ( Ib+Iw)  
WhereIbandIware the photon fluence in the region of 
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interest in bone and water respectively.   
 
3.0 Results and discussion  
3.1 Photon spectra, electron contamination and 
entrance dose 
The average energy for copper targets is 
between the average energy obtained using the 
original Varian targets and the carbon targets; see 
Figure 4. In addition, the low photons ratio increased 
in using the copper target compared with use of the 
original target. It was further found that by using a 
thin target (0.25 cm copper or 1 cm carbon) the 
number of primary electrons at the isocentre 
increased sharply.  
The entrance dose also increased when the 
FF was removed and also increased as the thickness 
or Z of the target decreased. Table 2 summarises the 
calculated parameters for each configuration. 
3.2 Image quality 
The acquired images obtained by the 
conventional and the copper target beams are shown 
in Figure 5. It can be noticed that the image acquired 
by the copper target beam has the better quality 
compared to that acquired by the conventional beam. 
The LC has improved by 31%. Clearly, this value 
will reduce if a thicker phantom is used.  
This modification is suggested to be useful 
for repositioning patients who undergo radiotherapy 
treatment for lung or head and neck cancers. This 
modification points to provision of a simple and low 
cost technique for acquiring MVCBCT images with 
better quality, being perhaps most important for 
developing countries. 
 
4.0 Conclusion 
The effect of using a copper target to 
acquire MVCBCT images was studied using the MC 
method. It was found that the low energy photon ratio 
was increased by a factor of 7.2 when the copper 
target was used as a replacement for the conventional 
target. The image quality for a simple phantom made 
of two cylindrical bones with 2 cm diameter placed 
inside 5 cm thickness water phantom was improved 
by 31%. It can be concluded that the quality of 
MVCBCT images can improved with a simple and 
low cost modification of the Varian copper target. 
 
Acknowledgements 
This Project was funded by the Deanship of 
Scientific Research (DSR), King Abdulaziz 
University, Jeddah, under Grant No. (1433/135/511). 
The authors, therefore, acknowledge with thanks 
DSR technical and financial support. 
 
 
 
Corresponding Author: 
Dr. FouadAbolaban 
Department of Nuclear Engineering 
King Abdulaziz University 
Jeddah 21589 
Kingdom of Saudi Arabia 
E-mail: fabolaban@kau.edu.sa 
 
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Life Science Journal 2013;10(4)                                                          http://www.lifesciencesite.com 
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Figure 1: The simulated geometry of the conventional 6 MV Varian Clinac 2100C. 
 
Figure 2: The PDD curves of the conventional 6 MV Varian linac beam simulated (squares) and obtained from BJR-
suplement-25 (circles) for a 10 × 10 cm
2
 field-size in a water phantom. 
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http://www.lifesciencesite.com   1628                     lifesciencej@gmail.com 
 
Figure 3: Conventional LINAC irradiating two cylindrical bones with 2 cm diameter placed inside a water phantom 
with 5 cm thickness at 100 cm from the target. 
 
 
Figure 4: The photon spectra in air for all configurations at the isocentre. 
 
Table 2: The average photon energy, low energy photon ratio, electron contamination ratio and the entrance dose for 
all configurations. 
 FF FFF Cu(0.5 cm) Cu(0.25 cm) C(1 cm) C(2 cm) 
Average photon energy (MeV) 2.5 2.1 1.9 1.8 1.6 1.2 
10-150keV photons ratio(%) 0.33 1.03 2.39 3.55 6.06 5.16 
Electron contamination ratio (%) 0.09 0.25 0.06 6.17 41.07 0.03 
Entrance dose (%) 65.7 84.4 72.6 90.2 84.4 72.9 
 
           
(a)                                                                                                        (b) 
Figure 5: The acquired images of two cylindrical bones with 2 cm diameter placed inside water phantom with 5 cm 
thickness at 100 cm from the target by (a) the conventional beam and (b) the copper target beam. 


Improving MVCBCT image quality using a Cu target with flattening filter-free LINAC

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Improving MVCBCT image quality using a Cu target with flattening filter-free LINAC

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