卒業論文

題目：Basic research for

development of modulated spiral beam scanning system

in particle therapy

(粒子線治療における変調型スパイラルビームスキャニング

照射法の開発へ向けた基礎的研究)

大阪大学医学部保健学科放射線技術科学専攻

（指導：医用物理工学講座 小泉 雅彦 教授）

05C12018 坂田 愛美

（平成 27 年 12月 10 日提出）

- 1 -

Abstract

Purpose: The purpose of this research is to propose modulated spiral beam scanning system to

improve some problems of the spot scanning irradiation system for particle therapy, and to

demonstrate and evaluate this new method. We use electron beam in the present study because

electron can be used easily due to its light mass, and no secondary neutron.

Materials and Methods: We made the device which deflected electron beam, and named

“Proef”. Beam was emitted horizontally, and Proef was set up downstream of gantry head of

linac and was made a vacuum. Electron dose distribution was measured with darkness of

GAFCHROMIC FILM EBT2. Dose rate was 900 MU/minutes, and the irradiation time was 30

minutes. First, one spot to determine beam profile and another spot added -5 kV and -10 kV to

y axis an electrode plate to deflect the beams were irradiated to the films. Second, 3-spot for

flat dose distribution and 8-spot irradiation to compare measured data with simulation data were

irradiated to films. Finally, the electron spiral beam irradiation was done with the contour data

taken from a clinical patient tumor which was expanded in Fourier series, using wave form

generator and high-voltage power supply.

Results: In the first experiment, beam spot was shifted by 24 mm±1 mm from center of the

film to –x direction unexpectedly. When the voltage was generated, shift for -y direction was

15 mm±2 mm per 5 kV. Full width at half maximum (FWHM) was 24 mm±1 mm. The

difference between simulation data and measured data in one spot was 1.8 mm±0.3 mm and

the 8 spot irradiation difference was 1.4 mm±0.2 mm. That of spiral scanning was nearly 1

mm only in 70% darkness but there were large errors in lower darkness.

Conclusion: The newly device for the modulated spiral scanning irradiation of electron was

developed. It gave the result that measured data agreed with simulation data well in the

distribution of the darkness. This results imply that dose distribution we want can be realized,

and also suggest the possibility of this method for particle therapy.

1. Introduction

Particle therapy is one of the external beam radiotherapies using beams of protons,

neutrons, or heavier ions for cancer treatment. Ion beam has a Bragg peak where absorbed dose

is enhanced. Number of proton therapy facilities is rapidly increasing worldwide owing to the

well-known advantage of low integrated dose and ability to spare healthy tissues1). Broad beam

irradiation and pencil beam scanning (PBS) are the commonly used techniques for delivering

proton beams2). Broad beam and uniform scanning methods require patient-specific devices

such as a compensator and aperture.

In the PBS technique, the target is “painted” by sequential superposition of many

small-size PBS spots, which are magnetically directed to the target. The PBS technique has no

patient-specific devices that are necessary in broad beam irradiation. It is more economical and

the number of secondary particles including neutrons is much less than the conventional broad

beam method.

As the PBS technique, there are other two techniques. One is dynamic or continuous

line scanning whose beam is scanned continuously through the iso-energy layers3). The other

is raster scanning whose only difference to discrete spot scanning is that the beam is not

stopped when moving to the next spot4-5). It is a hybrid of discrete spot and continuous line

scanning. The problems using these two method are that they take longer treatment time with

above discrete spot scanning than with continuous irradiation, and have the rough edge in

beam turning point.

We proposed “Modulated spiral beam scanning system6)”, and focused on it to

improve the above problems of the methods above. The feature of the system is that a beam

draws a spiral trajectory based on the patient tumor shape, so the field edge was smooth. In

addition to this, continuous irradiation makes the treatment time short. The purpose of this

- 2 -

research is to demonstrate it in order to apply this method to particle therapy in the future.

Electron can be used easily due to its light mass, and no secondary neutron. We developed a

new device for modulated spiral scanning with use of electron beam, and investigated the

basic properties of it.

- 3 -

2. Materials and methods

2.1. Device

The device we made to deflect electron beam is named “Proef” (the proof device

for electron spiral beam scanning feasibility). Electron beam was used in the device because

electron can be used easily due to its light mass, and no secondary neutron. The geometry is

shown in Figure 1. The whole length of Proef is 1805 mm and its inside is vacuum by vacuum

pomp (Alcatel 2004 2004A Dual Stage Rotary Vane Vacuum Pump, Ideal Vacuum Products

LLC.). The degree of it was from 0.15 Pa to 3.0 Pa. Electron beam from linac is collimated

inφ= 5 mm and deflected by two sets of electrode plates. Their gap is 29 mm. One of the

horizontal plates (x direction) is connected to the high-voltage power supply (Model-502, Pulse

Electronic Engineering Co., Ltd.). The voltage is modulated by wave form generator (33522B

Waveform Generator Review, Agilent). They are recorded by logger (midi LOGGER GL900,

GRAFTEC Corporation). It also has two windows; from the upper one the irradiated

radiochromic film can be seen, and the lower one is lighting window. The film size was 76×76

mm.

Figure 1. The geometry of proef (side view)

- 4 -

2.2.1. Basic experiment

Electron beam of 5 MeV energy from a linear accelerator (ONCOR impression,

Siemens AG) at Osaka University Dental Hospital was emitted horizontally, and Proef was set

up downstream of gantry head of the linac head as shown in Figure 2. The dose rate was 900

MU/minute. Electron dose distribution was measured with GAFCHROMIC FILM EBT2

(Ashland Inc.).

The first irradiation was one spot irradiation to determine beam profile. The irradiation

time was 30 minutes. Next, we add -5 kV and -10 kV to a y axis electrode plate to deflect the

beams and irradiated to the second and third films respectively. The darkness of Gafchromic

film was read by scanner (GT-X970, EPSON) at the Osaka University Hospital, and analyzed

by the software MATLAB (The MathWorks, Inc.). MATLAB was also used for fitting by

Gaussian function (Eq.(1)) and calculated FWHM by Eq.(2).

G(x) = 𝑁𝑒𝑥𝑝 (−(𝑥−𝑥0)2

2𝜎2 ) (1)

FWHM = 2.35σ (2)

We had second spot irradiation and got a new beam profile. Simulation was done by

2D-Gaussian function. It was compared with the measured ones by a root mean square (RMS)

of the differences of the distance [mm] from the center to the edge of the darkness distribution

in the same level by Eq.(3).

RMS[x] =√1

𝑁∑ (𝑥𝑖)2𝑁

𝑖=1 (3)

- 5 -

Figure 2. Proef set up downstream of gantry head of the linac head

- 6 -

2.2.2. The second experiment

The 3-spot irradiation was done with 40 minutes/spot to investigate beam flatness. Its

planning spots are shown in Figure 3 (a). Flatness was evaluated with next formula by Eq.(4)6).

The flatness was measured from y = -12 to y = 31 (the distribution was nearly flat).

F = 𝑀−𝑚

𝑀+𝑚×100 [%] (4)

F: flatness, M: maximum dose, m: minimum dose

Next, eight spots were irradiated on another film for 30 minutes/spot. Its planning spots

are shown in figure 3 (b). The distribution of electron dose on the film was simulated in advance,

and was compared them with the measured ones. The difference was evaluated with distance

[mm] from the center to the edge of the darkness distribution in the same level of it.

(a) 3-spot irradiation (b) 8-spot irradiation

Figure 3. (a) shows the points of 3-spot irradiation which were planned to made its

distribution flat. (b) shows the points of 8-spot irradiation which were planned to correspond

with simulation data.

-10

0

10

20

30

-1 0 1

y[m

m]

x[mm]

-40

-30

-20

-10

0

0 10 20

y[m

m]

x[mm]

2.2.3. The spiral beam scanning

Spiral beam scanning needs an analytic form of function of tumor contour. Fourier

series is one of the method to express contour data with function. The tumor contour data of a

patient were presented by the polar coordinates, and transformed them to the formula developed

by Fourier series.

The planning irradiation trajectry based on this Figure 4 was inputted to wave form

generator and the wave data was transferred to high-voltage power supply. The figure was

scanned spirally from the center to outside, and vice versa repeatedly. The irradiation time was

two and a half hours. The difference between simulation and measured data was evaluated with

distance [mm] from the center to the edge of the darkness distribution in the same level of it.

Table 1. Input of high-voltage power supply

sample Rate

[Samples/sec]

Amplitude

[Vpp]

Offset

[V]

Channel 1 (x direction) 10 2.72 1.50

Channel 2 (y direction) 10 3.20 1.75

Figure 4. This figure shows the spiral trajectory from contour data. Vertical and horizontal

line express the figure size [mm]. The center of figure is 0 [mm] (both x and y direction).

y[m

m]

x[mm]

3. Results

3.1 Basic experiment

In the first experiment, beam spot was shifted by 24 mm±1 mm from center of the

film to –x direction unexpectedly. When the voltage is generated, it was shifted by 15 mm±2

mm per 5 kV to -y direction. The darkness of each spots are shown in Figure 5. These beam

profile in the first experiment is summarized in Table 2. It indicates that FWHM in x direction

in of (b) and (c) are larger than (a), and Maximum darkness of (b) and (c) are higher than (a).

Table 2. The beam profile of Figure 5

center of spot

x[mm]

center of spot

y[mm]

FWHM

x [mm]

FWHM

y [mm]

Maximum

darkness

(a) -22.5 4 22.8 23.2 1208

(b) -22.1 -13.5 26.4 24.7 1642

(c) -23.4 -26.7 25.8 23.4 1542

(a) 0 kV (y axis) (b) -5 kV (y axis) (c) -10 kV (y axis)

Figure 5. Measured spot irradiations when 0 kV, -5 kV and -10 kV were added to y direction

electrodes respectively. Vertical and horizontal axis are position from center of Gafchromic film.

The gray scale bar shows darkness of the film.

3.2. The second experiment

The three spots were irradiated at 24.5 mm, 12 mm, and -0.5 mm from y = 0 [mm], and

were shown in Figure 6 with three horizontal dashed lines. The sizes of spots (FWHM, x

direction) in Figure 6 (a) were 31.7 mm, 32.8 mm, and 41.6 mm. The three spots were shifted

to –x direction similarly to the first experiment. Flatness on a vertical white line (x = -25 [mm])

in (a) is 4.8% (≦10%) as shown in Figure 6 (b). The range for the flatness calculation was from

y = -12 to y = 31.

(a) 3-spot irradiation (b) darkness distribution along cutting plane

on vertical white line in (a)

Figure 6. (a) shows the 3-spot irradiation to investigate the distribution flatness in y direction.

Dashed lines shows the y positions of irradiation spots. Vertical white line in (a) is x = -25 [mm],

and it passes through center of each spot. The gray scale bar shows the percentage of darkness

of the irradiated film (maximum darkness is 100%). Vertical and horizontal axes are position

from center of Gafchromic film. (b) shows darkness distribution along cutting plane on vertical

white line in (a). Vertical axis is darkness, and vertical two lines show the range for flatness

calculation from y = -12 to y = 31. A thick curve for the range shown in (b) is a fitting curve of

measured data. Horizontal axis in (b) represents y axis in (a).

y

A spot irradiation in Figure 7 (a) shows comparison between simulation data and

measured data. The difference was evaluated with distance [mm] from the center to the edge of

the darkness distribution in the same level of it. The difference (mean ± standard deviation) is

1.8 mm ± 0.3 mm.

Figure 7 (b) is the same as Figure 7 (a), but for 8-spot irradiation. The difference (mean

± standard deviation) is 1.4 mm ± 0.2 mm in 8-spot irradiation.

Both of these spots were shifted to –x direction similarly to the first and second

experiment.

(a) One spot irradiation (b) 8-spot irradiation

Figure 7. The darkness distribution of the irradiations, and the white and black contour curves

are simulation data. The gray scale bar shows the percentage of darkness of the film by the

irradiation (maximum darkness is 100%). Vertical and horizontal axes are position from center

of Gafchromic film. The gray scale bar shows darkness of the film.

3.3 The spiral beam scanning

Figure 8 shows the darkness distribution of the spiral irradiation, and contour curves

is the simulation data. The difference between the measured data and simulation data was nearly

1 mm only in 70% darkness, while the differences in the lower level were larger than that of

70% (Table 3). Figure 9 shows the voltage waveform was constant for the experiment.

Table.3 Differences between darkness distribution and simulation contour curves

darkness 30% 50% 70% 90%

difference 7.1mm 2.7mm 1.1mm 1.9mm

Figure 8. This figure shows the darkness distribution of the spiral irradiation. The white and

black contour curves are simulation data. Four missing corner was track of the tape to fix the

film to Proef. The gray scale bar shows the percentage of darkness of the film by the irradiation

(maximum darkness is 100%). Vertical and horizontal axes are position from center of

Gafchromic film.

Figure 9. This figure shows the voltage waveform in the spiral experiment (the graph was

shortened in the middle part). The figure shown in Figure 4 was scanned spirally from the center

to outside and vice versa repeatedly when this voltage was added. The vertical axis is voltage

[kV], and the horizon axis is the time we experimented in. The green wave shows the voltage

outputted in channel 1, and the orange wave shows the voltage outputted in channel 2.

Channel 1

Channel 2

Time

Volt

age

[kV

]

4. Discussion

The beam profile and the relationship between position shift and voltage were obtained.

It was found that the darkness and FWHM of x direction were larger when high voltage to –y

direction was added as shown in Figures 5 and 6 (a). It implies that the spot size was larger than

that without voltage, and its shape shrank for y direction. We guess that the electron density of

the spot was more concentrated.

Dose flatness was obtained in 3-spot irradiation. Sifts of the three spots in –x direction

were also observed in the former experiment. This was caused because collimator in Proef was

not attached perpendicular to the vacuum duct.

The dose distribution in Figure 7 agreed with the simulation data well. On the other

hand, measured data and simulation data in the spiral experiment (Figure 8) were different at

the low-dose region. One of the reasons is owed to inaccuracy of the simulation beam profile

in Figure 7 (a). It is assumed that our simulations were simply based on 2D-Gaussian function.

The spot irradiation generally follows a Gaussian distribution except at the low-dose region

where there is a broad non-Gaussian tail or ‘halo’5).

There were large errors in lower darkness of Gafchromic film in the spiral scanning

experiment. In the electron beam, it was found that the edge was less sharp and complex figures

ware difficult to express because of the large spots. It is also difficult to prepare stable high

voltage sources and vacuum device. The demonstration for spiral scanning was succeeded in

even under this condition.

5. Conclusion

The newly device for the modulated spiral scanning irradiation of electron was

developed. It gives the result that measured data agreed with simulation data well in the

distribution of the darkness. The results imply that dose distribution we want can be realized,

and also suggest the possibility of this system for particle therapy.

6. Acknowledgments

This work has been done in collaboration with Prof. M. Fukuda and Mr. S. Hara

(Research Center for Nuclear Physics, Osaka University), Dr. M. Takashina (Training

Plan for Cancer Professionals, Graduate School of Medicine, Osaka University) and Koay

HuiWen (University of Science-Malaysia).

Technical support has been given from Dr. I. Sumida (Osaka University Hospital) and

Mr. H. Kitamori (Osaka University Dental Hospital).

References

1. DeLaney TF, Kooy HM, Proton and Charged Particle Radiotherapy, Lippincott

Williams & Wilkins, Philadelphia, 2008.

2. Both S, Shen J, Kirk M et al., Development and Clinical Implementation of a Universal

Bolus to Maintain Spot Size During Delivery of Base of Skull Pencil Beam Scanning

Proton Therapy, Int J Radiation Oncol Biol Phys, 90(1), 79-84, 2014.

3. Lomax A, Kirch K, Graeff C, Towards the treatment of moving targets with scanned

proton beams: Experimental verification of motion mitigation techniques with Gantry

2 at PSI. Available from:

http://e-collection.library.ethz.ch/eserv/eth:7833/eth-7833-02.pdf

4. Haberer T, Becher W, Schardt D and Kraft G, Magnetic scanning system for heavy ion

therapy, Nucl Instr Meth Phys A, 330(1-2), 296, 1993.

5. Furukawa T, Inaniwa T, Sato S et al., Performance of the NIRS fast scanning system

for heavy-ion radiotherapy, Med Phys, 37(11), 5672, 2010.

6. Fukuda M, Okumura S, Arakawa K, Simulation of spiral beam scanning for uniform

irradiation on a large target, Nucl. Ittstr. Meth. A396, 45-49, 1997.