JAERI-Conf 95-021

3a4-1
A TREATMENT BEAM CONTROL SYSTEM FOR
IRRADIATION GATED BY RESPIRATION OF A PATIENT
K. Noda, S. Minohara, M. Torikoshi, M. Kanazawa, E. Takada,N. Araki, A. Noda*, A. Itano**, K. Sato***, H. Ogawa and S. Yamada

National Institute of Radiological Sciences, Chiba 263, Japan*Institute for Chemical Research, Kyoto University, Uji 611, Japan**Regional Hygiene Division, Hyogo Prefectural Government, Kobe 650, Japan···Research Center for Nuclear Physics, Osaka University, lbaraki 567, JapanAbstract
A beam control system for irradiation treatment
gated by respiration of a patient has been developed atHIMAC to minimize an unwanted irradiation to normal
tissues around tumor. The system employs rf-knockout
extraction with gate function. Preliminary experimental
results, which were carried out with moving phantom
and ·simulating signal of respiration, are encouraging.
1. INTRODUCTION

Clinical trial has been successfully progressing
since June 1994, after commissioning of a heavy ion
accelerator complex, HIMAC, dedicated to medical
applications [1 ].
A high irradiation accuracy is required in heavyion therapy because of the high dose localization. In
treating a tumor moving along with respiration of apatient, in particular, damage to normal tissues aroundtumor is inevitable without beam control that gates
irradiation according to respiration. A beam delivery
scheme, which can respond to irregular respiration,
should be applied in order to minimize the unwanted
irradiation to normal tissues around tumor. At HIMAC,
therefore, such a system has been developed [2,3].
The design considerations and the preliminary
experimental results are reported in the paper.
2. BEAM CONTROL SYSTEM
A beam control system for the irradiation gatedby respiration requires essential design considerations asfollows. (1) Beam extraction method should respondquickly to a trigger signal according to irregularrespiration. (2) Operation pattern of synchrotron should
give maximum irradiation dose rate. (3) An aborting
system of residual beam should be provided in order toavoid undesired activation by unused beam.
2.1 Extraction method
Concerning the irradiation gated by respiration,
it is important to start and stop beam extraction

promptly according to the beam "on/off" signal. One of
the suitable extraction methods for this purpose is beam
extraction using a transverse rf field resonated with ahorizontal betatron tune, while a separatrix is kept
constant, which is called "rf-knockout extraction". The
transverse rf electric field is applied with frequency andamplitude modulation. The frequency modulation
increases an extraction efficiency because it broadens afrequency band width corresponding to a horizontal
tune spread at a resonant extraction. The amplitude
modulation is applied to control a spill envelope of anextracted beam. Advantages in the present method are1) prompt response to start and stop of the beam
extraction because of using an rf electric field with afaster response compared with magnetic elements, and2) a small emittance in the horizontal direction due toa constant separatrix. The results from experiments
showed that the response is within 1ms and thehorizontal emittance of beam extracted by the present
method was reduced by about 70% compared with thatby the ordinary extraction method. The extraction
efficiency in the present method was more than about
85% which was comparable with that in the ordinary
method. The details were reported in Ref. [2].
2.2 Operation pattern of synchrotron
To optimize an operation pattern of synchrotron,
it is assumed that a respiration pattern is independentof an operation pattern of synchrotron, and anirradiation for treatment is continued infinitely. Under
the condition, an irradiation dose rate is maximized at
an extraction duty factor of 50%, because a beam canbe extracted as long as the extraction period iscoincident with an irradiation period permitted by therespiration pattern. If the extracted beam intensity canbe infinitely increased, on the other hand, the dose rateis increased as increasing the duty factor. At HIMAC,therefore, the dose rate is maximized at the duty factorof 50% in the 0.3Hz operation of synchrotron. The
effective dose rate in this operation can be kept at theoperational value if the extracted beam intensity isincreased by 70%, which will be easily realized.
All of the accelerated beam should be extracted-439-JAERI-Conf 95-021

as soon as the extraction signal is generated, to reduce
effective irradiation period. On the other hand, anirradiation period more than 10 times longer than thatof wobbling magnets is required to obtain a uniform
dose in the lateral distribution. The amplitude of atransverse rf field is determined so that all of theaccelerated beam is extracted during about 400ms inorder to satisfy the above requirements.

The P80_20 in the fixed phantom is naturally less thanthat in the gated irradiation, because the phantom in thelatter case moves somewhat during the irradiation due
to applying a sinusoidal wave as the simulating signal.
However, the P80_20 in the gated irradiation isconsiderably reduced to 6.1mm from 19.4mm in theungated irradiation.

2.3 Beam aborting system
In the irradiation gated by respiration, theresidual beam has to be aborted around synchrotron,
because the accelerated beam should not be extracted
from synchrotron as long as a "beam off'' signal isgenerated. Decelerating the residual beam to aninjection energy as a beam aborting system is proposed
to avoid unwanted activation around synchrotron. As aresult of the preliminary test, the deceleration efficiency
was about 80%. Details will be presented elsewhere[4].
Summarizing the above considerations, anoperation patten of the beam control system for theirradiation gated by respiration is schematically shownin Fig. 1.

(d)

(e)

3. EXPERIMENTAL RESULTS

An irradiation gated by respiration waspreliminarily experimented by using a phantom moving
along with a simulating signal of respiration [3]. In theexperiment, synchrotron is operated by a cycle of0.3Hz with a duty factor of 45%. Carbon beams withthe energy of 290 and 400MeV/n are extracted fromsynchrotron by the rf-knockout method, and delivered
through an irradiation system to an isocenter at atreatment room. Concerning the experimental setup in
the treatment room, a phantom placed at the isocenter
is moved with a stroke of 25mm and, is driven by a0.33Hz sinusoidal signal that pretends respiration. The
simulating signal is generated by a sensitive strain
gauge set on the phantom. A collimator with anaperture of 40mm square is placed in front of thephantom in order to define an irradiation field, and is
450mm distant from the phantom.
Figure 2 shows a typical result of a beam
extraction gated by the simulating signal. As can beseen in the figure, the beam was successfully extracted
in only the permitting irradiation period. A penumbra
size was also obtained by measuring a density of an
exposed X-ray film attached with the phantom moving
along with the simulating signal. Three cases, i.e, afixed phantom, a gated irradiation to moving phantom,
and an ungated irradiation to moving phantom wereinvestigated as shown in Fig. 3. The penumbra size
P80_20 in three case were obtained as 2.4, 6.1 and19.4mm, respectively, where P80_20 is defined at thedistance of the lateral dose falloff from 80% to 20%.

(f)

:i:t:

I:

:: :: ::~)~I

Fig. 1 Schematic operation pattern of the system forirradiation gated by respiration. (a) Respiration signal,(b) Permitting irradiation signal, (c) Operation patternof synchrotron, (d) Flat-top signal, (e) Extraction
signal, (t) Beam spill, (g) Circulating beam intensity.
4. CONCLUSION
The beam control system for irradiation treatment
gated by respiration is developed. The beam control
system's ingredients are; rf-knockout extraction that canrespond to a respiration signal quickly, a 0.3Hz, 50%
duty operation of synchrotron that maximizes dose rate,
and a beam deceleration as a beam aborting system.
As preliminary results of experiment, anextraction gated by a simulating signal of respiration
was successfully achieved. The penumbra size was alsomeasured by using a phantom moved along with thesimulating signal, and was considerably reduced to
6.1m from 19.4mm in the ungated irradiation. The
system will play an important role in clinical study
such as treatment for a lung or liver cancer moved
along with respiration of a patient.

-440-

JAERI-Conf 95-021(lsec/div .)Fig. 2 Beam extraction gated with the simulating signal of respiration in 400MeV/n carbon beam.
(a) Simulating signal of respiration, (b) Beam spill, (c) Operation pattern of synchrotron, (d) Permitting irradiation
signal.

..

100.

.

-~,.,, . . . . ......................l\, .......\ :-----+------1 (a). (b),,\,.

. . i. / /:.'!i /80\~-

1.......

r;r..!.fr'~ 60.........

"'=,:
i······· .

,r<.

~\ ".¥.···_;>{~) .

,I .i/1. :' ...............,...........! . . . . . . . . . . . . ~.' ·················· ..·····!· ·························· \·· ..........\ , ·····(·····[
I······

~~

40

j i

20~.......

:{ / :/r - \' - -\ ..\...

\i J_ Ps~;):0 2.4mm/ i)\\1\. . . . ~\. 1\.,:;,',:~::

0-4 0. ·3 0·2 0·1 0100204030[mm]Fig. 3 The lateral dose distribution in 290MeV/n carbon beam. (a) fixed phantom, (b) gated irradiation to moving
phantom, (c) ungated irradiation to moving phantom.
5. ACKNOWLEDGEMENTS

6. REFERENCES

The authors would like to express their thanks
to MD. H. Kato and Mr. T. Ishii of Division ofRadiation Medicine, and Mr. A Fukumura of Divisionof Radiation Research at NIRS for valuable discussions.
They are also grateful to the crew of AcceleratorEngineering Corporation for skilful operation ofHIMAC and to Dr. K. Kawachi and members of theDivision of Accelerator Physics and Engineering atNIRS for their warm support and encouragement.

[1] K. Sato et al., Nucl. Phys. A588,229(1995).
[2] K. Noda et al., Proc. 4th EPAC, London, 1994,
pp.982-984.
[3] S. Minohara et al., Proc. 12th JAMP, Tokyo, 1995,
pp.38, in Japanese.
[4] M. Kanazawa, et al., to be submitted.

-441-