Response of electric portal imaging device to energy
spectrum of therapeutic photons
Poster No.:
C-1432
Congress:
ECR 2014
Type:
Scientific Exhibit
Authors:
H. Takei , Y. Watanabe , T. Isobe , K. Takada , N. Shigematsu ,
1
2
2
2
3
4
5
1
2 1
E. Sato , H. Hara , H. Muraishi , T. Hasegawa ; Tokyo/JP,
2
3
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Sagamihara/JP, Tsukuba, Ibaraki, JP/JP, Tsukuba/JP,
5
Sagamihara Minami-ku/JP
Keywords:
Quality assurance, Image guided radiotherapy, Radiation therapy /
Oncology, Physics, Dosimetry, Digital radiography, Radiation
physics
DOI:
10.1594/ecr2014/C-1432
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Aims and objectives
The electric portal imaging device (EPID) is now being used for in vivo dosimetry. The
fluence of photons that penetrate the human body is calculated from the pixel value of
the EPID image. The pixel value is converted from the absorbed dose in a scintillation
layer. This dose depends on the energy spectrum of therapeutic photons [1], so the pixel
value of the EPID image is also affected by the energy spectrum. The dependency of the
absorbed dose to the energy spectrum was evaluated to account for the change in the
energy spectrum to provide an accurate calculation of in vivo dosimetry.
Methods and materials
A linear accelerator (Trilogy, Varian) and an EPID (aS1000, Varian) at the University of
Tsukuba Hospital were used: the structure of the EPID is shown in Fig. 1. The aS1000
consists of a1mm copper plate overlying a scintillating layer of gadolinium oxysulphide
phosphor screen, and a 40×30 cm2 (1024×768 pixel) amorphous silicon array. The
human body was simulated by an inhomogeneous phantom consisting of water and bone
2
equivalent materials. The phantom had a 40×40 cm surface and a 20 cm depth and
consisted of a sandwich structure with 5 cm thick bone equivalent material at the center
and 5 cm thick water equivalent materials on both upstream and downstream ends. A
3
2
40×40×20 cm solid water phantom was used for a comparison. A 10×10 cm area of the
phantoms was irradiated with 6 MV photons and EPID images were acquired. The Monte
Carlo Simulation (MC; Geant4.9.4 patch04) was used to evaluate the dose absorbed in
the scintillation layer per detected photon (Ed) as a function of the energy of the incident
photon on the surface of the EPID (Ei). An agreement of the EPID images between the
measured and the MC was confirmed in our previous study [2]. The pixel value of the
EPID image was considered as a conversion from Ed. Ratios of Ed were compared for the
inhomogeneous phantom and the solid water phantom (Ed,i/Ed,w) or without a phantom
(Ed,i/Ed,0). Ed,i, Ed,w and Ed,0 were normalized by the number of detected photons.
Page 3 of 8
Fig. 1: A structure of the EPID (aS1000, Varian).
References: Oncology Center, Keio University Hospital - Tokyo/JP
Results
Fig. 2 shows the energy spectra of the incident photons on the surface of the EPID
without a phantom, with the solid water phantom, and with the inhomogeneous phantom.
The spectra are normalized so that the highest peak represents 100%. The spectra with
phantoms showed shifts in the peak to higher energy because photons with low energy
are absorbed by phantom more than those with higher energy that is represented as the
linear attenuation coefficient up to the energy of therapeutic photons. The values of Ed
as a function of Ei and the detection efficiency as a function of Ei are shown in Fig. 3
and Fig. 4. Ed,i/Ed,w and Ed,i/Ed,0 were 0.980±0.008 and 0.810±0.008 respectively. Ed is
associated with an amplitude of the EPID signal per photon while the detection efficiency
has a linear relationships with the number of photons detected. The response of EPID
to the energy spectrum is a combination of these characteristics. Significant changes
were seen in the energy spectrum between photons with and without phantoms when
Ed,i decreased by 19% compared to Ed,0. The energy spectrum needs to be considered
for accurate calculation for in vivo dosimetry since it affects the pixel value of the EPID
image to an extent that cannot be ignored.
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Fig. 2: Energy spectra of the incident photons without a phantom (solid line), with a
solid water phantom (dashed line) and with an inhomogeneous phantom (dotted line)
on the surface of the EPID.
References: Oncology Center, Keio University Hospital - Tokyo/JP
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Fig. 3: Energy deposit in the scintillating screen per photon as a function of the
incident photon energy on the surface of the EPID.
References: Oncology Center, Keio University Hospital - Tokyo/JP
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Fig. 4: Detection efficiency of the EPID as a function of the incident photon energy on
the surface of the EPID.
References: Oncology Center, Keio University Hospital - Tokyo/JP
Conclusion
The response of the EPID to the energy spectrum of therapeutic photons was evaluated
for an accurate calculation for in vivo dosimetry.
Personal information
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Hideyuki Takei, Ph.D
Oncology Center, Keio University Hospital, Tokyo, Japan
E-mail: [email protected]
References
[1] C. Kirkby and R. Sloboda: Med. Phys. 32 (8), 2649-2658, 2005.
[2] H. Takei, Y. Watanabe, et al: Japanese Journal of Medical Physics Vol. 32 Supplement
No. 3, 287-288, 2012.
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