Proceedings of IBIC2012, Tsukuba, Japan

MOPB78BEAM SPOT MEASUREMENT USING A PHOSPHOR SCREENFOR CARBON-ION THERAPY AT NIRS
K. Mizushima#, E. Takeshita, T. Furukawa, T. Shirai, K. Katagiri, Y. Hara, K. Noda,
National Institute of Radiological Sciences, Chiba, Japan
A two-dimensional beam imaging system with aterbium-doped gadolinium oxysulfide (Gd2O2S:Tb)
phosphor screen and high-speed charge coupled device(CCD) camera has been used to measure the beam spot
for scanned carbon-ion therapy. The system can take theimage of the beam spot with the frequency of 50 Hz.
Using this system, the time stability of the unscannedbeam spot size and position was verified in the isocenter.
This system was also used to check a beam alignment byobserving a shadow, which appears on the beam spot
image, of the steel sphere located on the reference axis.

INTRODUCTION
In charged particle cancer therapy with pencil beamscanning, the time stability of the beam position and size
is important factor to provide the prescribed dose
distribution. The large fluctuations of them are capable ofincreasing damage to normal tissues in the vicinity of thetumor and producing a hot or cold dose region in thetarget volume. To verify their stability at the isocenter inthe treatment room, National Institute of RadiologicalSciences (NIRS) [1] in Japan developed a twodimensional (2D) beam imaging system.
NIRS has performed the three-dimensional scanning
irradiation with a carbon-ion pencil beam [2] since May
2011. The carbon-ion beam is provided from the Heavy
Ion Medical Accelerator in Chiba (HIMAC) synchrotron
using the RF-knockout slow extraction method [3]. Thebeam energy is changed from 430 MeV/u to 140 MeV/u
stepwise by the multiple-energy synchrotron operation[4], and small changes of the beam range are controlled
by inserting the PMMA plates with various thicknesses.
In the HIMAC synchrotron, the flattop is extended inorder to extract the beam for a long time. The beam size
variation due to emittance growth, therefore, should becared during a long extraction period. The beam position
drift is also cared although the beam position is controlledwith the feedback system integrated into the control
system of the scanning magnet power supply.
The 2D beam imaging system developed at NIRS canobtain the 2D beam profiles with the frequency of 50 Hz
and observe the stability of the beam position and size at
the isocenter in the treatment room. This system consistsof a phosphor screen and high-speed charge coupleddevice (CCD) camera. This system construction isidentical to that of the beam monitor system installed in
the high-energy beam transport line at HIMAC [5]. The
2D beam imaging system also functions as a beamalignment adjustment system by setting a steel sphere on_________________________________________

the reference axis. The beam alignment can be checkedby observing a shadow of the steel sphere on the beam
spot image.
Using the 2D beam imaging system, the fluctuations of
the unscanned-beam spot position and size were observedin the isocenter to verify the time stability of the delivered
beam for scanning irradiation. On the other hand, themisalignment of the beam has been routinely checked forquality assurance. In this paper, we describe the 2D beamimaging system and the measurement results using it.

2D BEAM IMAGING SYSTEM
System Configuration
A schematic drawing of the 2D beam imaging system isshown in Fig. 1. A terbium-doped gadolinium oxysulfide(Gd2O2S:Tb) phosphor screen is mounted at an entrance
surface of the beam inside the dark box. The light radiatedfrom the phosphor screen is carried to the high-speed 8bit CCD camera (Type XG-H035M, KEYENCE, Japan),for protecting the camera from the radiation damage, via a
mirror placed at a 45 degree to the beam direction. The
typical observed image of the beam spot is shown in Fig.
2. The spatial resolution in this system is about 0.2
mm/pixels.

Figure 1: Schematic drawing of the 2D beam imagingsystem.

Figure 2: Typical image of the beam spot measured by
the 2D beam imaging system.

#mizshima@nirs.go.jp

ISBN 978-3-95450-119-9
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1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○AbstractMOPB78

Proceedings of IBIC2012, Tsukuba, JapanThe CCD camera is controlled by the special controller(Type XG-7500, KEYENCE, Japan). It employs threeprocessors for a fast image processing, and it is operated
through the Programmable Logic Controller.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Beam Alignment Adjustment
The 2D beam imaging system is also utilized for thealignment adjustment of the beam by using an acrylic
phantom. The setup for adjusting the beam alignment isshown in Fig. 3. A steel sphere is embedded in the surfaceof this phantom, and its position is on the axis of thereference coordinate system. The misalignment of thebeam can be measured by taking an image of the beam
spot while setting this phantom.
The steel sphere generates a shadow, as shown in Fig.4,
on the beam spot image. The beam misalignment iscalculated by deriving the centres of the beam spot andthe steel sphere. The algorism to obtain the two centres isindicated in Fig. 5. At first, the standard deviations of thebeam image including the shadow are calculated from thehorizontal and vertical projection profiles. In the next step,
the difference map is created by subtracting the pixel
values of the original image from the 2D Gaussiandistribution, which is derived from the above standarddeviations. After that the negative values in the difference
map are replaced zero, the centre of the beam spot is
obtained from the addition map of the original image andthe difference map. This addition map is the anticipatedimage map in the case of without the steel sphere. The
centre of the steel sphere is also calculated by subtracting
the original pixel values from the addition map.

Figure 3: Photograph of the 2D beam imaging systemwith a steel-sphere-mounted acrylic phantom. This is
the setup for the beam alignment adjustment.

Figure 4: Typical image of the shadow on the 2D beam
spot profile. It is created by the steel sphere located on
the reference axis.

Figure 5: Algorism to calculate the beam misalignment.

MEASUREMENT RESULTS
The measurement of the time stability of theunscanned-beam position and size was carried out at ascanning beam course in the HIMAC treatment room. The
2D beam imaging system was set so that the phosphor
screen corresponded with the isocenter. The carbon-ion
beam was provided from the HIMAC synchrotron using
the multiple-energy operation with extended flattops. The
beam-energies were 430, 400, 380, 350, 320, 290, 260,230, 200, 170 and 140 MeV/u. The rate of the extracted
beam was 1 × 108 particles per second.
The beam was extracted for 20 seconds on eachenergy-flattop, and its position and size were measured
with the frequency of 50 Hz all the time. The result of thetime stability measurement is shown in Fig. 6. The
fluctuation of the beam position was observed during
extraction. The horizontal and vertical position
fluctuations were in the range of 0.4 mm and 0.2 mm,respectively. The horizontal fluctuation was larger thanthe vertical one, and this is because the horizontal beam
position is strongly affected by the current fluctuation of
the synchrotron magnets due to the extraction mechanism.
On the other hand, the slow drift of the beam position was
not observed owing to the feedback control system of thescanning magnets. The measurements of the unscannedbeam position and size stability were carried out at otherthree courses. Their results were equal to that indicated inFig. 6.

Figure 6: Time stability measurement of theunscanned-beam position and size.

ISBN 978-3-95450-119-9
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Proceedings of IBIC2012, Tsukuba, JapanThe beam alignment was checked using the 2D beamimaging system and the steel-sphere-mounted acrylic
phantom at the four beam courses. The calculation of thebeam misalignment is performed by the beam image
analysis program, as shown in Fig. 7. This program
derives the centres of the beam spot and the shadow
created by the steel sphere from the algorism indicated inFig. 5. The beam misalignment is calculated from thedifference between the two centres. The results of thederived misalignments are shown in Fig. 8. They weremeasured for the eleven beam-energies as the timestability measurement of the unscanned-beam positionand size. The beam misalignments were less than thetolerance of 0.5 mm in all the scanning beam courses.
The beam alignment measurement is routinely carriedout at NIRS. If the misalignment is over the tolerance, thecurrent of the steering magnets placed in the upstream ofthe scanning magnets is adjusted so that the beam spot
centre corresponded to that of the steel sphere.

MOPB78CONCLUSION
A 2D beam imaging system with a terbium-doped
gadolinium oxysulfide (Gd2O2S:Tb) phosphor screen andhigh-speed CCD camera has been used to measure thebeam spot for scanned carbon-ion therapy at NIRS. Thesystem enables us to obtain one image of the beam spot
every 20 milliseconds. The small fluctuations of theunscanned-beam spot position and size were confirmed in
the isocenter to verify the time stability of the delivered
beam for scanning irradiation. The beam imaging system
also functions as a beam alignment adjustment system bysetting a steel sphere at the isocenter. For quality
assurance, the beam alignment is routinely checked byobserving a shadow of the steel sphere on the beam spot
image, and it was confirmed that the misalignment of thebeam was smaller than the tolerance of 0.5 mm.

ACKNOWLEDGMENT

REFERENCES[1] Y. Hirao et al., Nucl. Phys. A 538, 541 (1992).
[2] T. Furukawa et al., Med. Phys. 37, 5672 (2010).
[3] M. Tomizawa et al., Nucl. Instr. and Meth. A 326, 399(1993).
[4] Y. Iwata et al., Nucl. Instr. and Meth. A 624, 33 (2010).
[5] E. Takeshita et al., in: Proceedings of IPAC’10, Kyoto,Japan, 2010, 3218.

Figure 7: Screenshot of the beam image analysis program.

Figure 8: Measurement results of the beam misalignment.

ISBN 978-3-95450-119-9
Transverse Profiles, Screens & Wires Monitors

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○The authors would like to express their thanks to themembers of the Accelerator Engineering Corporation(AEC) for their skillful operation of the HIMAC
accelerator complex. This work was carried out as a partof the Research Project with Heavy Ions at NIRS-HIMAC.