JAERI-Conf 95-021

3a4-4
DOSIMETRY SYSTEM FOR HEAVY-ION RADIOTHERAPY
Tatsuaki KANAI, Yasuyuki FUTAMI, Hiromi TOMURA and Naruhiro MATSUFUJI
Division of Accelerator Physics and Engineering,
National Institute of Radiological Sciences,9-1, Anagawa 4-chome, lnage-ku, Chiba-shi, 263 CIDBA JAPAN.
facility is presented and discussed in detail.
Abstract
This paper describes a dosimetry system forthe heavy-ion radiation therapy installed at
HIMAC facility. Daily check of the dosemonitor, a species of the accelerated particles,
their energy are carried out every morning. And
the dose calibration of the monitor for eachpatient condition are also carried out everymorning.
The variations of the monitor
responses were less than 2 % during 3 monthsof treatment term. In order to shorten the time
for the check, a simplified dosimetric system
for the dose calibration of the monitor wasinstalled from this autumn.
1. Introduction
A radiation therapy using a carbon beam
has been started at HIMAC facility from lastyear[1].
The heavy-ion radiotherapy is
expected to be superior to the conventionalradiation because of its excellent doselocalization
and
its
high
biologicaleffectiveness.
Then, it is important toirradiate patients as previously planned by atreatment planning system. Around 18 times ofexposures are given to the same patient in theheavy-ion radiotherapy. And it takes over one
month to finish the treatment. It is necessary toguarantee daily exposure dose to be the plannedexposure. In order to guarantee these things,
the check of the dose monitor, a species of theaccelerated particles, and their energy arecarried out every morning. The dose calibrationof the monitor for each patient condition are
also carried out every morning. These checkis very important for the assessment of theirradiation port.
In this paper, the dosimetry system for theheavy-ion radiation therapy installed at HIMAC

BEAM

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10PatientITFig. 1. Illustration of the therapeutic beam port
2. Monitor system
In the HIMAC irradiation course for theradiation therapy, 5 monitors are installed.
Fig. 1. shows the illustration of the therapeutic
beam port of HIMAC and indicates the locationof the monitors in the beam course[2].
Monitors which are shown as up-stream 1,2 anddown-stream 1,2 are the parallel plate
ionization chamber. The up-stream chamber
has two signal electrodes and three high voltage
electrodes of the parallel plate ionizationchamber. The one signal is fed into a high--448-

JAERI-Conf 95-021

speed amplifier and used for monitoring thetime structure of the beam. The other output
signal is fed into IIF converter which output apulse for every 1000 pC. The pulse train isinput to a preset counter which is connected toa beam shutter for controlling the exposure
dose to the patients. We have another dose
controlling monitor, SEM ( SecondaryEmission Monitor ). We have anxiety thatbeams may have a big spike in the beam spill.
In that case, accurate dose can not be measured
by the ionization chamber monitor because ofrecombination effects in the big spike. Then,
we used the secondary emission chamber forthe back-up of the ionization chamber.
At just up-stream of the beam collimator, aThe
down-stream monitor was placed.
monitor is also a parallel plate ionizationchamber, and has one signal plane which is
sandwiched by two high voltage electrodes.
On one side of the signal plane, there are 29
signal electrodes of 20 mm in diameter whichThis
are distributed in the guard earth.
ionization chambers are used for roughly
monitoring the uniformity of the irradiation
field. The other side of the signal plane of thedown-stream chamber is a signal electrodewhich cover the whole uniform irradiation field.
This output is used as another back-up monitor
of the exposure dose.
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3. Dose calibration system
Standard procedure of the dose calibration
using the above system is as follows;
1) Depth dose distribution of a spread Bragg
peak which has 6 em width is measured by astandard dosimeter. The standard dosimeter isa parallel plate ionization chamber, which isfabricated by Far West Technology, Co.,. We
use a binary filter to change a depth in water inthe measurements of the depth dose
distributions. The binary filter consists of 9
PMMA (Polymethyl methacrylate; Lucite)
sheets of 0.5, 1, 2, 4, 8, 16, 32,64 and 128 mm
thickness and 10 x 10 em area. Inserting thesesheets in the beam course, the target thickness
is changed. The thickness of the PMMA istransferred to water equivalent thickness.
2) The measured dose distribution is comparedwith a calculated depth dose distribution.
From the comparison, variations of the residual
range of the carbon beam at the irradiation site
and absolute value of the output of the standard
chamber are recorded for the check of the beamenergy, and the species of the particles. Fig.2
shows the variation of the range of the 350
MeV/u carbon beam checked by the350 MeV/u carbon(treatment room A)..........

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Using these monitor system, the irradiationdose to the patients are controlled andmonitored.

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3.2 1 o- 505101520253023Date ( July 1995 )Fig. 2.

Daily change of the monitor calibration factor and the peak shift-449-

JAERI-Conf 95-021

Thecompanson with the calculation.
variation of the dose at 0 absorber position in
the measurement of the depth dose distributionis also shown in the figure.
3) The information of the treatments, theexcitation current of the wobbler magnets,
thickness of the scatterer, sort of the ridge filter,
thickness of the range shifter and so on, aresend to the irradiation control computer. And
the devices are adjusted according to thetransferred information. The thickness of thebinary filter is set so as for the residual range ofthe carbon beam to be half of the width of thespread-out Bragg peak. The standard chamber
is set to the center of the spread-out Bragg peak,
center of the target volume in the simulated
phantom. Dose calibration of the monitor
chamber is again performed under thistreatment condition several times. Thesecalibration factors are used for the real
exposure to the patient.

distribution of that day.
From these simplified procedure of thedose calibration, it is possible to shorten thecalibration time without making the accuracyof the dose calibration worse.
References[1] Tujii, H., et al., XXI PTCOG meeting(1994).
[2] Kanai, T., et al., XXI PTCOG meeting(1994).

4. Simplified Calibration
In the actual clinical trials, it takes about 30
minutes for taking the depth dose distribution,
and about 5 minutes for taking each dose
calibration factor of each patient. It takes anhour and half for the total calibration procedureof 12 patients. This calibration procedure isnecessary whenever the initial energy of thecarbon beam is changed. We use 290 and 350
or 400 MeV/u carbon beams in one day at
IDMAC clinical trials. At that case, it takes 3
hours for the calibration. In order to shorten
the calibration time, we are going to adapt asimplified calibration procedure from thisautumn. The procedure is as follows~
1) The depth dose distribution is measured by amulti-layer ionization chambers m oneexposure.
2) The dose calibration measurement for theeach patient condition is performed only at theThe ratio of the dosefirst treatment.
calibration factor to the entrance dose of thedepth dose distribution is recorded and used for
The dose calibration
the next treatments.
factor for the later treatment is obtained by theratio times the entrance dose of the depth dose-450-