JAERI-Conf 95-021

3a4-3
PHYSICAL STUDIES OF BEAM DELIVERY SYSTEMIFOR PROTON AND HEAVY ION TREATMENT
Akio Higashi, Tatsuaki Kanai", Hiromi Tamura", Masahiro Endo",
Akifumi Itano, Hiroshi Karashima and Yoshio Hishikawa
Charged Particle Treatment, Public Health and Environment Department,
Hyogo Prefectural Government, Kobe, Hyogo, Japan*Research Center of Charged Particle Therapy, National Institute of Radiological Sciences,Chiba, Japan
Abstract
We report design and optimization of beam delivery
system for proton and heavy ion particles, which will be
constructed in charged particle therapy facility in Hyogo
Prefecture. Penumbra sizes were calculated for different
systems to optimize device parameters under considering
effects of multiple scattering and energy loss of beam. It was
found that penumbra size of carbon ion beam was about threetimes smaller than that of proton, and that devices which
affect physically in particle delivery system should be placed
as upstream as possible to reduce penumbra size.
1. Introduction
It is reported here the results of physical investigation forbeam delivery system using proton and heavy ion beam toconstruct facility of charged particle therapy in Hyogo
Prefecture. In this facility, the three fixed beam ports which
deliver proton and heavy ion (helium and carbon) beam and arotatable gantry using proton are planned for treatment issue.
The beam delivery system which include such beam ports andgantry have not yet been constructed in Japan.
In the design of the beam delivery system, importantdevices are wobbler magnets, scatterers, ridge filters andrange shifters, because they scatter beam particles and affectpenumbra size. Position of these devices are investigated,assuming sizes of the devices which are slightly different fromthose used in Heavy Ion Medical Accelerator in Chiba(HIMAC) in National Institute of Radiological Sciences(NIRS) but structures of them are almost same as those inHIMAC. Penumbra of the delivery systems which weredesigned under specification were calculated. Since it isnecessary for accurate treatment that penumbra are to be as
small as possible, the goal of their size is about 2 mm.
2. Design of the beam delivery system and calculation forpenumbra designed
In calculating penumbra, the method developed in NIRS
was employed. Physical meaning of the calculation is asfollows!). Particle distribution is obtained as an approximatedsolution of the Boltzmann equation. In this case, multiple
scattering and energy loss are assumed when particles passthrough material in the devices of the delivery system. Fermi
and Eyges showed that radial deflection y and its angle <1> ofthe particles passing through materials under thoseassumptions could be calculated2). At the formulation, thequantity D which corresponds to the emittance squared in thebeam transport are to be written as

where <yZ:> and <q?> are the variances ofy and <j>, respectively
and <y<j» is cross term. It is important that effects of multiple
scattering and energy loss are already included in them. D iscalculated by <yZ:>, <<j>Z:> and <y<j» which are values on thematerial at each grid point segmented small enough along thebeam direction. Penumbra Pso...2o are to be written as follows;
Pso...20 = 1.68 Oo~,Lc,oo~,2 = D/<y2> + <y2> ( <y<j»/<y2>- 1/Lwi,

where Lc and Lw are the distances from isocenter to collimatorand wobbler magnet, respectively. Here the quantitative
definition of the penumbra is the distance between 80 % dosepoint compared to the dose ofisocenter and 20% point ofthat
on lateral dose distribution. This calculation method havemuch advantage for its analytically solved results than theusually used Monte-Carlo method, because it is easy tounderstand about the effect of positions of the devices andmaterials and takes much less times to calculate. For example
of position of the devices, it is important to place a collimator
to a patient body as close as possible. The results of thismethod for beam delivery systems in HIMAC have been foundto agree within 1 mm or less, which is about 10 % of the
penumbra size measured by X-ray photograph.
Figure 1 shows an example of fixed beam port used for
proton and heavy ion beam. Calculations for this system weremade for 165 and 230 MeV energies of proton and 320 MeV/u
energy of carbon ion beam. Figure 2 shows an example ofrotatable gantry for proton only. Calculations for this gantry-445-JAERI-Conf 95-021

were made for 165 MeV energy of proton beam. Ma'<imumrange of each kind of beam through these delivery systems areabout 17 em for both 320 MeV/u carbon ion and 165 MeV
proton beam and about 30 em for 230 MeV proton,respectively. Distance, Lc, between collimator and isocenter
was set to be 40 em.
3. Results and discussions
Firstly, for the fixed beam delivery system shown in theFigure 1, field radius, maximum range and spread out Bragg
Peak (SOBP) width were set to be same in carbon ion andproton beam. The maximum range and SOBP width weretaken as 17 em mentioned above and 6 em, respectively.
Dependence of penumbra size on field radius which wasadjusted by scatterer thickness and radius of wobbler wereestimated. It was found that the dependence was small but theabsolute value of penumbra in the case of proton beam wereabout three times larger than those of carbon ion beam in thesame range. The results are shown in Figure 3. It was also
demonstrated that the heavier ion, such as carbon, can perform
more accurate treatment.
In next step, effects of scattering in compensator andinside of a patient body were estimated. For simple studies,water was placed after a collimator and its thickness waschanged. Water is often used in estimation of scattering incompensator and human body, because density of water isalmost the same as polyethylene which is the material ofcompensator and as a human body. Figure 4 shows thedependence of penumbra size on water thickness in thiscalculation. It was found that the size of penumbra caused byscattering in the body was smaller than that by the delivery
system. It was also shown that the effect of scattering in thecompensator was much smaller than that in the body becauseof difference of path length. As is in the Figure 3, absolute
penumbra size of proton beam in the Figure 4 was also foundto be about three times larger than that of carbon ion.
In the last study, penumbra size was calculated bychanging the position of ridge filter in the gantry shown in theFigure 2 for proton. The results are shown in Figure 5. It canbe seen that the distance between scatterer and ridge filter islonger, the penumbra size is larger. The dependence ofpenumbra size on the positions of devices which scatter beam
particles is not only for ridge filter position but also for thoseof scatterer and range shifter. Scatterer, ridge filter and range
shifter should be placed closely each other and as upstream aspossible, and the distance from range shifter to the isocenter is
to be taken as long as possible. However, it is much difficultin working upon the ridge filter, because the thickness andgaps of the ridges should be much smaller. Selection formaterial which is easily worked upon or oscillating bar ridge
filter and rotating spiral ridge filter are to be taken into
account.
Three-dimensionally conformed (3-D) irradiation system
is under planning in this facility, because it has muchadvantage for dose localization. Searching parameters ofdevices in beam delivery system for adjusting irradiation field
can also be simplified using the 3-D system. Dynamic 3-D
system without devices which scatter particles and absorb
energy of the beam will be installed in this facility to use
maximum range of initial beam energy and minimum
penumbra size by initial emittance.
References
1) H. Tomura et al., Abstract Book of the Fifth Workshop on
Heavy Charged Particles in Biology and Medicine, GSI(1995).
2) L. Eyges, Phys. Rev. 74 (1948) 1534.

Wobbler magnet (X)Wobbler magnet (Y)Compensator

100cm

Scatterer

Figure I. Fixed beam delivery system for proton and heavy ion beam. Total length shown here is 940 em.

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Wobbler magnet (X)Compensator~4lOOcm

Scatterer

Figure 2. Rotatable gantry for proton beam . Total length shom1 here is 350 em.

o. 8...-----·-11"0. 75 0. 6'-./ 0. 5~ 0. 46 0. 30010.20... 0. 1~~12C320MeV/u......... P 165MeV--.- p 230MeV/'o.

002015
10Radiation field radius (em)
5Figure 3. Dependence of penumbra size on field radius at 6 em of SOBP width.
~~12C

320MeV/uport

0. 8~~12C"5 0. 6

320MeV/uwater'-./--.-~12C~ 0. 4total
165MeV port~/'o.

I0oo, 0. 2~P__...- ...--1111!-llll!!

0...

---*-- p 165MeV water

0101614
12Water thickness (em)320MeV/u18---- p 165MeV total

Figure 4. Dependence of penumbra size on water thickness. Water is placed dom1stream of collimator.

0. 8"5 0. 6'-./I~P 165MeVI~ 0. 4/'o.

I0oo, 0. 20...

00200
150
100
50
Distance from port entrance
to R. F. position (em)Figure 5. Dependence of penumbra size on ridge filter position in gantry for proton.

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