Proceedings of IBIC2012, Tsukuba, Japan

MOPB76EVALUATION OF A FLUORESCENT SCREEN WITH A CCD SYSTEMFOR QUALITY ASSURANCEIN HEAVY-ION BEAM SCANNING IRRADIATION SYSTEM
Y. Hara#, T. Furukawa, K. Mizushima, S. Sato, T. Inaniwa, T. Shirai and K. Noda,
National Institute of Radiological Sciences, Chiba, JAPANE. Takeshita, Gunma University Heavy-Ion Medical Center, Gunma, JAPANTwo-dimensional dosimetry system was developed forquality assurance (QA) of therapeutic scanned ion beams
at HIMAC. This system consists of a fluorescent screen
and a charge coupled device (CCD) camera. To evaluate
the performance of this system, we carried out a few
experiments for QA procedures. The verification of thissystem was also carried out by comparing the film
dosimetry. As a result, we confirmed that this system
could be used as the system for QA procedures oftherapeutic scanned ion beams.

INTRODUCTION
Heavy-ion beams such as carbon-ion beams haveattracted growing interest for cancer treatment due to theirhigh dose localization and high biological effect at theBragg peak. To make the best use of these characteristicsand provide flexible dose delivery, three-dimensional(3D) pencil beam scanning [1-3] is an ideal irradiation
technique. At the Heavy Ion Medical Accelerator in Chiba(HIMAC), it has been utilized for treatment since 2011[4]. In a dynamic delivery system using the 3D scanningsystem, it requires additional quality assurance (QA)
procedures to ensure a consistent and safe doseprescription, as compared with broad-beam delivery
system. Since the accuracy and quality of the planned
dose for the treatment depends on the accurate depositionof individually weighted pencil beams, any change ofscanned beams will result in a significant impact on theirradiation dose. Thus, the QA procedures and tool formaking refined measurements for the verification of theposition and size of pencil beams must be developed. For
this purpose, a few types of QA tool have been developed,
such as an ionization chamber array and a radiographicfilm. These systems allow an efficient check of theabsorbed dose in the treatment field at many points andperform a direct comparison with the planned dose atthese points. However, the spatial resolution of theionization chamber array is not so high. Althoughradiographic film is a very useful tool due to its highspatial resolution and suit for the measurement of theintegral dose, overall data processing with the system istime consuming. Instead of these systems, we developed aquick verification system using a fluorescent screen witha charge coupled device (CCD) camera, which we called
the QA-SCN [5], originally proposed by Boon et al [6, 7].
___________________________________________#y-hara@nirs.go.jp

In this paper, the results of the QA measurements
obtained by using the QA-SCN are described. The
performance of the QA-SCN is evaluated from theviewpoints of utility as the system for QA procedures oftherapeutic scanned ion beams. The verification of theQA-SCN was also carried out by comparing the film
dosimetry.

MATERIALS AND METHODSThe technical details of the QA-SCN system, e.g.,
design of the QA-SCN, the control system and theoff-line image processing, was reported previously [5]. A
simplified explanation only is given here.

Figure 1: Schematic of the QA-SCN system layout.
The QA-SCN system was used to verify the 2D
irradiation field deduced from the relative fluence on theisocenter without the water phantom. Figure 1 shows aschematic layout of the QA-SCN system. The QA-SCN
consists of a fluorescent intensifying screen (TypeHG-M2, Gd2O2S:Tb, Fujifilm Corp., Japan), a CCD
camera (Type BU-41L, 1360×1024 pixels, Bitran Corp.,
Japan), a mirror, camera controllers and a dark box toprotect against surrounding light. The CCD camera
allows the measurements of 2D light output with a largedynamic range and low thermal noise, due to a decreaseof the operating temperature of the CCD by a cooling unit.
The QA-SCN is set at the isocenter. The distribution offluorescent light is reflected by the mirror, located at 45
degrees relative to beam axis, and is observed by theCCD camera, which was installed at 90 degrees relative
to the beam axis to reduce the background from theradiation. The distance between the fluorescent screen
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1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○AbstractProceedings of IBIC2012, Tsukuba, Japanand the mirrror is 505 mm
m, and the disstance betweeen themirror and thhe CCD camerra is 535 mm.. The viewingg field
and the aperrture of the dark
dbox coveered an irradiiation
field of 250×250 mm2, which corrresponded too thespecificationn of the irradiaation system. Also, to veriffy theperformance of the QA-SC
CN, the film was
w attached oon theQA-SCN andd irradiated withw similar irraadiation patterrns.
After the irradiation, alla of the detected image s aretransferred to data servver at once,, and the immage
processing iis started. Fiirst, the backkground dataa are
subtracted frrom the originnal image. To suppress the spikenoises, the mmedian filter is applied to the image after
subtraction. Simultaneouusly, the pixel
pspacingg isconverted to the actual sppacing as milliimeters. To reeducethe variationns of output caused
cby lens vignetting aand adifference inn thickness of the screeen, the flat fieldcorrection iss applied. Fiigure 2 show
ws the profille ofuniform irrradiation fielld with and without thesecorrections. The overall level of thee response iss notaffected by thhese correctioons.
Raw data
BG corr
MedianFF corr

Brightness [a.u.]

50000
40000

Figu
ure 3: The results of spot irrradiation measured by thee
QA--SCN.
1.5
1.0

LargeMiddle
Small

0.5

30000

0.0
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-1.0

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-1.5

10000

-1.55 -1.0 -0.5

0-100

-50

050

0.000.5

1.0

1.5

X [mmm]

100X-pposition [mm]Figure 2: Prrofiles of irraadiation field by a scannedd ion
beam with aand without eaach correction
n. The dashedd line
shows the pprofile withouut correction. The dottedd line
shows the prrofile with baackground coorrection only.. The
continuous line shows the profile with backgrroundcorrection, mmedian filter. The triangle shows the prrofile
with backgroound correction, median filter
fand flat fieldcorrection.

Figu
ure 4: Diffeerences betw
ween the measuredm
anddpresccribed positio
ons for three different irrad
diation fields,,±1200, ±80, ±40 mm.
m1.5

430 MeV/n
290 MeV/n
140 MeV/n

1.00.5REESULTS AN
ND DISCCUSION
Several typpes of QA inspections musst be perform
med tocheck the beam position, 2D
2 intensity modulation
mannd theeffect of hysteresis for thherapeutic sccanned ion beeams.
The scanned beam possition is dirrectly affectss thedelivered doose distributiion. Thus, wew confirmedd theaccuracy of the position forf scanned beam by usingg spot
irradiation. T
Typical resultss of spot irradiiation measureed by
the QA-SCN
N are shownn in Fig. 3. Figure 4 sshowsdifferences between thhe measured and presccribedpositions forr three different irradiation fields, ±120, ±80,±40 mm. Thhe results shoow the accuraacy of ±0.3 m
mm at
1-sigma. To verify the eneergy dependen
nce of the scaanned
beam positioon, the measuurement was alsoacarried ouut by
using the spoot irradiation. As shown in Fig. 5, differeences
between the measured and prescribed positions for threedifferent enerrgies obtainedd by using small irradiationn field
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Y [mm]c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○60000

w0.2 mmm at 1-sigma.
are withinY [mm]

MOPB76

0.0-0.5-1.0-1.5-1.55 -1.0 -0.5 0.000.5

1.01.5

X [mmm]Figu
ure 5: Diffeerences betw
ween the measured
mandd
presccribed positions for three ddifferent energ
gies, 430, 290,,
140 MeV/n.
Too verify the unniformity of tthe 2D irradiaation field, wee
measured the unifform irradiatio
ion. Figure 6 shows resultss
D uniformity measurementt. The upper anda right-handdof 2D
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Proceedings of IBIC2012, Tsukuba, JapanAs shown in Fig. 9, the differences between theuniformity of 2D irradiation by using the QA-SCN andthe film were less than 1.2%. From these results, weconfirmed measurements by the QA-SCN were good
agreement with the film dosimetry.
Overall verification of the scanned beam can berealized by checking the 2D delivery with intensity
modulation [8]. The checking irradiation pattern
introduced by Flanz [8] was employed and measured by
the QA-SCN. As shown in Fig. 10, the 2D irradiation
with the intensity modulation was successfully performed.
The measured distribution was good agreement with theexpected one, i.e. 1.5% at 1-sigma.
1.5

QA-SCNFILM

1.0

Y [mm]figures in Fig. 6 show the results for a typical
one-dimensional distributions selected form the 2D
irradiation measurements at both the horizontal andvertical irradiation lines for the treatment room F. The
fluctuations of the uniform irradiation were small, 2% at
the standard deviation. To check the spacing of each spot,
we also fitted the uniform irradiation field measured byQA-SCN with multi Gaussian functions. The beam sizeand position, which are the parameter of the Gaussianfunction, were deduced from the measurements obtainedby spot irradiation. Figure 7 shows the measured andfitting profiles of uniform irradiation field for threedifferent irradiation fields. In any fields, we confirmed thespot spacing determined by fitting is within less than 0.1
mm of the prescribed position.
The performance of the QA-SCN was also verified bycomparing the film dosimetry. To check the propriety ofthe beam position, we compared the results of spot
irradiation measured by the QA-SCN and the film
dosimetry in Fig. 8. In both measurements, differences
between the measured and prescribed positions are within
0.2 mm at 1-sigma.

MOPB76

0.5
0.0
-0.5
-1.0

1.0FHCFVC

0.0-1.5 -1.0 -0.5 0.0

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0-1000.1727

Y-position [mm]0.5

X [mm]1.0

1000.5

X-position [mm]-50050

0.0-100-500.3453

00.51800.6906

50Figure 8: Differences between the measured andprescribed positions obtained by using small spotirradiation field. The rhombus shows differences obtainedby the QA-SCN. The square shows differences obtainedby the film.

0.86331001.036
1.105
1.00.50.8

00.1648

Y-position [mm]Relative Output

1.0

100

1.0

1.2

X-position [mm]
-50
050

0.0

-100-100

Large
MiddleSmall
Fitting
Fitting
Fitting

QA-SCN
FILM

0.0

0.5Figure 6: Result of 2D uniformity measurement obtainedby using large uniform irradiation.

-5000.3297
0.49450.6594500.82420.6
1000.40.9891
1.055

0.20.0-100-50050100

X-position [mm]Figure 7: Fitting results of three different uniformirradiation fields using multi Gaussian functions.

Figure 9: Result of 2D uniformity measurement obtainedby using middle uniform irradiation. The continuous line
shows the profile measured by using the QA-SCN. The
dotted line shows the profile measured by using the film.

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Copyright ○-1.50.5

MOPB76

Proceedings of IBIC2012, Tsukuba, Japan

1.00.5
measurement

0.0calculation
1.00.51000.0-100

X-position [mm]-500500-100
Y-position [mm]0.1563-500.3125
0.468800.6250

500.7813
0.93751.000

100Figure 10: Measured distribution in 2D intensity
modulation. The dotted line shows the prediction.

CONCLUSIONS

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○irradiation field, the verification of the spacing at eachspot and the check of intensity modulation in 2D delivery,
can be measured by using the QA-SCN accurately.
Compared with the result of the QA-SCN and the film, inboth measurements, the accuracy of the beam position
was within 0.2 mm. The differences between theQA-SCN and the film by using 2D uniform irradiation
were less than 1.2%. As a result, we confirmed that theQA-SCN could be used as a useful tool for QA
procedures of therapeutic scanned ion beam.

We evaluated a fluorescent screen and a CCD camera
system, so-called the QA-SCN, used as useful tool for
quality assurance of therapeutic scanned ion beams.
Several types of QA inspections such as the accuracy ofthe scanned beam position, the uniformity of the 2D

REFERENCES[1] E. Pedroni et al., Med. Phys. 22 (1995) 37.
[2] Th. Haberer et al., Nucl. Instrum. Methods. A 330(1993) 296.
[3] T. Furukawa et al., Med. Phys. 37 (2010) 5672.
[4] T. Shirai et al., “Recent progress of new cancer
therapy facility at HIMAC”, IPAC 2011[5] E. Takeshita et al., Nucl. Instrum. Methods. B 269(2011) 2936.
[6] S. N. Boon et al., Med. Phys. 24 (1998) 464[7] S. N. Boon et al., Med. Phys. 27 (2000) 2198.
[8] J. Flanz, “Quality assurance, accelerator and beamdelivery”, PTCOG49 2010.

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