=Paper=
{{Paper
|id=Vol-2005/paper-18
|storemode=property
|title=Isocenter and Field of View Accuracy Measurement Software for Linear Accelerator
|pdfUrl=https://ceur-ws.org/Vol-2005/paper-18.pdf
|volume=Vol-2005
|authors=Aleksei E. Zhdanov,Leonid G. Dorosinskiy
}}
==Isocenter and Field of View Accuracy Measurement Software for Linear Accelerator==
https://ceur-ws.org/Vol-2005/paper-18.pdf
Isocenter and Field of View Accuracy
Measurement Software for Linear Accelerator
Aleksei E. Zhdanov1 and Leonid G. Dorosinskiy1
Ural Federal University named after the first President of Russia B. N. Yeltsin, Mira
19, 620002 Yekaterinburg, Russia
jjj1994@yandex.ru, l.dorosinsky@mail.ru
Abstract. For radiotherapy, Linear Accelerator (Linac) quality assur-
ance is necessary to provide precise and accurate radiation treatment.
Besides treatment plans and patient positioning, machine status is ano-
ther vital issue that affects radiotherapy. In this paper, we implemented
and discussed the Isocenter and Field of View Accuracy Measurement
Software for Linac. All data and images were taken by Brainlab Vero
Linac of Erlangen University Clinic.
Keywords: Image processing, linear accelerator, quality assurance pro-
gram, star-shot analyzing algorithm
1 Introduction
In general, quality assurance means all planned or systematic actions for guar-
anteing the given requirements. In radiotherapy, quality assurance refers to all
procedures that ensure consistency of medical prescriptions and practical fulfill-
ment [1].
According to radiation therapy workflow, treatment planning system TPS
plans, the treatment details based on assumption of real Linac parameters; also,
during irradiation, patient positions are compared to radiation isocenter for pa-
tient positioning. For accuracy considerations, consistency of treatment planning
assumption, patient positioning, and actual machine status are critical, which
also means that regular assurance of machine quality is necessary.
Besides dosimetric accuracy, the machine quality assurance can also increase
the probability of recognition and rectification accidents or fault in case when
they do occur. Minor incidents can occur anytime, as well as by prompt recogni-
tion, we can modify sequential fractions and, thus, reduce overall consequences.
The objective of machine QA is to ensure exposure of normal tissue during
irradiation be kept ALARA (as low as reasonably achieved), so as to integrate
patient safety by reducing machine uncertainty.
Usually, quality assurance includes a certain types of measurements: isocen-
ter accuracy, field of view (FOV) accuracy, multileaf collimator MLC leakage,
dynamic MLC banks position, leaf position, one picket test, and MLC leakage
ratio. The first two types of measurements are the most important for delivering
a certain radiation dose to the object.
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2 Method and Material
2.1 Film
In this article, we use EDR2 (Extended Dose Range) film for evaluation of dose
exposure. The EDR2 film developes in case exposed in normal environment,
thus, during measurement, the film should be covered by opaque paper, and in
this article, we make use of such property for field reference. Before irradiation,
we puncture reference point (light field vertex or longitudinal direction) with
needle on cover, so as to develop a small dot for reference for the analysis.
The EDR2 film needs developing procedure before analysis, and we have
developed the film by automatic developing device, but considering average cost,
it is quite worth to substitute it by instant self-developing for the EDR2 film.
2.2 Electronic Portal Imaging Device (EPID)
EPID is used for several processes. For example, patient positioning, verifica-
tion and dosimetric verification of IMRT, and Linac quality assurance. Quality
assurance is based on EPID and offers high spatial resolution, fast image acqui-
sition, and digital output. It is considered as an accurate tool for both patient
and machine QA in radiation therapy [2].
2.3 Isocenter Accuracy Measurement
In this article, we measure the accuracy of beam isocenter with rotation of gantry
and rings. We set the MLC window width to be identical 10 mm and irradiate
from different direction and compare the area of isocenter sphere with expected
value [3].
Isocenter sphere for gantry-rotation. We clamp the film between two plastic
blocks to fix and place it corresponding to the expected beam isocenter by laser
(Fig. 1). Then we irradiate with the MLC window size of 10 × 50 mm2 and from
gantry angle of 0, 70, 140, 210, 280, and 350 degree. After that, film is scanned
and analyzed.
Isocenter sphere for ring-rotation. Similarly to the gantry-rotation test, we
make the film positioning direction towards the beam portal. We change the
MLC window size to 10 × 150 mm2 and irradiate from different ring angles.
Film is also scanned for the analysis step.
2.4 FOV Accuracy Measurement
In this test, we measure correlation of the light-field, as well as the irradiation
field and we choose 50 × 50 mm2 and 100 × 100 mm2 field for test. First, we
place the film towards beam portal and puncture four corners corresponding to
the light-field on film, then we change the MLC window size to the same and
irradiate with dose of 300MU. The film is later analyzed by scanner after being
developed.
157
Fig. 1. Film setting for isocenter sphere measurement (gantry)
3 Result and Analysis
3.1 Isocenter Accuracy Measurement
Figure 2 shows the initial image by scanning the film. We use computer-aided
image analysis based on MATLAB [4] for measuring isocenter sphere, which
consists of three main steps.
1. Enhancing image contrast (Fig. 3a).
2. Fitting central axis of each strip (Fig. 3b).
3. Calculating smallest hitting circle based on line configuration of central axis
(Fig. 3c and Fig. 3d).
Based on the smallest hitting circle coming from algorithm in Fig. 3d, we have
calculated the diameters of isocenter sphere are 1.56 mm for gantry rotation and
0.54 mm for ring rotation.
3.2 FOV Accuracy Measurement
Figure 4a shows original image after scanning the both the film and Fig. 4b, as
well as Fig. 4c, shows the light-field and irradiation field that we extract from
the initial scanning image. We have detected the field corners and borders, as
shown in Fig. 5.
By analyzing Fig. 5a and Fig. 5b, the developed program measures the border
length of field and displays the results of calculations in the table, as shown in
Table 1.
4 Conclusion
In isocenter accuracy assurance test, diameter of the gantry-rotation isocenter
sphere is 13% excessive to expected value and in the ring-rotation isocenter
158
sphere is 27% excessive. This implies slight offset of beam isocenter from gantry-
rotation isocenter and ring-rotation isocenter. In the field correlation test, dif-
ference of edge length of actual irradiation field and light-field are within 2%,
so, we can conclude that irradiation field conforms with to the light-field quite
well.
Thus, in the paper, we have implemented and discussed the Linac quality
assurance software of two important parameters.
Fig. 2. Films of isocenter accuracy measurement (gantry-rotation)
159
Table 1. FOV detection result
Vertex coordinate (cm) Border length (cm)
Light field (50 × 50mm2 ) Light field (50 × 50mm2 ) Irradiation field (50 × 50mm2 )
(7.92, 5.48),
(12.79, 5.41), 4.87, 4.96
5.02, 5.05
(7.85, 10.45), 5.04, 5.03
(12.90, 10.45)
Light field (100 × 100mm2 ) Light field (100 × 100mm2 ) Irradiation field (100 × 100mm2 )
(5.28, 14.41),
(15.23, 14.33), 9.95, 9.83
10.14, 10.13
(5.33, 24.24), 9.91, 9.86
(15.24, 24.19)
a) b)
c) d)
Fig. 3. Film Isocenter circle of gantry rotation after image processing; a) contrast
enhancing, b) fitting central axis, c) the smallest hitting circle, d) the smallest hitting
circle (zoom-in)
160
a)
b)
c)
Fig. 4. Correlation of the light-field and irrradiation field; a) original film, b) extracted
corner of light-field, c) extracted area of irradiation field
161
b)
a)
Fig. 5. Features detected from initial image: a) detected irradiation field borders, b)
detected light-field corners
5 Acknowledgements
All data and images were taken by Brainlab Vero Linac of Erlangen Univer-
sity Clinic. The experiments, which is shown in this paper were supported by
Prof. Dr. rer. nat. Christoph Bert of Friedrich-Alexander University Erlangen-
Nurnberg, Erlangen.
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