Development of a computer-based daily QA tool using an electronic portal imaging device

unkindnesskindUrban and Civil

Nov 15, 2013 (3 years and 9 months ago)

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Development of a computer
-
based daily QA tool using an
electronic portal imaging device

S Lim
1
,2
,
B Yi

1
, S Ahn

1
, J Kim

1
, S Lee

1
, S Shin

1
, S Kwon
2
,
and E Choi

1

1
Department of Radiation Oncology, Asan Medical Center, University of Ulsan College of M
edicine, Korea

2
Department of
Medical
Physics,
Kyonggi

University, Korea


Abstract

Dosimetric properties of an amorphous silicon electronic portal imaging device (EPID) for the computer
-
based routine quality
assurance (QA) of the linac were developed
in
this study
. A phantom consists of acrylic plate was placed on the EPID and it aligned
to the light fields of the collimator. Every corner of the field square of the phantom, the lead wire
embedded

parallel to the beam
direction in order to mark the referen
ce field size of the portal image. The
portal image
s

from the EPID transferred
as a DICOM
format to the computer and it were analysed by the software, developed in our department. The
symmetry
, the energy

and the field
size of the beams from the linac coul
d be verified at once. To assure the QA system, the ion
-
chamber and the films (X
-
Omat V2,
Kodak, USA) were used. The
verification

performed everyday. The discrepancy between the measurements of the EPID agrees well
with the measurements of the film. It was

found that this QA tool using the EPID could be substituted the film test which is time
-
consuming for the daily routine QA.

Keywords

computer
-
based, daily QA
,
portal image, EPID



Introduction

C
o
nventional clinical practice in radiotherapy involves
align
ing the treatment unit light field with skin marks on the
patient as the final confirmation that the patient is correctly
positioned with respect to the radiation beam.[1] The
assumption is that the light and radiation fields are congruent.
To verify that
the light and the radiation fields agree with each
other and the radiation beam constancy, the x
-
ray film is
conventionally used.[2]

The verification is, however, time
-
consuming involving film processing and digitization, and
comparison to the electronic p
ortal imaging device (EPID).
Frequent film calibraion and processor quality assurance is also
neccessary.[3] The developement of EPID has made it a
popular tool for on
-
line treatment verification and transmission
dosimetry measurement. The treatment qualit
y assurance (QA)
systems using EPIDs have been studied by many groups for
static fields and dynamic fields.[4
-
11]

The purpose of this study is to develope a daily QA tool
using an EPID. In this study the dosimetric properties of an
amorphous silicon EPID

for verification of daily beam
constancy were investigated and a convenient software tool
was developed for performing the task. These properties
included the beam symmetry, the field size error, and the light
and the radiation fields congruence. The vari
ation of the beam
energy spectrum is also studied by measuring the dose with
different amounts of attenuation. Additionally the wedged
beams were verified using the software against an established
reference wedged beams. Results of this verifications are n
ot
presented in this paper.



Material and methods

Phantom design


A phantom was designed 25
×
25

cm
2

size with 2 mm
thick of arcrylic plate and underneath the plate four 3.1 cm
long pillars were used for sustaining the plate with a gap of 5
cm from the EPI
D.

The lead wires were embeded parallel to
the beam directions with considering of the divergence at
every corners of the light field squares, which would be
shown in the image as points for light field congruence. The
isocenter is on the plate of the phan
tom. This daily QA tool
were designed to verify the beam with field size of 20
×
20
cm
2
, but 10
×
10 cm
2

and 16
×
16 cm
2

are also available as
occasion demands. It was designed that each light fields
could be aligned with the scale on top of the phantom as
shown

figure 1. To verify the constancy of the energy of the
beam, a several pieces of lead attenuators were stacked on
the phantom with thickness of 10 cm water equibalent for
the beams. The EPID (PortalVision aS500, Varian, Palo Alto,
CA) that was used in thi
s study has a resolution of 512
×
384
pixel and a pixel pitch of 0.78
×
0.78 mm2.[12] The EPID
was positioned at SSD 105 cm.


Figure 1:

Illustration of the daily QA phantom on the EPID
.

The
phantom is positioned at SSD 100 cm with sustaining by four
pillars and the EPID positioned at SSD105 cm. The light field could
be aligned with scale on the phantom.


Daily QA Software


The software to verify the images from the EPID was
developed in this study using the IDL (Research Systems, USA,
v
ersion 6.0). After the irradiation, the images from the EPID
were transfered to the local computer as a DICOM format. The
software designed to analyze the filed size, the field symmetry,
and the constancy of a beam energy at one time. Additionally it
was d
esigned to verify the wedged beam for all photon
energies. The algorithm in the software made it possible to
find the edge and the center of the irradiated field in the image
and align the image. The various attenuation blocks on the
phantom were used to
verify the variation od the beam energy
spectrum.


Verification


To check the daily constancy of the field sizes and the
symmetry of the beam, 6 MV and 15 MV photon beams with
10
×
10 cm
2
, 16
×
16 cm
2
, and 20
×
20 cm
2

square fields were
irradiated to the phantom

everyday. The portal images
extracted to the DICOM format, which were analysed by the
software. The markers of lead wires in the phantom represent
the field size of the light. The program compares the field size
for light field and for irradiated field a
utomatically. To assure
the accracy of this QA tool, ion
-
chamber and films (X
-
Omat
V
-
2, Kodak, USA) were used. The profiles for this study were
compared to that of films. The constancy of the energies could
be verified at the same time. The beams that thro
ugh the
attenuator which made of lead were compared.
Non
-
uniform
beams such as wedged beams also could be verified with this
QA tool.




Results and discussion

The edge of the radiation field was automatically detected by
the algorithm in the software. And

it find the center of the field
and display the center and edge on the image. The light and the
radiation fields congruence, the actual field size, the field size
error, the beam symmetry, and the energy spectrums are
calculated and the result is display
ed on the right side of the
software window (figure2). In case of the field size is too small
to cover the attenuators for energy spectrum assessment, the
error message will be appeared in the message box.

Figure 3 shows profiles of 10
×
1
0 cm
2

field, ir
radiated to 6
MV photon beam which aquired by EPID and the films The
agreement between the EPID profiles and film measurements
was within 2% for the square field. The daily variation of the
beam symmetry and the actual field size were shown table 1 .
These

results show that the EPID could be a useful device for
verification of static field delivery, and for quality assurance
purpose such as the beam profile constancy, the symmetry, and
the energy. The memory effect that is caused by latent energy
in the gad
olinium oxysulphide phosphor for the previous
irradiations is less than 0.2%.[12] With the interval between
image acquisition in this study, the effect is not so significant
that could be ignored.



Figure
2
:

The user interface of the Daily QA software developed in Asan medical center
.

The symmetry errors, the actual field size for x
and y axis, the variation of the energy of the beam, and the light and the radiation congruence are displayed by one c
lick.

0
50
100
-10
-5
0
5
10
O ff Axis (cm )
Relative dose (%)
Film
this study
Figure 3:

Relative dose profil
es of 6 MV photon beams with
10×10cm2 rectangular fields measured by film and EPID. The solid
line is the profile of film scan and the dotted line is from this study.
The curves are normalized at each center of the profiles.




6 MV

Symmetry (%)

Field Si
ze (cm)

X

Y

X

Y

Day 1

-

0.05


0.53

20.5

20.5

Day 2

1.23

-

1.73

20.5

20.5

Day 3


-

0.39

-

0.93

20.5

20.2

Day 4

-

0.33

-

0.72

20.4

20.3

Day 5

1.00

-

1.30

19.9

19.6


15 MV

Symmetry (%)

Field Size (cm)

Day 1

-

0.27

-
.020

20.
3

20.4

Day 2

-

0.32

-

0
.63

20.5

20.2

Day 3

-

0.34


-

0.86

20.5

20.2

Day 4

0.58

-

0.84

19.9

19.9

Day 5

0.15

-

0.79




Table 1:

Symmetry errors and actual filed sizes for 6 MV and 15 MV
of photon beams with 20×20 cm2 fiels size.




Conclusion

Setup and image acquisition time f
or all photon energies was
less than ten minutes. The image analizing time was less than
one minute. The software developed in this study made it easy
to verify the symmetry, the field size, and the energy of the
beam in a minute. In this study the daily Q
A tool with an EPID
was an efficient daily QA tool.






References


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and
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199
9

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