JAERI-Conf 95-021

3a4-2
DEVELOPMENT OF 3-DIMENSIONAL IRRADIATION SYSTEM
FOR HEAVY-ION RADIATION THERAPY
Yasuyuki FUTAMI, Hiromi TOMURA, Naruhiro MATSUFUJI and Tatsuaki KANAIDivision of Accelerator Physics and Engineering, National Institute of Radiological Sciences,9-1, Anagawa 4-chome, Chiba-shi, 263 Japan

Abstract
A three-dimensional irradiation system using abroad beam was installed at HIMAC facility for aheavy-ion radiation therapy. The thickness of thewedge absorber and the shape of the radiation field
made by the multi-leafcollimator were changed during irradiation in order to sweep the Bragg peak onlyin the target area. In this report we discuss thethree-dimensional irradiation system of HIMAC andalso results of preliminary irradiation tests using 12 Cbeams.

1Introduction

It is one of the most important goal of any radiotherapy to shape the treatment volume as exactly as
possible to the tumor as the target volume. In mostcases a sharp cutoff of the dose is needed in order tospare the surrounding healthy tissue or critical structures to a maximum extent. It is necessary to use
a flat-top depth-dose distribution extending several
centimeters, even tens of centimeters to encompass
solid tumors. For this purpose heavy ion beams aredelivered to the treatment volume by the beam scan. ning method [l] or the broad beam method. The
clinical port at HIMAC facility is designed by thelatter method and shown in fig.l. The system con-(c~(b)....~~

beam(d) (e)~ . / (f) !9>

11[0II It Target

compensator (g) and a monitor system. The wobbler magnets and the scatterer are used for spreading
the beam uniformly over the radiation field[ 2J. For
2-'dimensional irradiation, the ridge filter is used tospread the sharp pristine Bragg peak. A spread-out
Bragg peak (SOBP) is made by superposing shifted
Bragg peaks with suitable superposing ratio. Irregular shape of radiation fields are obtained by blocking the uniform beam with the multi-leaf collimator.
Fine adjustment of heavy ion range in patient is performed by the range shifter. And the compensator isused for adjusting position of the distal part of thetarget.
For a tumor conform treatment, a 3-dimensional
irradiation using a synchronized fast sweeping technique is also possible in HIMAC irradiation systemand presently developed. This novel technique implies some important advantages. The 3-dimensionaldose distribution can be shaped exactly to the tumor
volume in order to prevent particles from depositing their biologically very effective dose outside thetarget volume. Using the sweeping technique, complex 3-dimensional scanning procedure is no longer
needed saving irradiation time. And the system canbe extended from the present 2-dimensional irradiation system reducing the costs. In the following theprinciples of the 3-dimensional irradiation are discussed and the first results obtained with the system
at HIMAC are presented.

2Method of 3D irradiation

Figure 2 illustrates the 3-dimensional irradiation
system installed in HIMAC. An uniform beam 1s
untrorm

I~beamMain Monita X-ray Tube,
Ught Localizer,

L.aSer Poirter

Figure 1: Schematic diagram of HIMAC irradiation
port. Devices labeled (a)""(g) are explained in text.
Multi-leaf

sists of a pair of wobbler magnets (a), a scatterer(b), a ridge filter (c), a range shifter (d), a pair ofwedge absorber (e), a multi-leaf collimator (f), aWedgeAbeorbercollimator

Figure 2: Illustration of Three-Dimensional Irradiation.

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made by the wobbler magnets and the scatterer in thesame way of the 2-dimensional irradiation. "Slightly"
spread-out Bragg peak (we call it s-SOBP in thispaper) is made by the ridge filter. The s-SOBP isshifted by inserting the wedge absorbers in the beamcourse. By combination of the two wedge absorbers,
the total thickness of the absorber is uniform in theradiation field. A SOBP that should be conformed toa target shape (we call it total-SOBP in this paper)
is made by superposing shifted s-SOBP by changing the absorber thickness during the irradiation withsuitable superposing ratio. During the sweep of thes-SOBP in the target region, needless part of the irradiation field can be cut by adjusting the irradiation
field with the multi-leaf collimator.
In order to realize this conformation therapy, we
have to develop the fast movements of the wedge absorbers and the multi-leaf collimator and their synchronism. Specifications of the wedge absorber andmulti-leaf collimator installed at HIMAC are shownin fig.3.
[Wedge Absorber]beam ~_~

a rarge shifteras~
20-130 mm
Thickness,,, ,,,,,,,,,,,,,,,,,,,., Changing Speed 'Z1 mm..S, ,,,,,,,,,,,,,,,,,,u,,,,,,,,,,,,;,,),!,,Q Accuracu 0.125 mm~~

li''' ' "' ' ' "' ;;! ''"'''''''"'""''''-

Sliding Wedge Absorber75
hight750
width270
Sliding Speed (max)1.25
Position accuracy

mmmmmm'smm

in Polymethyl methacrylate (PMMA) material. To
monitor the beam profile in the target material, aZnS screen was inserted between PMMA blocks and ,placed almost parallel but slightly tilted to the beamdirection. The movements of the wedge absorbers,
the multi-leaf collimator and also the beam profile
along the beam axis were recorded by video camera.
In each slice scintillation light from the Zns screen
lay almost within the target area and results weresatisfactory.
For the quantitative test of the performance of the3-dimensional irradiation, a depth dose distribution
was measured for 10 em width total-SOBP using 290
MeV /n carbon beam. The SOBP was designed tomake a uniform biological dose distribution[ 3J usinga thick ridge filter (ridge filter A). The 5 mm widths-SOBP was made by a thinner ridge filter (ridgefilter B) that was used to make a uniform biologicaldose distribution. Three types ofradiation fields weremade:(a) 10 em width SOBP directly made by 2D irradiation using the ridge filter A(b) 5 mm width SOBP made by 2D irradiation
using the ridge filter B(c) 10 em width total-SOBP made by 3D irradiation u.sing ridge filter B.
In each irradiation the physical dose was measured
with a standard ionization chamber. The preliminary results for the depth dose distribution are shownin fig.4. Open circles, diamonds and closed circles

SOBP by 3-d irradiation ~ ~~~~=i~cm

150 x 220 mm
Field Size
No. of Leaves 23 x 2Iron
Leaf Material
Pitch of Leaves 6.5 mm Posision accuracy 0.5 mm
80 mm..s
Leafthickness 140 mm Sliding speed

o.o 0~....__..........5::!:-'-............__._1,..,0............__._........,.=-=:...._;o;;;....._,;I20
Depth in Water

Figure 3: Specification of wedge absorber and multileaf collimator.

3<> sobp-Smm----cal-lOom-cal-Smm···-cal""3d10cm<-5mm

3 ·5[Multi-leaf Collimator]

Measurement of dose distribution

The first test of the 3-dimensional irradiation confirming with eyes was carried out using 290 MeV /ncarbon beam. The target volume was assumed tobe a ball of 7 em in diameter, which is embedded

Figure 4: Measured and calculated physical dose distributions.
represent measured physical dose of (a), (b) and(c), respectively. Lines are calculated depth dose
distribution[3J. The measured dose by 3D irradiation were slightly larger than those by 2D irradiation.
However, measured and calculated results by 3D irradiation were agreed among themselves. This showsthat the total-SOBP realized by 3-dimensional irradiation method can be improved by optimizing theshape of the s-SOBP.

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JAERI-Conf 95-021

To reproduce the 2D SOBP ((a) in fig.4) using the3D irradiation, a new ridge filter for the modulateds-SOBP of 5 mm in width was designed. Figure 5shows the calculated results for the physical dose distribution. It is seen that the total-SOBP of the modResnlls- ( a ) cal-lOcm (2D)----·(b) cal-5mm-2D( unifo:rm Bio:""dose)

3 _5 ,--~~.,.,.-~~,---,-~rll:!!~:mbio-dose)(c)[1] Th. Haberer et al., Nucl. Inst. Meth. A330
(1993) 296.

2.5II)..!!!
.s::. Cl) 1.0
D.

References-(e)mod-3d10cm<-5mm~ r-~~·-;'-~;;.:;,-~~-·

i j ::::g(e)(a)t(b) -··- . __ )._/s 0.5 ----------~d?

The authors would like to thank the other membersof Division of Accelerator Physics and Engineeringof NIRS for their helpful collaboration. They arealso indebted to the staff of Accelerator Engineering
corporation for their support.

----· (c)cal-Jd-(d)=dulated 5mmcu Gl 3.othe dose measurement methods.

•[2] H. Tomura et al., Proc. lOth Sympo. on Accelerator Science and Technology, Hitachinaka, October, 1995.

, . -------------··

14.5
1515.5Depth in Water (em)[3] T. Kanai et al., NIRS HIMAC report HIMAC003 (1992) 1.

Figure 5: Calculated physical dose distributions (a)for 10 em field by 2D only, (b) for 5mm uniform biological dose distribution, (c) for 10 em field by 3D
with (b), (d) for modulated !)mm field, (e) for 10 em·field by 3D with (d).
ulated ridge filter is closer to the 2D results (within± 4.6 %) than those of the ridge filter B. Needlessto say that the further study is needed to optimize
the shape of the s-SOBP and other test experiments
using the 3-dimensional irradiation method with themodulated ridge filters are planed. In order to evaluate the quality of the radiation field made by the 3dimensional irradiation (uniformity, penumbra), themonitoring systems for the dose distribution have tobe developed. For this purpose we have planed to develop a proportional chamber (for 1-d.imensional distribution), a silicon strip detector (for 2-dimensionaldistribution) and a pile of ionization chambers (for3-dimensional distribution).

4

Concluding remarks

An irradiation system of 3-dimensional conformation therapy was designed and installed at HIMAC
facility. The 3-dimensional irradiation for a ballshaped target of 7 em diameter was performed using
290 MeV/n carbon beam. The wedge absorbers andthe multi-leaf collimator were controlled satisfactorily during the irradiation for conformation therapy.
Preliminary results of a dose distribution of the 10 em
SOBP made by superimposing 5mm width SOBP using the sliding wedge absorbers was obtained to checkthe function of the wedge absorbers.
In order to improve this system, we plan to develop
the optimizing methods to design a ridge filter and-444-