Filmless Evaluation of the Mechanical Accuracy of the Isocenter in Stereotactic Radiotherapy

Abstract

The Winston-Lutz test verifies the mechanical accuracy of the isocenter in stereotactic radiotherapy. A lead ball inside a small beam is exposed to film applying different combinations of the gantry angle and the table angle. The increasing replacement of films by digital images requires alternative imaging methods. The suitability of two different electronic portal imaging systems and of a system based on digital luminescence radiography was investigated.
The imaging systems included the portal imaging devices BEAMVIEW PLUS and OPTIVUE1000 (both Siemens Medical Solutions, Erlangen, Germany) and the luminescence system KODAK ACR 2000 RT (Eastman Kodak Comp., Rochester, NY, USA). 6-MV photons from the linear accelerators PRIMUS and ONCOR (both Siemens Medical Solutions) were applied. First, only the small beam covering the lead ball was exposed. Second, an additional bigger open beam part in a certain distance to the small beam was applied.
For all three investigated imaging systems, which are using preprocessing imaging software, only for the beam arrangement with additional open beam parts, the lead ball could be detected inside the small beam. Only for the application of a dosimetric software tool to the luminescence system, the metal ball inside the small beam became visible without an additional open beam part.
Applying the proposed beam arrangements, the Winston-Lutz test can be done by digital and filmless imaging systems, thereby saving time as well.

Full text

76 Strahlenther Onkol 2007 · No. 2 © Urban & Vogel
Strahlentherapie
und Onkologie
Original Article
Filmless Evaluation of the Mechanical Accuracy
of the Isocenter in Stereotactic Radiotherapy
Peter Geyer, Hilbert Blank, Carsten Evers, Thomas Leichtner, Horst Alheit1
Background and Purpose: The Winston-Lutz test verifies the mechanical accuracy of the isocenter in stereotactic radiotherapy.
A lead ball inside a small beam is exposed to film applying different combinations of the gantry angle and the table angle. The
increasing replacement of films by digital images requires alternative imaging methods. The suitability of two different elec-
tronic portal imaging systems and of a system based on digital luminescence radiography was investigated.
Material and Methods: The imaging systems included the portal imaging devices BEAMVIEW PLUS® and OPTIVUE®1000 (both
Siemens Medical Solutions, Erlangen, Germany) and the luminescence system KODAK ACR 2000 RT (Eastman Kodak Comp., Roch-
ester, NY, USA). 6-MV photons from the linear accelerators PRIMUS® and ONCOR® (both Siemens Medical Solutions) were applied.
First, only the small beam covering the lead ball was exposed. Second, an additional bigger open beam part in a certain distance
to the small beam was applied.
Results: For all three investigated imaging systems, which are using preprocessing imaging software, only for the beam arrange-
ment with additional open beam parts, the lead ball could be detected inside the small beam. Only for the application of a dosi-
metric software tool to the luminescence system, the metal ball inside the small beam became visible without an additional open
beam part.
Conclusion: Applying the proposed beam arrangements, the Winston-Lutz test can be done by digital and filmless imaging sys-
tems, thereby saving time as well.
Key Words: Stereotactic radiotherapy · Quality assurance · Storage phosphor plate · Electronic portal imaging device
Strahlenther Onkol 2007;183:76–80
DOI 10.1007/s00066-007-1659-4
Filmlose Bestimmung der mechanischen Isozentrumsgenauigkeit in der stereotaktischen Strahlentherapie
Hintergrund und Ziel: Der Winston-Lutz-Test verifiziert die mechanische Genauigkeit des Isozentrums in der stereotaktischen
Strahlentherapie. Abbildungen einer Bleikugel innerhalb eines kleinen Bestrahlungsfeldes erfolgen für verschiedene Gantry- und
Tischwinkelkombinationen auf Film. Die zunehmende Abkehr von der Filmanwendung erfordert den Einsatz alternativer bildge-
bender Verfahren für diesen Test. Die Eignung zweier verschiedener elektronischer Portal-Imaging-Systeme und eines Systems der
digitalen Lumineszenzradiographie wurde untersucht.
Material und Methodik: Als bildgebende Systeme wurden die Portal-Imaging-Systeme BEAMVIEW PLUS® und OPTIVUE®1000
(beide Siemens Medical Solutions, Erlangen) und das Lumineszenzsystem KODAK ACR 2000 RT (Eastman Kodak Comp., Rochester,
NY, USA) eingesetzt. Die Bestrahlung erfolgte mit 6-MV-Photonen an den Linearbeschleunigern PRIMUS® und ONCOR® (beide
Siemens Medical Solutions). Die Belichtungen für die untersuchten Systeme erfolgten zum einen nur mit dem sehr kleinen Feld,
das die abzubildende Metallkugel enthält. Zum anderen wurden in räumlicher Trennung zu diesem kleinen Feld zusätzliche und
deutlich größere Feldanteile aufbelichtet.
Ergebnisse: Für die drei untersuchten bildgebenden Systeme und die Bildsoftware mit unbeeinflussbarer Vorbearbeitung war nur
bei den Feldanordnungen mit zusätzlichen offenen Feldbereichen eine Detektion der Kugel innerhalb des umgebenden kleinen
Feldes möglich. Allein eine Dosimetriesoftware für das System der Lumineszenzradiographie erlaubte die Erkennung der Kugel
innerhalb des kleinen Feldes ohne zusätzliche offene Feldteile.
Schlussfolgerung: Die vorgeschlagenen Feldanordnungen erlauben die Durchführung des Winston-Lutz-Tests mit digitalen, film-
losen Bildsystemen. Damit kann der Test auch in kürzerer Zeit durchgeführt werden.
Schlüsselwörter: Stereotaktische Strahlentherapie · Qualitätssicherung · Speicherfolie · Elektronisches Portal-Imaging-
System
Received: August 9, 2006; accepted: December 6, 2006
1 Department of Radiotherapy and Radiooncology, University Hospital Carl Gustav Carus, University of Technology, Dresden, Germany.

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Introduction
Stereotactic irradiation techniques based on linear accelera-
tors are widely established for intracranial [6, 8, 9, 15, 19, 21,
22, 27] and extracranial lesions [28, 36], also including intensi-
ty-modulated radiotherapy (IMRT) [36]. This high-precision
therapy requires a stringent and extensive quality assurance.
The evaluation of the geometric accuracy of the isocenter is
mostly based on films [10, 18, 31, 33, 34], though other meth-
ods like a laser application are described [14]. A common test
for a fast evaluation of an overall isocenter accuracy based on
film is known as the Winston-Lutz test or joint-center test.
Here, a metal ball is positioned at the isocenter and a film be-
hind the ball is exposed by a small beam, using different com-
binations of the angles for gantry and treatment table [18, 26,
30]. The small beam is larger than the metal ball. If the ball is
imaged inside the beam for all exposures, then the difference
between the beam size and the ball diameter is a measure for
the mechanical accuracy of the isocenter.
Due to the replacement of films by digital imaging sys-
tems there is a need for adapting the test procedures. Such al-
ternative techniques are electronic portal imaging devices [1,
3, 5, 7, 11, 20, 23, 35] or the digital luminescence radiography
[2, 4, 13, 16, 17, 32, 37].
The present study investigates the suitability of these two
imaging techniques by applying them to the Winston-Lutz test.
Material and Methods
The Winston-Lutz test of our department uses a lead ball of
3 mm diameter and X-OMAT V XV2® film (Eastman Kodak
Comp., Rochester, NY, USA) applying either a circular beam
of 5 mm diameter or a square beam of (6 × 6) mm2. The beams
are shaped by a conical collimator or by the micro-multileaf
collimator (micro-MLC) M3 (both devices manufactured by
BrainLAB AG, Heimstetten, Germany). These devices were
mounted at the linear accelerators PRIMUS® and ONCOR®
(both Siemens Medical Solutions, Erlangen, Germany). The
basic version of the test includes eight combinations of table
and gantry angles: table angle 0° with gantry angles 0°, 90°, 180°
and 270°, table angle 90° and 270° with gantry angles 0° and
180°. The collimator angle is set to 0° except for the application
of the micro-MLC and a table angle of 0°. Here, the collimator
angle has to be set to 90° due to the mounting position of the
film holder at the micro-MLC. The exposure is done by 6-MV
photons applying 100 monitor units (MU) per image to the
film. The exposed film is evaluated manually, thereby applying
the criterion, whether the ball is completely located inside the
small beam or not. To get quantitative results on the location
and accuracy of the isocenter, other tests are used in our de-
partment [12]. The Winston-Lutz test is done before a radio-
surgery treatment or before a fractionated stereotactic irradia-
tion with remarkable high single doses (≥ 10 Gy).
First, the film was replaced by storage phosphor plates of
the system KODAK ACR 2000 RT (Eastman Kodak Comp.),
that is destined for digital luminescence radiography in radio-
therapy [13, 24]. For the presented investigations only the
“portal scan” mode and the highest scanning resolution of
2,048 pixel/line were applied. Prior to the irradiation, the stor-
age screen of the size (24 × 30) cm2 was inserted into a light
tight envelope and mounted in the film holder. Two different
software components were used for image processing, view-
ing, and evaluation. The first component was the Kodak Ra-
diation Oncology Software (version 5.0, Eastman Kodak
Comp.), that includes an operator-independent preprocessing
[13, 24]. Further, the Kodak Radiation Oncology Beam Do-
simetry Package (version 1.0, Eastman Kodak Comp.) was ap-
plied to the images. This tool is destined for two-dimensional
dosimetric purposes and does not show any inherent prepro-
cessing [25, 29]. This means that the signal (expressed in
counts) of a pixel at the storage foil keeps its dependence on
the exposed dose value to this pixel. The images were dis-
played at a 10-bit gray scale monitor (KODAK Direct View
2MP Monochrome display, Model #DV2MM, Eastman Ko-
dak Comp.). The eight combinations of gantry and table angle
were exposed to one plate applying 2 MU per beam. To evalu-
ate the processing of the exposed plates, every other plate was
finally irradiated with an additional beam of a bigger size and
1 MU. This additional beam was either shaped by the mi-
cro-MLC with a (3 × 3) cm2 size or by a conical collimator with
4 cm diameter and was positioned near the small side of the
plate (Figure 1). Only for the image evaluation by the Dosim-
Figure 1. Detail of a storage phosphor plate image of the Winston-Lutz
test, which contains four combinations of gantry and table angles.
The image was processed by the Kodak Radiation Oncology Software.
The image shows the four (6 × 6) mm2 fields with the lead sphere in-
side (at the left and the right edges of the image) and the additional
(3 × 3) cm2 beam at the top of the image.
Abbildung 1. Ausschnitt eines Speicherfolienbildes mit vier Kombina-
tionen aus Gantry- und Tischwinkeln des Winston-Lutz-Tests. Die Bild-
bearbeitung erfolgte mit der Kodak Radiation Oncology Software. Das
Bild zeigt die vier (6 × 6)-mm2-Felder, die die Bleikugel enthalten (an
der rechten und linken Bildseite), und das zusätzliche (3 × 3)-cm2-Feld
an der Bildoberkante.

Geyer P, et al. Filmless Evaluation of Stereotactic Isocenter Accuracy
78 Strahlenther Onkol 2007 · No. 2 © Urban & Vogel
etry Package, different values of 2, 5 and 10 MU were applied
to the (6 × 6) mm2 beam.
Second, the film application was replaced by two different
portal imaging systems. The OPTIVUE®1000 (Siemens Medi-
cal Solutions) is mounted at the linear accelerator ONCOR®.
This imaging system uses a high-resolution amorphous silicon
detector with an active area of (40 × 40) cm2, a resolution of
1,024 pixel in both directions, and a pixel depth of 16 bit. This
flat panel was operated by the software COHERENCE Ther-
apist Workspace (version 1.0.657, Siemens Medical Solutions).
The second investigated portal imaging system was the
BEAMVIEW PLUS® TI (version 2.2, Siemens Medical Solu-
tions) that is attached to the accelerator PRIMUS® [11]. This
portal imaging device contains a fluorescent intensifying
screen viewed by a CCD camera. The evaluation of these por-
tal imaging systems was performed only by applying the mi-
cro-MLC to the beams of the Winston-Lutz test. Again, two
beam arrangements were compared. First, only the small
beam of (6 × 6) mm2, located at the central axis of the beam,
was exposed. Second, this small beam was accompanied by
two bigger openings in the micro-MLC beam setup. Each of
these two rectangular openings had a size of about (2 × 5) cm2
and its border was separated from the small square beam edge
by 2.25 cm. One of the rectangular openings was located left
from the small central square and between the central axis
plane and the maximum micro-MLC opening toward the gan-
try. The other rectangular opening was positioned right from
the central square and between the central axis plane and the
maximum beam border of the micro-MLC directed away from
the gantry (Figure 2). Each micro-MLC beam arrangement
was exposed with 3 MU. Neither this monitor unit value nor
the beam shapes were varied. The variation of the monitor
units is limited by the COHERENCE software to values be-
tween 1–3.
The primary jaws of the linear accelerator were set to (5 ×
5) cm2 for the conical collimator and to (9.4 × 9.4) cm2 for the
micro-MLC.
The time for the application of the test with the different
image detectors was also measured.
After their evaluation at the monitors, the processed
images obtained by the storage foil and the portal imaging
systems were printed on paper for documentation and stored
in the PACS of the department. Both types of images were
stored as raw data (before processing by the operator) as well
as processed data.
Results
For both investigated electronic portal imaging devices, the
lead ball inside the circular or square field could be evaluated
only if additional larger fields were exposed (Figure 2). The
same was the case if the storage foil system KODAK ACR
2000 RT in combination with the preprocessing Kodak Radia-
tion Oncology Software was used (Figure 1). Only if this sys-
tem was applied in combination with the Kodak Radiation
Oncology Beam Dosimetry Package, the lead ball was visible
without the exposure of additional open beams.
In all other images of only the small beam, the brightness
of this opening was increased during the primary and observ-
er-independent processing in such a way, that the lead ball
became completely invisible.
The shape of the square beam, defined by the micro-MLC,
changed at the images slightly into a rectangle (Figures 1 and
2). This might mainly be caused by the single focusing type of
the MLC with rounded leaf edges. This MLC design results in
different penumbra values for the direction perpendicular to
the leaves and for the direction parallel to the leaves. The ratio
of the beam length parallel to the leaves to the beam length
rectangular to the leaves (the direction with the steeper pen-
umbra region) was influenced by the imaging device. It changed
from about 1.1 (film), 1.15 (storage foil evaluated with Kodak
Radiation Oncology Software, amorphous silicon panel), to
1.25 (storage foil evaluated with Kodak Radiation Oncology
Beam Dosimetry Package). The exposures with different mon-
itor units in the case of the evaluation of the storage screen with
the Kodak Dosimetry Package resulted in a linear increase of
both beam diameters with increasing monitor unit values. The
beam length for 10 MU was 10–20% larger than for 2 MU. The
Figure 2. Detail of an OPTIVUE®1000 image of one combination of
gantry and table angles of the Winston-Lutz test. The image shows
the (6 × 6) mm2 field with the lead sphere inside (in the center of the
image) and parts of the two additional open beams at the left and the
right edges of the image.
Abbildung 2. Ausschnitt eines OPTIVUE®1000-Bildes einer Kombinati-
on von Gantry- und Tischwinkel des Winston-Lutz-Tests. Das Bild zeigt
das (6 × 6)-mm2-Feld, das die Bleikugel enthält (in der Bildmitte), und
Teile der zusätzlichen offenen Felder an der rechten und linken Bild-
seite.

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79
Strahlenther Onkol 2007 · No. 2 © Urban & Vogel
span of this increase was due to a different processing by the
operator. Despite these findings, a qualitative evaluation of
the images became possible if the processing by the operator
avoided a remarkable deformation or distortion of the small
beams, shaped by the micro-MLC or the conical collimator.
The resulting parameters of center and width were about 2,700
and 1,500 for the ACR system, using the Kodak Radiation
Oncology Software. In the case of the portal imaging system
OPTIVUE®1000 values of center and width of about 15,000
(span 9,000–25,000) and 60,000 (span 43,000–63,000) were
found to be optimal. Changing an image from the normal gray
scale to the reverse gray scale (black and white inversion) im-
proved the visibility of the metal ball in some cases.
The comparison of the image quality based on the used
detectors was quite difficult due to the different viewing con-
ditions (without a monitor in the case of film or with different
monitors for storage foil and portal imaging) and the wide
range of window level settings in case of the latter mentioned
detectors. However, it was estimated that the film showed the
best image quality, followed by the storage screen, the amor-
phous silicon detector, and the conventional portal imaging
system.
The application of the portal imaging system for the
Winston-Lutz test does not require a film holder. Therefore,
limitations of the collimator angle or collisions due to the
film holder are avoided. The possibility of a collision between
the portal imaging device and the treatment table has to be
considered.
The preparation for the test, including the adjustment of
the metal ball with respect to the room lasers, took about 10
min, independent of the imaging detector. The eight expo-
sures, including four different gantry angles and three differ-
ent table rotations, took 20 min using film, 17 min for the stor-
age foil, and only 10 min for the portal imaging systems. The
minor time saving in case of the storage screen compared to
the film is due to the reduced exposure time (2 MU instead of
100). The portal imaging systems showed a remarkable
time-saving effect because shifting the film or the screen in the
film holder and mounting and taking apart the film holder
during exposures became unnecessary.
Discussion
A method is presented that applies the digital luminescence
radiography and electronic portal imaging devices to the
Winston-Lutz test in the field of stereotactic irradiation. The
electronic portal imaging devices included a system based on a
fluorescent intensifying screen as well as an amorphous silicon
panel. The proposed method is based on additional open
beams either at the storage phosphor plate or in the beam ex-
posed to the portal imaging detector. Due to a lack of informa-
tion by the manufacturers on the internal image processing of
these systems, it is only assumed that the additional open beam
areas prevent an overmodulation of a gain or brightness pa-
rameter during the observer-independent processing. Only
the dosimetric software for the storage foil system was found
to obtain usable images by exposing only the small beam. This
result suggests that the imaging software should also include a
module without any preprocessing. Besides the image, that
was processed by the operator, a raw image should be stored
as less-processed as possible to consider the influence of the
image processing. Both imaging techniques do not require
films and offer the advantages of digital processing. However,
preliminary findings show an influence of the image process-
ing on the evaluated beam size and shape. The qualitative
evaluation, that was considered in this work, was obtained by
a processing, that resulted in a nearly square beam shape with-
out any curved sides. Since it is quite easy to center a pre-
defined geometric contour inside an imaged shape, the follow-
ing method could improve the used qualitative evaluation. At
the monitor, a predefined square or circle with the nominal
beam size is centered to the beam shape image, and a pre-
defined circle with the diameter of the metal ball is centered to
the image of the ball. Now, the criterion of the test is applied
to the two predefined contours instead of to their images. Fur-
ther investigations are necessary if the imaging devices are to
be used to obtain quantitative results.
The handling of the storage screen is similar to film in-
cluding the need for shifting the screen in the film holder dur-
ing the exposures. The application of the portal imaging de-
vice avoids this reason for entering the treatment room,
thereby saving about 10 min compared to the application of
film or storage foil.
Acknowledgments
The authors wish to thank the Health Imaging group of the Kodak
Ltd., Stuttgart, Germany, for the support of these investigations by
providing the Kodak Radiation Oncology Beam Dosimetry Package.
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Address for Correspondence
Dr. Peter Geyer
Department of Radiotherapy and Radiooncolgy
University Hospital Carl Gustav Carus
University of Technology Dresden
Fetscherstraße 74
01307 Dresden
Germany
Phone (+49/351) 458-4195, Fax -4339
e-mail: peter.geyer@mailbox.tu-dresden.de