The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

OVERALL QUALITY COMPARISON OF C-BAND AND X-BAND
MEDICAL LINACS.
M. El-Ashmawy, M. Uesaka, H. Iijima, T. Imai, N. H. Quyet
Nuclear Engineering Research Laboratory, University of Tokyo, JAPAN.
E. Tanabe, AET Japan, Inc.
INTRODUCTION
Some tumours can be effectively treated withcombination of low and high-energy photons or ofphotons and electrons. The single machine that can
provide both low and high energy x-rays together with awide range of electron energy is the electron linearaccelerators. Accordingly, the electron linear accelerator
has become the dominant device for radiation therapy
since it was introduced in the 1950’s. There are morethan 5500 electron Linacs are used for medical purposes
worldwide.
Most of medical Linacs use S-band frequency but such
Linacs cannot fit to modern advanced treatment
techniques such as Tomotherapy [1] and Stereotactic [2]radiotherapy, which allow physicians to locate the tumour
position during treatment and enable for beammodification based on real time analysis.
For that reason the need of new generation of electron
Linac that fit to new radiotherapy techniques becomes
necessary. C-band and X-band Linacs are promising new
generation of electron Linacs that fulfill the ambitions ofradiotherapists to use a fast, accurate, and cheap radiation
therapy tool.
In this proceeding we will highlight on the overall
comparison of S-band, C-band, and X-band medical
accelerator and special concern will be devoted to C-band
type because of its promising applicability in modern
treatment techniques.

disadvantage in medical S-band Linac, which necessitateslarge space for installation.
LINAC STRUCTURE

Fig. 1. Tomotherapy concept [3]S-BAND MEDICAL LINAC VS. C-BAND &X-BAND MEDICAL LINAC.
The S-band (3 GHz) has been the prevailing frequency
for Linacs in medical and industrial applications. This ismainly attributed to the availability of stable high power
microwave sources in S-band frequency range.
Unfortunately, S-band Linacs are usually long, e.g. only
4.5 MeV bi-periodic on-axis coupled structure has 74 cm
length [4], and 14 MeV bi-periodic on-axis structurecomposed of 30 accelerating cells with 29 coupling cells
is 145 cm long [5]. Such long, and heavy Linacs cannot
be used in the modern treatment techniques such asTomotherapy where the accelerator structure is built inrotating ring, see fig. 1 or Stereotactic therapy where theaccelerator structure is built in rotating arm, see fig. 2. It
is also worth to note that the RF power source (klystron,pulsar, and modulator) in S-band frequency is fairly stable
but it is seriously huge and expensive; this is anotherFig. 2. Stereotactic concept

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Accordingly, and due to the need of Linac structuresthat fulfil new treatment technology demands, higher RF
frequencies C-band (5.7 GHz) and X-band (11.4 GHz)
have been proposed for electron Linacs to be used in such
sophisticated treatment techniques. C-band and X-band
frequency have been chosen for new Linacs due to thefollowing advantages:••••

Compact size: leads to less weight, which enables
the Linac motion to be controlled more precisely.
Higher shunt impedance hence;
The beam energy is increased for fixed input RFpower
Higher breakdown voltage threshold levels:
consequently this leads to higher field gradient
between the accelerating cavities and hence;

For that reasons C-band and X-band are recommended
for new generations medical Linacs.

C-BAND MEDICAL LINAC VS. X-BAND
MEDICAL LINAC.
The earlier X-band Linacs uses magnetron as RF powersource. Such type of RF source in this high frequency
ranges is low powered and unstable. Recently X-band
klystron become available but still difficult to make them
operating reliable at higher peak power due to several
shortcomings such as s tighter mechanical tolerance andhigher power transmission loss. Meanwhile the HighEnergy Accelerator Research Organization (KEK) inJapan has developed C-band components and acceleratorsthat bring solution to the aforementioned problems. This
includes:•••C-band Klystron
Pulsed modulator for C-band klystronC-band accelerating structure

Also, since the accelerator structure will be built inrotary gantry, that is necessitates to use RF rotary joint tofeed the RF power from RF power source to theaccelerator structure; this RF rotary joint still is notavailable in X-band frequency while it is currently
available in C-band frequency.
Accordingly, and due to the several technicaladvantages of C-band frequency technology rather than
X-band, C-band Linac could be the best choice for thefuture application in radiotherapy.
Tanabe, et al. [6] proposed compact high power
accelerator for medical application, which utilizes thehybrid standing wave structure. Table 1 summarizes thedesign parameters of proposed structure.

SW structure because of its high shunt impedance, highphase stability versus temperature variations, short length,
and high efficiency of RF power conversion to electronenergy gain. C-band (5712 MHz) has been chosen to forthe advantages mentioned in the previous chapter. In
following chapter we will represent the simulation resultsof 9 MeV C-band, 24 cm long standing wave acceleratorsfor medical application as a case study for C-band Linac
design.
Table 1: specification of C-band accelerating structure
proposed by Tanabe et al.
Accelerator lengthStructure.

23Hybrid SW

Number of accelerating cavities.
Frequency.

95712

Energy (MeV).
Beam current.

10600

Beam pulse width (µsec).
Repitation rate.

4240

RF peak power (MW).

11140

Shunt impedance (MΩ/m).
Load line (MeV).

15-8i

SIMULATION RESULTS OF 9 MEV
C-BAND LINAC
The structure consists of 9.5 cells as follows: a) 3 onaxis coupling standing wave cells for buncher section.
The on-axis coupling cells has been chosen for thebuncher in order to suppress the effect of non axial
symmetric electromagnetic field in the cavity. b) 6.5 sidecoupling cells for the regular accelerating section.
Simulation is carried out using PARMELA andSUPERFISH codes.The beam hole diameter (2b) is 4 mmwith total length of cell (L) 2.62 cm; the gab (g) between
the two successive noses is 1.3 cm, which equal tol/4 λ of the wavelength of the resonance frequency. Fig. 3shows a schematic diagram of the resonance cavity.

g ≈ λ/4b

GENERAL STRATEGY OF THE C-BAND
ACCELERATOR DESIGNL ≈ λ/2

In this chapter and thereafter we will highlight on thedesign of the C-band medical Linac with describing acase study for a simulation of C-bnad structure. Insteadof travelling wave structure we chose the side-couplingFig.3. C-band resonance cavity
The cell structure has been optimized using SUPERFISH
code and the resonance frequency was calculated at π/2
mode. Table 2 represents the cavity parameters.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Table 2: Cavity parameters calculated by SUPERFISH
code

Table 3: specification of the simulated C-band
accelerating structure

Effective Shunt impedance

56.1 MΩ/mTotal structure length (cm)24Transient time factor

0.66

Number of buncher cavities

3 (On-axis coupling)

Quality factor

9991

6.5 (Side-coupling)

Peak to average ratio (Emax/E0)

2.99

Number of regularaccelerating cavities
Frequency

5712 MHz

Beam energy (MeV)

9

Beam pulse width (µsec)

4Effective Shunt impedance

56.1 M Ω/m

The simulation has been carried out taking inconsideration space charge effect with 105 electrons perbunch with 104 bunches per pulse. The beam pulse width
supposed to be 4 µ seconds. Fig. 4 represents the beamprofile in x-y plan and fig. 5 represents the energy
spectrum at the end of accelerator structure.

SUMMARY

Fig. 4. Beam profile in x-y plan at the end of Linacstructure

C-band and X-band Linac structure could be the best
choice in advanced radiotherapy treatment such as
Tomotheraby and Stereotactic because of its advantages
such as: compact, and high shunt impedance, highbreakdown voltage. X-band structure still shows someshortcomings in X-band Klystron, and in RF rotary joint
as well, consequently C-band is preferred to be applicablein the near future due to availability of stable RF powersource (klystron type).
Using SUPERFISH and PARMELA codes we carriedout preliminary simulation to optimize the cavity structure
at C-band frequency and this enable us to design hybrid
standing wave compact size, 24 cm long, electron
accelerator with final energy 9 MeV. The simulation
results indicate that more enhancements in cavity
parameters are required to increase the effective shunt
impedance, which is important factor in determining therequired power of the klystron.

REFERENCES

Fig. 5. Energy spectrum at the end of Linac structure
Table 3 summarizes the specifications of simulated Cband accelerator structure. Simulations results indicatesthat the structure needs some modification to make it
closer to the structure proposed by E. Tanabe, et al. intable 1.

[1] T.R.Mackie, et al. “Tomotheraby: A new concept forthe delivery of dynamic conformal radiotherapy”
Med. Phys. 20, 1709-1719 (1993).
[2] W. Lutz, et al “A system for sterotactic radiosurgery
with linear accelerator” Int. J. Radiation Oncology
Biol. 14, 373-381 (1988).
[3] http://www.psl.wisc.edu/tomo.html[4] N. Nepal, et al. “Design study on standing wave
linear accelerator” proceeding of PAC, 2802-2804(2001)[5] J. Qingxiu, et al. “14 MeV single section SW wave
guide for medical accelerators” proceeding of PAC,
1218-1220 (1997).
[6] E. Tanabe et al. “Medical Applications of C-Band
Accelerator Technologies” proceeding of LINAC,627-629, (1998).