Proceedings of IBIC2012, Tsukuba, Japan

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DEVELOPMENT OF THE NEW TYPE MLIC WITH PMMA PLATES AND
GRAPHITE ELECTRODES
S. Iwata, N. Shinozaki, A. Takubo, C. Kobayashi, AEC#, Chiba, Japan
S. Fukuda, NIRS, Chiba-shi, Japan
T. Kanai, Gunma University, Heavy-Ion Medical Center, Maebashi-Gunma, Japan
Abstract
The MLIC (Multi-Layer Ionization Chamber) that has a
lot of ionization chambers stacked in the depth direction
is a useful detector for measuring the depth dose
distribution (DDD). By using the MLIC, the measurement
time and the amount of beam for dosimetry are drasticallydecreased. In HIMAC (Heavy-Ion Medical Accelerator inChiba), the MLIC has been effectively used for QA(Quality Assurance) measurement of heavy-ion
therapeutic beam since 2002. We are developing a new
type MLIC that has electrodes made of graphite on thesurface of the polymethyl-methacrylate (PMMA) plates
for particle therapy. The purpose is to obtain the same
results as the DDD measured in water. We will report on
the progress of the development.

water, under irradiation of the heavy-ion beam. In such
effect, it is expected that the measurement result of theexisting MLIC is different from that of the DDD in water.
We are developing the new type MLIC (See Figure 1)
that can output the equivalent data of the DDD in water.
This MLIC has the electrode substrates made of PMMA
and graphite. The constituent atoms of these materials are
only hydrogen, carbon, and oxygen. Therefore, the effectof the fragmented particles generated inside the new type
MLIC will be close to the water phantom. However, wehave not yet reached that make a specific assessment for
the results of the new type MLIC, for now. In this paper,
the progress of its development is described.

In heavy ion therapy, the quality of the beam has to bechecked every day. The DDD is an important indicator ofthe beam condition. However, the DDD measurement
needs much time with the existing method that used thedosimeter moving step by step in the depth direction. The
MLIC can measure the DDD at once. It has a lot ofmeasurement points regularly arranged in the depth
direction. Therefore, it does not need time for changing
the depth position of the dosimeter. In addition, theamount of the beam used for the measurement of theDDD could be reduced.
In HIMAC, the MLIC has been used since 2002. It hascontributed to the increase of the number of treatment by
shortening the measuring time of the DDD. Morespecifically, the measuring time was reduced to about 3
minutes from about 20 minutes. It does not include thetime to set up and to put away the detector.
From the experience in using the MLIC, it has beenrecognized as useful means for quality assurance of theheavy-ion therapeutic beam in HIMAC. On the otherhand, it was found necessary to improve the accuracy ofthe dosimetry with the MLIC. In external beam therapy,
the dose distribution in water is treated as a reference.
Therefore, it is preferable that the output of the MLIC
closes in the DDD in water. However, the MLIC used in
HIMAC is not so because it has the electrode substrates
made of glass epoxy plate (FR4) and copper foil. Theseelectrode substrates also have been used for an energy
absorber of the heavy-ion beam. The materials of them
are composed of higher atomic number elements thanwater. The varieties of the fragmented particles generated
by these materials are different from ones generated in

215 mm

160 mm
480 mmFigure 1: The picture of the new type MLIC.

DESIGN OF THE NEW TYPE MLIC
The MLIC has a lot of ionization chambers stacked in
the depth direction. The ion chambers of the new type
MLIC are parallel plate type. Each electrode substrates
are placed orthogonal to the beam traveling direction. The
momentum of the beam particles are lost by passing
through the electrode substrates. In essence, progresses tothe back side of the MLIC, the beam particles slow down.
The amount of charge collected by each stackedionization chambers has varies depending on the depth
position. Therefore, we will get the DDD as the Bragg
peak. The measuring range of the new type MLIC isabout 280 mmWEL (Water Equivalent Length) that
corresponds to 400 MeV/u carbon beam.
Figure 2 shows the structure of the new type MLIC.
And, Table 1 is the specification of it made for Gunma
University Heavy Ion Medical Center.

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Copyright ○INTRODUCTIONMOPB54

Proceedings of IBIC2012, Tsukuba, Japan
Table1: Specification of the new type MLIC
Size of the casing(without handles)

Thickness of the aperture window

Width 215 mm
Height 160 mm
Depth 480 mm
Aluminum alloy
with anodized
about 18 kg
ȭ128 mmPMMA
2.32 mmWEL

Number of channels
Measuring Range
Resolution of depth position
Thickness of PMMA plates
Thickness of graphite electrodes
Air gap
Size of the signal electrodes
Sensitive volume
Operating high-voltage

64 chabout 280 mmWEL
4.3-4.4 mmWEL
3.5 mm
10-20 ȣm3 mm
3 mm square
27 mm3+0.6 kVMaterial of the casing
WeightAperture size
Material of the aperture window
64 ch
280 mmWELa) Inside of the new type MLIC
The stacked ionization chambers are parallel
plate type. Each electrode substrates are placedorthogonal to the beam traveling direction.
ZSignal electrodeof 3 mm square

Xb) The array of the signal electrodes
The signal electrodes were placed regularly thebeam traveling direction at the center of theirradiating area.

The MLIC outputs weak current signals, therefore, themeasuring system must be careful to noise. The MLIC,
the high-voltage power supply, and the measuring module
must have a common ground level that is grounded at asingle point. Figure 3 shows an overview of the cabling.
The guard electrodes enclosing the signal electrodes are
grounded indirectly through the measuring module, andinsulated from the casing of the MLIC in order to prevent
creating the ground-loop.
MLIC casing

4th ch output

3rd ch output

2nd ch output

Dose

1st ch output

PMMA䞉䞉䞉䞉䞉䞉䞉䞉

Dose distribution

Core of the SHV coaxial cable

Shield of the SHV coaxial cable

Graphite

HV plateEnergyloss

Signal plate

Energyloss

SHV connecter

Signal plate

Energyloss

HV plate

Energyloss

Signal plate

AirAirAirAir
HV gap Signal gap HV gap Signal gap HVplate
plate
plate
plate
plate

HV platec 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Y

Insulated
Measuringmodule(A/D converter)

Depthc) Arrangement of the electrode substrates
The HV (high voltage) plates and the signal
plates are stacked alternately. This figure shows
the overview of the DDD measurement.
Figure 2: The structure of the new type MLIC.

GND line
HV lineSignal line

HVpower
supplyTo the data acquisition system
Groundingat a single point

Figure 3: Overview of the cabling.

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Voltage characteristic
200

Operating voltage

Integrated Charge [pC]

250

150
100
50Correction of the beam divergenceThis correction means geometrical correction.
Irradiation beam tends to diverge, therefore, the amountof the beam that is received at a signal electrode depends
on the depth position of it.
The depth size of the MLIC is 480 mm, however thewater equivalent range is 280 mm. It means that the actual
position of the electrode substrate is different from thewater equivalent position. The outputs obtained at actual
positions have to be modified as measured at the waterequivalent positions. As shown in Figure 5, the base pointof beam divergence is called the virtual source position.
The fluence decreases in inverse proportion to the squareof the distance from the virtual source position. Then, thecorrection factor of i-th channel (Di) is given by Eq. (3).

0㻜㻞㻜㻜㻠㻜㻜㻢㻜㻜㻤㻜㻜㻝㻜㻜㻜㻝㻞㻜㻜

㻝㻠㻜㻜

Water
equivalentposition㻝㻢㻜㻜

Applied Voltage [V]Beamtraveling
direction

Figure 4: Voltage characteristic of the new type MLIC.

CALIBRATION OF THE MLICA calibration of the MLIC is needed in order tomeasure the DDD. The calibration of the MLIC has two
purposes, the one is to correct the individual difference ofeach channel, and other one is to enable that themeasurement results are treated as the DDD. The outputsof each channel of the MLIC mean only the amount ofcollected charge. The amount of the change is affectedfrom such as the dark current, the beam divergence, andatmospheric. Each effect has to be corrected before thecalibration. And then, the output data of the MLIC withthese corrections can be compared with the reference of
the DDD that was measured using the water phantom.

Correction of the dark current
The MLIC and the measuring module have dark current.
In proportion to the time required for the measurement,
the component of the dark current will be added to theintegrated charges.
This correction requires the data of the dark current thatwas measured before the irradiation. At i-th channel of theMLIC, the obtained signal without irradiation (Ci,dark) is
the data of the integrated dark current. The required time
for measurement of dark current is defined as t. The darkcurrent of i-th channel (Ii,dark) is given by Eq. (1).

Ii,dark =Ci,darkt(1)

The corrected value of dark current (Xi) is given by Eq.
(2), Where T is the measuring time of the beam and xi isthe output of i-th channel at that time.

Xi = xi - Ii,dark T(2)lwaterlactualActualposition

Virtual sourceposition

Figure 5: The beam divergence.
2lli,water  Di =   i,actual  2       (3)The actual positions of each electrode substrates weremeasured relative to the MLIC casing at the time ofmanufacture. Further, by using the beam, the waterequivalent positions of them were measured from theshifting amount of the Bragg peak.

Correction of the condition of atmosphericThis correction cancels the effects of atmosphericpressure and temperature. The correction factor (A) isgiven by Eq. (4). This factor is the same for all channels.

A=PrefPK
Kref(4)

P: actual value of atmospheric pressure [hPa]Pref : reference of atmospheric pressure [hPa]
K: actual value of temperature [Kelvin]
Kref : reference of temperature [Kelvin]
The value (Xi,correct) that is applied all corrections isgiven by Eq. (5).

Xi,correct = A Di Xi(5)

The dark current correction should be done before othercorrections that are described later.

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Copyright ○On the normal operation, the applied high-voltage tothe new type MLIC was determined to be +600 V fromthe measured data (see Figure 4). The voltage
characteristic was flat above +200 V, so the applied
voltage must be more higher. Other hand, considering therisk of discharge, the voltage value must not be too high.

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Proceedings of IBIC2012, Tsukuba, Japan

Figure 6 shows the corrected data of the MLIC. Thisdata was measured by irradiating carbon beam of 380
MeV/u.

MLIC0.60.5

Dose [Gy]Corrected data of the new type MLIC
600

Integrated Charge [pC]

Water phantom

SOBP 60 mm500
4000.40.3

Tail0.2

PlateauCenter ofSOBP0.1300

0200010050100150200250300

Depth [mmWEL]

0050100150200250300

Figure 8: SOBP 60 mm (carbon 380 MeV/u).

Depth [mmWEL]

Figure 6: The corrected data of the new type MLIC.

The corrected data of the MLIC was compared with theDDD measured by using water phantom as reference
value. In this case, both have to be obtained in the same
beam irradiation conditions. In addition, at the time ofcomparing, the water equivalent positions of eachchannels of the MLIC have to be matched the depth
position of the water phantom. The ratio between the two
was used as calibration constant. The calibration constantof i-th channel (ki) is given by Eq. (6).

ki =Wref(6)

Xi,correct

Wref : reference value measured by using water phantom
If the new data (element: yi) is measured under arbitrary
condition of irradiation, the DDD (element: Yi,calib) will beobtained by using Eq. (7).

Yi,calib = ki Yi,correct = ki A Di (yi - Ii,dark T)(7)

MEASUREMENT OF THE DDD
Using the new type MLIC calibrated by above method,
we measured some DDD under different conditions ofirradiation.

Inserting RSF 100 mm

Water phantom
MLIC

1.4
1.2

The DDD of Figure 7 was measured by irradiating
carbon beam of 380 MeV/u with 100 mm range-shifter(RSF). Figure 8 shows the result of spread-out Bragg
Peak (SOBP) of 60 mm. In each figures, the measuring
data of the water phantom layered on the MLIC data.
Table 2 shows the relative displacement of the MLIC datafrom the water phantom data in Figure 8.

CONCLUSION
The new type MLIC showed a trend that meets ourexpectation. The displacement from the reference that wasmeasured by using the water phantom was about a few
percent as shown in Table 2, however, it is only
evaluation in limited conditions of irradiation. To get
confirmation of water equivalent, it has to been tested inmany conditions.
In the case as to check the difference of the beam
quality in the day-to-day, the new type MLIC is not
required water equivalent. So it may be used in thecurrent state.

ACKNOWLEDGMENT
This project was conceived by Professor Kanai. We aredeveloping under the supervision of Dr. Fukuda. We havebeen given full support from NIRS.

REFERENCES

1Dose [Gy]c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Calibration of the MLIC

Table2: Relative displacement of the MLIC data in Fig.8
Subject of comparison
Displacement
Plateau region (average)+0.4 %
Center of SOBP (Depth: 201.5 mm)+1.1 %
Tail part (average)-4.7 %

0.8
0.60.40.20050100150200250300Depth [mmWEL][1] M. Shimbo et al., Development of a multi-layer ion
chamber for measurement of depth dose distributions ofheavy-ion therapeutic beam for individual patients Nippon
Acta. Radiol. 60 274-9 (2000)[2] K. Yajima et al., Development of a multi-layer ionizationchamber for heavy-ion radiotherapy Phys. Med. Biol. 54
N107-14 (2009)

Figure 7: Inserting RSF of 100 mm (carbon 380 MeV/u).

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