JAERI-Conf 95-021

PA40
Continuous Beam Monitoring for Charged Particle Therapy
M.Torikoshi, H.Ogawa, K.Noda, E.Takada, T.Kanai,K.Narita*, T.Fuk:uzumi*, T.Kohno**
National Institute of Radiological Sciences, 4-9-1 Anagawa, Chiba 263, Japan*Accelerator Engineering Corporation, Konakadai, Chiba 263, Japan**Department of Energy Sciences, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226, Japan

Abstract
Continuous beam monitoring can ensure beamstability during beam irradiation for charged particle
therapy .It can also detect the beam which is not synchronized with a patient's respiratory in case of treatment ofan organ moving with respiratory. A MWPC was adapted
to these purposes. It was used in air not to disturb beam.
The effective thickness of the MWPC with respect to thebeam is only that of wires. Therefore, scattering the beamand losing the energy of the beam in the MWPC issufficiently small so that they do not affect charged particle therapy. The MWPC was proved to be useful by use
for biology experiments.

Introduction

at an isocenter. The second is detection of the beam
which is not coincident with a patient's respiratory during
the treatment of an organ moving with the respiratory.
When such a beam spill is detected, the treatment isinterrupted in order to make beam extraction coincident
with the respiratory. Thus the continuous monitoring
shall be one of the activities of quality assurance for thecharged particle therapy.

Beam monitor for medical use
A multi-wire proportional chamber was in use forthe continuous monitoring. The MWPC is same as thatdeveloped for beam monitors used at the high energy
beam transfer lines (HEBT) of the IDMAC3). A pictureof the MWPC is shown in Fig.l.

The Heavy Ion Medical Accelerator in Chiba(IDMAC) at National Institute of Radiological Sciencesis the first heavy ion accelerator complex dedicated forcharged particle therapyl.2). Charged particle therapy
severely requires controlling dose of beam and safetyassurance. In the IDMAC, the dose is measured by using
two monitors, an ion chamber and a secondary electronmonitor, at the same time. Occurrence of any abnormal
conditions interlocks the beam irradiation through aglobal interlock system.
Furthermore, the continuous beam monitoring isone of strong means to ensure beam quality as follows.
The first is assurance of stability of beam profile andposition, because they affect beam intensity distribution

Fig.l MWPC used in air

Neutron Shutter
Ridge Filter

Scatterer

Compensator
Dose Monitors Range ShifterFig.2 Configuration of the BIOLOGY beam course- 139-

JAERI-ConfRare gases or organic gases are generally used toobtain high amplification factor of the MWPC. In thesecases, thin windows are necessary on the MWPC to havegas tight structure. Our purpose is focusing to use theMWPC as a sort of non-destructive monitors, so that theMWPC should have no window on it. Eventually, air isused as the sensor gas of the MWPC. Therefore the beampassing through the MWPC has a probability to collidewith only the wires. We describe the effects of the collision as an average thickness of the wires, that is approximately 8xl0-2g/cm2. This is corresponding to waterequivalent thickness of only 0.8mm. This value is negligibly small in comparison with thickness of a margin on
a target volume in a body.
The MWPC was installed at the end of BIOLOGY
beam line as shown in Fig.2. The MWPC is locatedbetween the end of beam duct and Wobbler magnets. The
beam intensity is uniformly distributed at the isocenter bya pair of Wobbler magnet and a scatterer. Each wobbler
magnet sweeps sinuously the beam in horizontal orvertical direction. Combination of both motions of thebeam results in rotational motion. The rotating beam isscattered by the scatterers. In this way, the beam of maximum diameter 22cm become uniform withitn ±5% at theisocenter. Scattering effect at the MWPC affects theuniformity. However, the effect evaluated by a calculationis only less than lxl0-3%. Eventually the MWPC can be
regarded as the non-destructive monitor for the charged
particle therapy.

Electronics
We used the same electronics with that used for thebeam profile monitors of the HEBT3). Generated current
on a wire of the MWPC is integrated on a capacitor in theintegration circuit.
The electronics gives us good performance asfollows. Typical noise level is 1 digit of full scale 2048digit. The noise level is independent of the time width ofintegration. It means that leak currents in electronicelements do not contribute to the noise, or are very small.
A simple estimation shows that the leak currents aresupposed to be less than O.lnA each channel. Cross talkbetween adjacent channels is less than -60dB. Because of
the low noise level and the small cross talk, the beamprofiles can be observed clearly even though the peakheight of the profile is less than 0.1 Y.

Tests and results
In order to test the MWPC under various beam
conditions, the MWPC was installed in BIOLOGY beamcourse as mentioned above. The BIOWGY beam course
is used for biology experiments and not therapy.

95-021

Fig.3 shows dependence of the gas amplification
factor on applying voltage. It was measured at C6+
290MeV/u beam. The amplification curve is normalized. . . . . .r. . . . . . . r. . . . . . .r. . . . . . .l. . . .a····r! . · · · ·a..

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Fig.3 Dependence of amplification
on applied voltage

to 1 at plateau region. Because the amplification factor
should be 1 at the plateau region. The curve shows theamplification factor is about 20 at -2.7kV the maximum
voltage of the MWPC. The amplification factor 20 issmall compared with that of same type profiles monitors
used at the HEBT. They gain more than 5,000 fillingwith Ar-C02 mixed gas4). Comparatively small amplification factor results from quenching effect due to oxygen.
An oxygen component of air strongly quenches electron
avalanche even under a considerably strong electric field.
Charged particle therapy is performed using the beam
intensity of 3.6-3.2x108pps. In order to observe such abeam, the MWPC does not need to obtain a wide intensity range. This condition does not require the MWPC tohave high amplification factors.
Dynamic range of this MWPC system spans from1 to 2,000. This results from that the range of amplification factor of the MWPC is from 1 to 20, and the dynamic range of the electronics is from 0.1 Y to lOY as
mentioned already. Then the intensity range of observablebeams is estimated, for instance ofC6+ 290MeV/u beam,
to be from 3x106pps to 6x109pps. It is sufficiently wide
to cover the intensity range necessary for charged particle
therapy or related measurements except for measuring
LET.
A beam profile of C6+ 290MeV/u beam observedby the MWPC is shown in Fig.4. The MWPC is locatedat downstream from the last triplet quadrupole magnets of
the BIOLOGY beam course. The triplet quadrupole- 140-JAERI-Conf 95-021

magnets are forming 10mm diameter beam at theisocenter 11m downstream. Therefore, the beam has an
approximate round shape as shown in the figure.

Although an exposure of a film to the beam andscanning the density measured the beam uniformity,
difference of the uniformity was undetectable as expected.
High voltage of -1kV was continuously applied tothe MWPC for about 3 months. An approximate amountof total beams was 1.314 particles. No damage due toradiation is found on the MWPC at present.

4Conclusions
325

101520253035

ChannelFig.4 Beam profiles

A range of He2+ 150MeV/u beam in water wasmeasured by using a water column. The measurements
were carried out with and without the MWPC in the beamcourse. Both results are plotted in Fig.5. Circles indicate
the results of the measurement with the MWPC, andsquares indicate those without the MWPC. Difference
between both Bragg peak positions was not detected. Itmeans the difference is less than position resolution ofthe measurement.

We summarize the results of the MWPC andthese tests as follows.
(1)The MWPC worked stably in air and gained the amplification factor of about 20 at -2.7k.V. The total dynamic
range including that of the electronics is from 1 to about
2,000. It is sufficiently wide to monitor the beam forcharged particle therapy and related measurements.
(2)Disturbance of the beam was negligibly small so that
it was supposed not to affect charged particle therapy andrelated measurements.
We do not test the MWPC to detect the beam
which is not synchronized with a patient's respiratory,
because the irradiation coincident with a patient's respiratory is in a test stage at present. However, we know thatthe response of MWPC is sufficiently fast to interrupt
the irradiation within acceptable dose ambiguity.
We can conclude the MWPC system is working
as a non-destructive beam monitor with versatility toensure the quality of treatment. A damages due to radiation of a long term, however, must be watched continuously in future.

Acknowledgment
We would like to express our gratitude to theother members of Research Center of Charged Particle
Therapy of NIRS for helpful discussion. We also wish to
thank the members of Accelerator Engineering Corporation for their warm support.

He150MeVQ)

4en0"0References

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1. K.Sato et al., "Status report on HIMAC", Proc. 4tha:ia: 2

~20

140
Depth in water [mm]Fig.5 Dose spectra measured with andwithout the MWPC. Circles show
results of with the MWPC.

160

European Particle Accelerator Conf., London, 1994
2. S.Yamada, "Commissioning and performance of theHIMAC medical accelerator", Int. Conf. on High Energy
Accel., Dallas, 1995, to be published3. M.Torikoshi et al., "Development of beam profile
monitor for HIMAC", The 8th Symp. on Accel. Sci. andTech.,Saitama, 1991, pp.317-319
4. M.Torikoshi et al., "Performance of beam monitors
used at a beam transport system of HIMAC", Beam
Instrumentation Workshop, AlP Conf. Proc. 333,Vancouver, 1994, pp.412-41- 141-