2. This Code of Practice is based on a
calibration factor in terms of absorbed dose
to water of an ionization chamber in a
reference beam which is taken to be 60Co
gamma rays.
The Code of Practice applies to heavy
ion beams with atomic numbers between
2(He) and 18(Ar) which have ranges of 2–
30 g/cm2 in water.
The depth dose distribution of a mono
energetic heavy ion beam in water is shown
in figure
Introduction
3. For clinical applications of heavy ion beams
we have to generate ‘spread-out Bragg peaks
‘(SOBP) in order to include the complete
target volume inside the SOBP.
In clinical applications it is common to use
a biological effective dose instead of a physical
dose (absorbed dose to water) because of the
uniformity in its dose distributions.
The difference between the two kinds of
distributions can be compared from the figures
. In the physical dose distribution we can see a
lack of uniformity in the SOBP.
The use of a biological effective dose makes
it possible to compare results obtained with
conventional radiotherapy to those using heavy
ion radiotherapy.
4. Unlike the clinical applications ,the dosimetry of heavy ion is carried out by
determination of the physical dose using an ionization chamber calibrated in terms
of absorbed dose to water, ND,w,Qo.
The goal behind this approach is to achieve international consistency in
dosimetry by adopting the same formalism and procedures for all the radiotherapy
beams used throughout the world.
Unfortunately, Task Group 20 in 1984 by the Association of American Physicists
in Medicine is the only protocol available regarding dosimetry recommendations .
Thus there is a need for a new protocol which will be able to establish a global
consistency in the determination of absorbed dose to water with heavy ions and is
common to dosimetry protocols.
For an accurate determination of absorbed dose using an ionization chamber, we
need to know the energy spectra of the incident heavy ion beam, the projectile
fragments as well as that of the target fragmented nuclei.
5. Dosimetry equipments
Ionization chambers
Cylindrical and plane-parallel ionization chambers are recommended for use as
reference instruments in clinical heavy ion beams.
cylindrical ionization chambers are preferred for reference dosimetry for SOBP
width ≥ 2.0 g/cm2 and plane-parallel chambers are preferred for SOBP width < 2.0
g/cm2.
The reason for this preferences is the higher combined standard uncertainty on
Dw,Q for plane-parallel ionization chambers due to their higher uncertainty for pwall
in the 60Co reference beam quality.
In the case of cylindrical chambers ,graphite walled are preferred to plastic
walled chambers because of their better long term stability and smaller chamber to
chamber variations.
6. Since the depth dose distribution in the SOBP for heavy ion beams is not flat and
the slope depends on the width of the SOBP , measurement of an effective point of
the chamber, Peff, is important.
The reference point of the cylindrical chamber should be positioned at a distance
0.75 rcyl deeper than the point of interest in the phantom, where rcyl is the inner
radius of the chamber.
For plane-parallel ionization chambers, the reference point is taken to be on the
inner surface of the entrance window, at the centre of the window. This point is
positioned at the point of interest in the phantom.
The cavity diameter of the plane-parallel ionization chamber or the cavity length
of the cylindrical ionization chamber should not be larger than approximately half
the reference field size.
7. Phantoms and chamber sleeves
Water is recommended as the reference medium for measurements of absorbed
dose.
The extension of phantom should be at least 5 cm beyond all four sides of the
field size at the depth of measurement and should extend to at least 5 g/cm2
beyond the maximum depth of measurement.
In horizontal beams, phantom should have plastic window with a thickness twin
between 0.2 and 0.5 cm. In the case of non-waterproof chambers, a waterproofing
sleeve made of PMMA, with a thickness than 1.0 mm have to be used.
To provide equilibrium air pressure in the chamber ,the air gap between the
chamber wall and the waterproofing sleeve should be within (0.1–0.3 mm).
8. If it is possible, the same waterproofing sleeve that was used for calibration of user
chamber should also be used for reference dosimetry. Otherwise another sleeve of
the same material and of similar thickness should be used.
Use of plastic phantoms for reference dosimetry in heavy ion beams is not
recommended.
One of the reason for this is difficulty in availability of water to plastic fluence
correction factors, hpl, and other reason is difference in the fluence of heavy ions
including fragmented particles in a plastic phantom and that in a water phantom.
However, for routine quality assurance measurements plastic phantoms can be
used, provided a transfer factor between plastic and water has been established.
9. BEAM QUALITY SPECIFICATION
Unavailability of both experimental and theoretical data regarding spectral
distributions of heavy ion beams makes beam quality specification more
impractical.
The current practice for characterizing a heavy ion beam is to use the atomic
number, mass number, energy of the incident heavy ion beam, width of SOBP and
range.
DETERMINATION OF ABSORBED DOSE TO WATER
From the spread out Bragg peak of a heavy ion figure we have seen that
The depth dose distribution is not flat, and the dose at the distal end of the SOBP
is smaller than that at the proximal part.
The slope near the centre of a broad SOBP is rather small whereas that of a
narrow SOBP is steep.
The reference depth for calibration should be taken at the centre of the SOBP, at
the centre of the target volume.
Reference conditions for the determination of absorbed dose to water are given as
10.
11. Determination of absorbed dose under reference conditions
The absorbed dose to water at the reference depth zref in water in a heavy ion beam
of quality Q and in the absence of the chamber is given by
where MQ is the meter reading of the dosimeter in accordance with the
reference conditions corrected for the influence quantities like
temperature and pressure, electrometer calibration, polarity effect
and ion recombination etc
ND,w,Qo is the calibration factor in terms of absorbed dose to water for the
dosimeter at the reference quality Qo.
kQ,Qo is a chamber specific factor which corrects for differences between
the reference beam quality Qo and the actual beam quality being
used, Q
12. Recombination correction in heavy ion beams
Dose rate will be very high when beams are generated by pulsed scanning
techniques and so recombination effects must be taken into account.
The correction factor for general (Volume) recombination is obtained
experimentally by the two voltage method.
When general recombination is negligible, initial recombination should be taken
into account for heavy ion beams, especially when the dose is measured using
plane-parallel ionization chambers.
The collected ionization current should be fitted by the linear relation
l/icol = 1/i∞ + b/V
where V is the polarizing voltage applied to the chamber. The correction factor is
given by ks
ini = i∞ /icol.
13. Part of worksheet regarding calculation for Initial ion recombination
14. Values of kQ,Qo
Beam quality specifications are not currently used for the dosimetry of heavy-ion
beams and so kQ values depend only on the chamber type used.
Experimental values of the factor kQ,Qo are not readily available and, therefore, in
this report only theoretical values will be used.
The correction factor is defined as.
Since there is no primary standard of absorbed dose to water for heavy ion
beams is available, all values for kQ,Qo given in this Code of Practice are derived by
calculation and are based on 60Co gamma radiation as the reference beam quality Qo
at 60Co.
15. There is no information available on perturbation factors for ion chambers in
heavy ion beams, and so is assumed as unity.
The stopping-power ratios and W values for heavy ion beams are taken to be
independent of the beam quality, because of lack of experimental data.
The contribution of fragmented nuclei to stopping-power ratios and W values
are also assumed to be negligible.
Constant values of the stopping-power ratio and W value are therefore
adopted here for all heavy ion beams — these are 1.130 and 34.50 eV,
respectively.
As the stopping power ratio sw,air of heavy ions is so close to that of 60Co, the
kQ values for heavy ions are dominated by the ratio of Wair values and the
chamber specific perturbation factors
16. Values of kQ for various cylindrical and plane-parallel ionization chambers
in common use are given. Some of the chambers listed in this table fail to
meet some of the minimum requirements .However, they have been
included because of their current clinical use.
bcs there is a lack of primary standards for heavy ions
Forthe commonly used plastics PMMA and clear polystyrene, the nominal values ñ PMMA = 1.19 g/cm3 and ñ polystyrene = 1.06 g/cm3 may be used for the calcula-tion of the water equivalent thickness of the window.
