Code of practice for reference dosimetry and relative dosimetry of low KV Xray beam

1. TRS-398
CODE OF PRACTICE FOR
LOW ENERGY KILOVOLTAGE X RAY BEAMS

2. Introduction
 In the kilovoltage X ray range, because of the dose fall off , there is
a limited availability of standards of absorbed dose to water.
 However, it is possible to derive calibration factors in terms of
absorbed dose to water from air kerma calibration factors.
 The dosimetry of low energy X rays has traditionally been based on
measurements in air of exposure or air kerma.
The exposure or air kerma is converted to absorbed dose to water
and absorbed dose at the surface of water is derived from this
measurement by applying a correction factor for the effect of
backscatter.
The IAEA Code of Practice includes measurements made in a full
scatter phantom, using a chamber that has been calibrated directly in
terms of absorbed dose to water while mounted in the phantom.

3. Dosimetry equipment
Ionization chambers
Ionization chambers for measuring low energy X rays must be of
the plane-parallel type with an entrance window consisting of a thin
membrane of thickness in the range 2–3 mg/cm2.
The thickness of the chamber should be sufficient to allow full
buildup of the primary beam (secondary electron spectrum).
This will also prevent secondary electrons generated
upstream(beam limiting devices) from entering the chamber.
 An additional plastic foil added to the window for beams above
50KV to provide full buildup of the primary beam .

4. The phantom and buildup foils need to be supplied together with
the chamber when it is sent for calibration and should be calibrated at
the same SSD and field size used for reference dosimetry in the clinic.
 Since there will be a large variation in the energy response from
chamber to chamber, a generic set of kQ,Qo values for a particular type
of chamber is not recommended.
The reference point of the chamber should be taken on the outside
of the chamber window at the window centre (or the outside of the
buildup foil if this is used).
The point remains the same for both the purpose of calibration at
the standards laboratory and for measurements under reference
conditions in the user beam.
 The point is positioned so that it aligns with the front surface of the
phantom because the calibration factor ND,w,Q is given in terms of the
absorbed dose to the surface of water.

5. Phantoms
The phantom must allow the chamber to be positioned in
such a way that its outside face of the chamber window is
aligned with the phantom surface.
A water phantom cannot do this purpose and so a plastic
phantom should be used .PMMA (Perspex, Lucite, etc.) is
acceptable.
 No dose or depth conversions are needed since the
phantom/chamber unit is calibrated in terms of absorbed dose
to water at the surface, irrespective of the type of plastic used.
The phantom should extend in the beam direction by at least
5 g/cm2 and in the lateral direction at least far enough beyond
the reference field size used to ensure that the entire primary
beam exits through the rear face of the phantom.

6. Beam quality specification
Choice of beam quality index
Half-value layer (HVL) and kilovoltage generating potential (kV)
are the beam quality parameters to characterize a kilovoltage X ray
spectrum for dosimetry .
HVL is taken as the primary beam quality index since it is often not
possible to match both the kV and HVL of each clinical beam with the
beams of the standards laboratory.
 Unfortunately, there is insufficient published experimental work to
indicate how calibration factors in terms of absorbed dose to water will
vary independently with HVL and kV.
Still a plot of air kerma calibration factor will give a light idea…

7. For a given HVL, the calibration factor varies over a range of
up to a little over 2%.
So a new quality index for kV X rays based on absorbed dose to
water (possibly a ratio of doses at different depths) would be
always welcomed.

8. Measurement of beam quality
The HVL varies with the distance from the X ray target due to
absorption of low energy X rays in air.
So the HVL for low energy X ray beams should be measured with
the chamber at the same SCD as will be used for measurements of
absorbed dose.
 The ideal arrangement is to place at about half the distance between
the X ray target and the chamber a collimating aperture that reduces
the field size to just enough to encompass the whole of the chamber.
The filters are added for the HVL measurement and the thickness
that reduces the air kerma rate to one half is obtained by interpolation.
 A monitor chamber is recommended in order to avoid the mistakes
due to variations in X ray output. If a monitor chamber is not
available, output variation can be detected by measuring the air kerma
rate without additional filters both at the beginning and at the end.

9. Determination of absorbed dose to water
Reference conditions
Influence quantity
Reference value or reference
characteristics
Phantom material Water equivalent plastic or PMMA
Chamber type Plane-parallel for low energy X rays
Measurement depth zref Phantom surface
Reference point of the
chamber
At the centre of outside surface of chamber
window or additional buildup foil if used.
SSD
Usual treatment distance as determined by
the reference applicator
Field size
3 cm × 3 cm, or 3 cm diameter, or as
determined by the reference applicator

10. Determination of absorbed dose under reference conditions
The absorbed dose to water at the water surface, in a low energy X ray
beam of quality Q and in the absence of the chamber, is given by
where MQ is the meter reading in accordance with the reference
conditions corrected for the influence quantities .
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
 The polarity and ion recombination corrections are difficult to
measure on the type of chamber recommended for low energy X rays
due to electrostatic distortion of the chamber window.
 However, the effects will be negligible as long as the polarity is
kept the same as was used for calibration.

11. Values for kQ,Qo
A thin-walled chamber on the surface of a phantom does not
represent a Bragg–Gray cavity.
 Also large chamber to chamber variations in energy response
makes it difficult to use generic values, measured for a particular
chamber type.
 Normally there will be only a single calibration factor ND,w,Qo
determined in a reference beam of quality Qo and one or more
measured factors kQ,Qo corresponding to the other calibration qualities,
Q.
In the case of low KV X-rays, from of a set of calibration factors
ND,w,Q ,one of the qualities should be chosen as the reference beam
quality Qo.

12.  The corresponding calibration factor becomes ND,w,Qo and the other
calibration factors ND,w,Q are expressed in terms of kQ,Qo using the
relation
 If the quality of the user beam does not match any of the
calibration qualities, the value for kQ,Qo can be interpolated.
 It is better to recalibrate the chambers at all qualities each time
because of the possible variation in the energy response of ionization
chambers to low energy Xrays.

13. Measurements under non reference
conditions
Central axis depth dose distributions
Though depth dose distributions may be obtained from the literature,
it can be measured by using the same chamber that was used for
reference dosimetry and a water equivalent phantom.
 For measurement purpose, thin sheets of water equivalent phantom
material designed for use with kilo-voltage X rays are placed over the
chamber in its phantom maintaining a constant SSD.
 Water equivalence of the material should be within a few per cent in
the energy range of interest .
This should be verified by comparison with published data and
manufacturer’s specifications.

14. Output factors
The output factor is the ratio of the corrected dosimeter reading at
the surface for a given set of non-reference conditions to that for the
reference conditions
Output factors are inevitable for all combinations of SSD and field
size used for radiotherapy treatment and must be measured for each
beam quality and each individual applicator.
Because of the significant scatter contribution from the inside of
an applicator, it is not sufficiently accurate to estimate output factors
for different applicators using the ratio of the backscatter factors
corresponding to the respective field sizes.
If a PMMA phantom is used, the response of the chamber to
different field sizes will not be exactly the same as that for a water
phantom, owing to the difference in backscatter.
However, because the output factor is a ratio of measurements,
this effect should not incur an error of greater than 1%.

17. Thank you…