Proceedings of IBIC2012, Tsukuba, Japan

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BEAM MONITORS OF NIRS FAST SCANNING SYSTEMFOR PARTICLE THERAPY
T. Furukawa, T. Inaniwa, S. Sato, E. Takeshita, K. Mizushima,
K. Katagiri, Y. Hara, T. Shirai, and K. Noda
National Institute of Radiological Sciences, Chiba, JapanAt National Institute of Radiological Sciences (NIRS),
more than 6500 patients have been successfully treated bycarbon-ion beams since 1994. The successful results oftreatments have led us to construct a new treatment
facility equipped with three-dimensional pencil beamscanning irradiation system. The commissioning of NIRS
fast scanning system installed into the new facility wasstarted in September 2010, and the treatment withscanned ion beam was started in May 2011. In thescanning delivery system, beam monitors are some of themost important components to safely deliver the dose tothe patient. In this paper, the design and thecommissioning of beam monitors in the delivery system
are described.

INTRODUCTION
Since 1994, more than 6500 patients have beensuccessfully treated with carbon-ion beams deliveredfrom Heavy Ion Medical Accelerator in Chiba (HIMAC).
To make optimal use of these characteristics and toachieve accurate treatment, three-dimensional (3D) pencil
beam scanning [1-3] is one of the sophisticatedtechniques in use. For implementation of this irradiation
technique, at HIMAC, a new treatment facility [4] wasconstructed. Figure 1 shows the treatment room of a newtreatment facility. After intense commissioning andquality assurance tests, the treatment with scanned ion
beam was started in May 2011.

method requires sophisticated beam monitoring andcontrol system to deliver well defined dose throughout thetarget volume safely with sufficient accuracy. In order tomeasure and control the dose of each spot, the main andthe sub ionization chambers are placed separately as flux
monitors. For monitoring of the scanned beam position, abeam position monitor, which is multi-wire proportional
chamber, is installed just downstream from the flux
monitors. This monitor can output not only the beam
position but also the 2D fluence distribution using
dynamic fast convolution algorithm.
In this paper, the design and the commissioning ofthese monitors in the delivery system are described.

NIRS FAST SCANNING SYSTEM
Layout of the scanning system [5] is shown in Fig. 2. It
consists of the scanning magnets, main and sub flux
monitors, position monitor, mini-ridge filter and range
shifter. To achieve the fast beam scanning at theisocenter, the distances from scanning magnets to theisocenter are designed to be 8.4 and 7.6 m, respectively.
The vacuum window is made of 0.1mm-thick kapton andlocated 1.3 m upstream from the isocenter. Beammonitors, mini-ridge filter and range shifter are installed
downstream of the vacuum window. Since the required
field size is 220×220 mm2 for the transverse directions,the effective area of the monitors are designed to be
240×240 mm2. Further, these monitors are designed tomeasure the beam having energies between 80 and 430
MeV/u, and having intensities between 1·107 and 1·109
particles per second (pps).

Figure 2: Delivery system of the new treatment facility.
Beam monitors described in this paper are assembled in
the region highlighted by red dot curve.

FLUX MONITORS
Figure 1: Treatment room of the new treatment facility.
The beam delivery with beam scanning can be used toachieve the desired dose distribution by magneticallydeflecting the beam across the target and by changing thebeam penetration depth. Thus, scanning irradiation

Configuration of Flux Monitor
The flux monitors, which are parallel-plate ionizationchambers, are placed just after the exit window of thevacuum duct, and they are operated in air at atmosphericpressure. Each flux monitor consists of a signal foil
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Copyright ○AbstractProceedings of IBIC2012, Tsukuba, Japandose was around 1%. From these results, we concluded
the position dependent calibration of the flux monitor is
not necessary. Output linearity of the flux monitor wasalso checked by changing applied count in 2D uniform
field delivery. The delivered dose was measured byreference ionization chamber of PTW30013 having
sensitive volume of 0.6 cc. As shown in Fig. 3, thedeviation from the linear relationship was less than 1%.
The measurement of the recombination was also carriedout. Although the typical beam intensity was 1-2·108
particles/s, the severe recombination was not observed up
to 6·108 particles/s.

BEAM POSITION MONITOR
Configuration of Beam Position Monitor
The beam position and profile are measured with amulti-wire proportional chamber (MWPC), which isinstalled just downstream from the main and sub flux
monitors. With the anode wire spacing of 2 mm, thisMWPC has 120 anode wires for x and y planes,
respectively. Anode-cathode distance is designed to be 3
mm to avoid any gain drops due to the space charge
effect. Diameters of the cathode and anode wires are 50and 30 μm, respectively. Figure 4 shows a schematic
drawing of this monitor system including its electronics,
consisting of I/V amplifiers and a digital control unit. The
I/V amplifiers, which have the amplification rate of 2·106
V/A, are directly connected to the monitor head. The
output signals of the I/V amplifiers are fed into the 12 bit
analog-digital converters (ADCs), which operate with thefrequency of 200 kHz.
ADC

Figure 3: Typical result of flux monitor commissioning,
left: position dependence measurement by film, right:
output linearity measurement.

Commissioning of Flux Monitor
The position dependence of the main flux monitor andthe scanning control was tested by using 2D uniform fieldirradiation. By applying the same count for regular grid
spots, 2D uniform irradiation was carried out. Theuniformity of the delivered field was measured by using
the 2D ionization-chamber array (OCTAVIUS Detector
729 XDR, PTW Freiburg) and the film. Figure 3 shows
results of 2D uniformity measurement by the film. We see
a uniform field distribution, indicating the main flux
monitor has rather small position dependence. In bothmeasurements, the standard deviation of the measured
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MWPC

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×120chI/V amp.

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2D profile

・・・・・・・・・・・・・・・・・・・・・・ ×120ch

・・・・・・・・・(anode), two cathodes and two grounded shielding foils.
For the cathodes, a wire mesh is employed to suppress theposition dependence of the output signal. The wires(diameter: 50 μm) used in the mesh are made of goldcoated tungsten, and they are aligned with the spacing of
1 mm. The signal foil is made of 25 μm-thick polyimide,coated with Cu and Au. The gap between the anode andeach cathode is 5 mm. With typical HV setting of –2500
V, the charge collection time is estimated to be 70 μs.
Although such response of the chamber might cause anerror in the dose delivery, the effect is considered to be
tolerable, because the rising time of the beam at the beam
ON timing is around 1 ms.
The output current from the flux monitor is digitized by
the current-frequency converter. This converter module
consists of the I/V amplifier (current to voltage amplifier)and VFC (voltage to frequency converter). Since cutoff
frequency of the I/V amplifier is set to around 100kHz,
the output response for 100 kHz input is –3 dB of theoutput response for DC input. The VFC can digitize thesignal from the I/V amplifier with the conversion ratio of
0.4 MHz/V, while the maximum output frequency of thedigitized pulses is 2 MHz. To cover the dynamic range ofthe expected beam intensity, this converter has threeswitchable ranges of 2MHz/100nA, 300nA and 1000nA.
In the beam tests, the beam intensity of 1.5·108 pps is
typically employed, providing digitized pulses of around
600 kHz under medium gain. Severe recombination of
more than 1% has not been observed in the measured
saturation curves of the flux monitor for different beam
intensities.

I/V amp.

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Copyright ○MOPB51

Controller 2(monitoring)

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img

MMI(display & log)×120ch×120chFigure 4: Schematic drawing of beam position monitor
and its electronics.
By using these digital data, this digital unit handles two
different tasks: 1) calculation of beam center and 2)
production of the 2D fluence map for each slice. The firsttask of the center calculation is carried out with arepetition rate of 200 kHz. When the beam position isrequired by controller 2, this unit provides the latest resultof the beam center calculation. In controller 2, received
data for the beam center are used for the interlock andirradiation log. Furthermore, this unit outputs an analog
signal of the beam position which is fed into an
oscilloscope to display the trajectory of the scanned beam.
On the other hand, the second task of fluence mapproduction for each iso-energy slice is carried out in theMiscellaneous and others

Proceedings of IBIC2012, Tsukuba, Japanfollowing steps. First, 11 channels around the beam centerfor each plane are selected. Second, by multiplying bothprofiles f(x) and g(y), the 2D image h(x,y) is calculated,while independence of each beam profile is assumed. This
temporary image (11×11) is repeatedly summed into afull size image (120×120) during one slice irradiation.
While the beam is OFF when changing the energy slice,
the latest 2D fluence map is sent and displayed on theman-machine interface (MMI), while the digital unit
prepares for the next slice, i.e. refreshing the image.
Having repeated this procedure, the 2D fluence map foreach iso-energy slice can be obtained.

Commissioning of Beam Position Monitor
Concerning the gain drop due to the space charge effect[6,7], the several types of gas mixtures are tested. Finally,
we decided to employ a mixed gas of 70% He and 30%
CF4. The measured gain curve are shown in Fig. 5.
Although the maximum gain was not so high, theintensity dependence was well suppressed compared withAr based gases.

The verification of the beam position monitor output
was carried out by comparing the film dosimetry. The
differences measured position between the monitor output
and the film were less than 0.5 mm. Further, the monitor
performance was evaluated by using the measurement
results. Figure 6 shows the beam center output during 2D
uniform scanning irradiation. The average position ofmonitor output for each spot agreed well with the preset
value within 0.5 mm. The standard deviation of thedifference between the preset position and 200 kHz
position output was less than 0.4 mm, corresponding to
the accuracy of this monitor including its electronics.
Consequently, the position monitor output can be used for
position monitoring with the tolerance of 2 mm.
On the other hand, the position output is routinely used
for online display of the beam scanning process. Figure 7shows the typical view of position monitor console.
Figure 8 shows typical oscilloscope display during thetreatment. These displays give important information tomedical staff during the irradiation process.

1010500

1000

1500

2000

Applied voltage [V]Figure 5: Measured gas gain with different beamintensities.

Figure 7: Online display of position monitor
measurement, left: online display of beam position, right:
measured fluence map for each iso-energy slice.

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Time [ms]Figure 6: Beam center outputs (solid line) from the beam
position monitor compared to the preset beam position(triangles). The average position between the spottransitions and the standard deviation are also plotted bybars. To clearly see the spot transition timing, the signalof transition is shown by the dashed line.

Figure 8: Typical oscilloscope display during treatment,
lower trace shows the enlarged view of upper trace. Red
line: synchrotron bending magnet pattern, yellow line:
beam flux monitor output, green lines: position monitor
outputs x and y.
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Copyright ○2E7pps
2E8pps
9E8pps100

Gas gain

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Proceedings of IBIC2012, Tsukuba, Japan

3D SCANNING BEAM DELIVERY
Verification of the scanning delivery system can berealized by checking the 2D delivery with intensity
modulation. The checking irradiation pattern introducedby Flanz [8] was employed and measured by using thefluorescent screen with CCD camera system [9,10]. As
shown in Fig. 9, we can see the 2D irradiation with theintensity modulation was successfully performed.
Overall check including the treatment planning system[11,12], the 3D dose conformation was tested bycomparing measured and planned dose distributions.
Typical result of comparison is shown in Fig. 10. Since
the measured distribution was good agreement with the
expected one, we can conclude that the beam control iscorrectly operating. The deviation was less than ±3%.
Further, the reproducibility of the delivered dose within±0.5% was also verified in these delivery tests.

Figure 9: Measured distribution in 2D intensity
modulation. Red dotted line shows the prediction.

SUMMARY

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○The scanning delivery system, including beammonitors, at the HIMAC new treatment facility wereconstructed and intensively commissioned. Consequently,
the system are routinely used for the carbon-ion
radiotherapy at NIRS-HIMAC.

ACKNOWLEDGMENT
We would like to express our thanks to staff at theAccelerator Engineering Corporation for their skillful
operation of the HIMAC accelerator complex. We aregrateful to members of Toshiba Corp. for construction ofthe scanning system.

REFERENCES[1] T. Kanai et al, Nucl. Instr. Meth. 214 (1983) 491.
[2] E. Pedroni et al, Med. Phys. 22 (1995) 37.
[3] Th. Haberer et al, Nucl. Instr. Meth. A 330 (1993)
296.
[4] T. Shirai et al, Proc. of IPAC11, 3604.
[5] T. Furukawa et al, Med. Phys. 37 (2010) 5672.
[6] K. Katagiri et al, Rev. Sci. Instrum., 82 (2011)053303-1.
[7] K. Katagiri et al, in these proceedings.
[8] J. Flanz, "Quality assurance, accelerator and beamdelivery", PTCOG49, 2010.
[9] E. Takeshita et al, Nucl. Instr. Meth. B, 269 (2011)2936.
[10] Y. Hara et al, in these proceedings.
[11] T. Inaniwa et al, Med. Phys. 36 (2009) 2889.
[12] T. Inaniwa et al, Phys. Med. Biol. 55 (2010) 6721.

Figure 10: Comparison between measured and planned
dose distribution. Upper trace: comparison on yellow line
in lower trace, dot and line show measurement and plan,respectively. Lower trace: comparison of iso-dose line,
solid contour shows measurement, while dashed contour
show planned dose distribution.

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