This document describes procedures for measuring absorbed dose to water for photon and electron beams using an ionization chamber and following IAEA TRS-398 protocol. For photons, absorbed dose was measured at a depth of 10 cm for 6 MV and 15 MV beams. The absorbed dose was 34.33 cGy and 38.63 cGy respectively after applying corrections. For electrons, absorbed dose was measured at the reference depth for 6 MeV, 9 MeV and 12 MeV beams and determined to be 40.51 cGy, 39.81 cGy and 38.71 cGy respectively. The results provide absorbed dose measurements in a water phantom but cannot be used to verify maximum dose due to lack of percentage depth dose data.

1. Master in Medical Physics ICTP 2015-2016
Absorbed dose in water for electron and photon beams
Francisco J.Hernández Flores∗
International Centre for Theoretical Physics
franciscohernandez_f2010@hotmail.com
August 26, 2015
Abstract
During the practice of radiotherapy it was evaluated the rate of absorbed dose to water
in a water phantom at reference condition for photon beams and electrons beams in terms of
references used during the commissioning of the treatment unit following the procedures of
IAEA Code of Practice TRS 398, taking into account the follow condition size of field of
10x10 cm2, the reference depth 5g/cm2 to electron beams and 10g/cm2 to photon beams,
measures readings were corrected for influences quantities linked to factors such as: polarity
effect (kpol), electron recombination (ks), pressure and temperature (kTP) and electrometer
factor (kelec). using all this factor with the reading was calculated the absorb dose in water
DW,Q for diferent energy of photon beam and electrom beams; the absorbed dose in water for
photon of 6 MV was 34.33 cGy and for the photon of 15 MV was 38.63 cGy,; for electron
beam was calculated for three energy de results were 40.51 cGy for 6 MeV, 39.81cGy for 9
MeV and 38.71 for 12 MeV of energy.
I. Introduction
It is recommended that quality assurance (QA) of the beam energy of radiotherapy
treatment machines is carried out at regular intervals, either weekly or monthly. The
standard for beam energy characterization is the measurement of depth ionization
curves or depth dose curves in a water phantom using a suitable detector, and usually a
measurement such as dose at depth is taken as the metric. However, such measurements
are time-consuming and impractical on a regular basis in the clinical environment, and
often sampling of the beam at various depths in a solid phantom is used as the QA
measurement to ensure that the beam energy is consistent with commissioning data. [3]
This practice only take into account one part of quality control of one treatment
unit, absorbed dose to water in a water phantom.
∗Radiotherapy Practical
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2. Master in Medical Physics ICTP 2015-2016
II. Theory
I. Determination of the absorbed dose to water for photon
It is assumed that the user has an ionization chamber or a dosimeter with a calibration
factor ND,w,Q0
in terms of absorbed dose to water at a reference quality Q0. The chamber
is positioned according to the reference conditions and the absorbed dose to water is
given by Equation 1 [1]
Dw,Q = MQND,w,Q0
kQ0
(1)
II. Correction for influence quantities
The calibration factor for an ionization chamber is valid only for the reference conditions
which apply to the calibration. Any departure from the reference conditions when using
the ionization chamber in the user beam should be corrected for using appropriate
factors.
II.1 Pressure, temperature and humidity
As all chambers recommended in this report are open to the ambient air, the mass of
air in the cavity volume is subject to atmospheric variations. The correction factor is
calculated by equation. 2
kTP =
273.2 + T
273.2 + T0
P0
P
(2)
should be applied to convert the cavity air mass to the reference conditions. P and
T are the cavity air pressure and temperature at the time of the measurements, and P0
and T0 are the reference values (generally 101.3 kPa and 20◦C).
II.2 Electrometer calibration
When the ionization chamber and the electrometer are calibrated separately, a calibra-
tion factor for each is given by the calibration laboratory. In this Code of Practice, the
electrometer calibration factor kelec is treated as an influence quantity and is included
in the product ?ki of correction factors. Typically, the calibration factor ND,w for the
ionization chamber will be given in units of Gy/nC and that for the electrometer kelec
either in units of nC/rdg.
II.3 Polarity effect
The effect on a chamber reading of using polarizing potentials of opposite polarity
must always be checked on commissioning. For most chamber types the effect will be
negligible in photon beams, a notable exception being the very thin window chambers
used for low energy X rays. In charged particle beams, particularly electrons,17 the
effect may be significant.
kpol =
|M+| + |M_|
2M
(3)
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3. Master in Medical Physics ICTP 2015-2016
where M+ and M_ are the electrometer readings obtained at positive and negative
polarity, respectively, and M is the electrometer reading obtained with the polarity used
routinely
II.4 Ion recombination
The incomplete collection of charge in an ionization chamber cavity owing to the
recombination of ions requires the use of a correction factor ks. Two separate effects
take place: (i) the recombination of ions formed by separate ionizing particle tracks,
termed general (or volume) recombination, which is dependent on the density of
ionizing particles and therefore on the dose rate; and (ii) the recombination of ions
formed by a single ionizing particle track, referred to as initial recombination, which is
independent of the dose rate. Both effects depend on the chamber geometry and on the
applied polarizing voltage. For beams other than heavy ions, initial recombination is
generally less than 0.2%. The recombination correction factor ks at the normal operating
voltage V1 is obtained from equation. 4 [1]
ks = a0 + a1
M1
M2
+ a2
M1
M2
(4)
III. Charge Measurement
The fully corrected charge reading from an ion chamber, M, is given by
M = MrawkskTPkeleckpol (5)
where Mraw is the raw ion chamber reading in coulombs, C, or the instrumentŠs reading
units (rdg).
IV. Determination of absorbed dose under reference conditions for electron
Beam
The absorbed dose to water at the reference depth zref in water, in an electron beam of
quality Q and in the absence of the chamber, is given by equation 6.
Dw,Q = MQND,w,Q0
kQ,Q0
(6)
where MQ is the reading of the dosimeter corrected for the influence quantities tem-
perature and pressure, electrometer calibration, polarity effect and ion recombination.
The chamber should be positioned in accordance with the reference conditions. ND,w,Q0
is the calibration factor in terms of absorbed dose to water for the dosimeter at the
reference quality Q0 and kQ,Q0
is a chamber specific factor which corrects for differences
between the reference beam quality Q0 and the actual beam quality Q. [1]
III. Material and method
Linear Accelerator Synergy of Elekta Oncology Systems, water phantom 3D with
automatic software connected to the console of the linear accelerator and computer that
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4. Master in Medical Physics ICTP 2015-2016
have installed the program drive the movement of the ionization chamber during the
scanner of data acquisition, electrometer PTW Unidos, barometer and thermometer, for
photon beam was used the ionization chamber type farmer and for electron beam was
used plane parallel chamber.
The head of the linear accelerator was adjusted to zero degree then proceeded to
align the phantom coincide reticle beam with the mark of the phantom, having properly
aligned the phantom ionization chamber was located following the instructions of the
code of practice of IAEA trs 398 for both photons and electrons beam.
For evaluate the absorbed dose in water for Photon beam were realized Three
measurements for each photon energy 6 MV and 15 MV were performed, standard
environmental conditions temperature and pressure was measured to evaluate the kTP
factor, three measurements with different polarity for evaluated the Polarity effect Kpol,
three reading with different polarity value for the ions recombination factor ks.
For evaluate the absorbed dose in water phantom for electron beam in accelera-
tor Synergy Agility were ionization chamber parallel plane, electrometer barometer
thermometer, phantom of water 3D and software Mephisto. we install the whole
system then aligns the phantom and finally the camera then proceeded to measure the
absorbed dose for the reference field at the reference depth for various energies of the
electron beam.
IV. Discussion and analysis of Results
I. First Practice Determination absorbed dose in water for Photon Beam.
The table 1 contains the reading measure for photon beams 6 MV and 15 MV delivery
50 MU in the console of linear accelerator with the camera positioning at 10 cm depth
and central axis of the beams.
N◦ Reading Reading [nC] Energy of beam [MV]
1 6.389 6
2 6.377 6
3 6.377 6
Average 6.381
1 7.280 15
2 7.279 15
3 7.281 15
Average 7.280
Table 1: Lecture Measure in referennce condition
The table 2 and table 3 show the data calculated for different Correction factor that
influence quantities in the reading. all these factor were measured using worksheet
from excel, the beam quality was obtained through interpolation.
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5. Master in Medical Physics ICTP 2015-2016
Energy [MV] kQ kpol ks ND,w[cGy/nC]
6 0.99 1.0005 1.0021 5.364
SSD [cm] Field [cm2] Zre f [cm] TPR20/10 kTP
100 10x10 10 0.6837 1.015
Table 2: Data from photon of 6 MV of energy
Energy [MV] kQ kpol ks ND,w[cGy/nC]
15 0.974 1.0003 1.0048 5.364
SSD [cm] Field [cm2] Zre f [cm] TPR20/10 kTP
100 10x10 10 0.6837 1.015
Table 3: Data from photon of 15 MV of energy
Using the data collected in table 1, 2 and 3 and use the equation 1 we can obtain
the absorbed dose in water for both energy of photon used in the practice the absorbed
dose in water to reference condition for photon of 6 MV was 34.33 cGy and for the
photon of 15 MV was 38.63 cGy.
II. Second Practice Determination absorbed dose in water for electron Beam.
The table 4 show the parameter used in measurement of electron beams for three
different energy, this parameter is useful to calculate the beam quality factor depending
of the type energy used. Theses parameters like: R50,ion, Zmax, Zre f and R50 all of them
were obtained during the measurement PDD for each one of energy. The beam quality
was obtained through interpolation take into account the R50 obtained in the software
mephisto for each energy.
Energy
(MeV) E0 (MeV)
Zre f
(gr/cm2)
Zmax
(gr/cm)
R50,ion
(gr/cm2)
R50
(gr/cm2) KQ
6 5.926 1.43 1.30 2.53 2.543 0.920
9 8.372 2.06 1.90 3.55 3.593 0.913
12 11.201 2.78 2.20 4.73 4.807 0.906
Table 4: Point of reference for electron beam depending of the energy
The table 5 show the data calculated for different Correction factor that influence
quantities in the reading. all these factor were calculated using worksheet from excel,
take the average of the three reading, follow the equation show in the section of theory
for each one of the factors.
Energy [MV] Field [cm2] kTP kpol ks ND,w[Gy/nC]
6 10x10 0.9992 1.0062 1.0030 1.35
9 10x10 0.9992 1.00312 1.0030 1.35
12 10x10 0.9992 1.00159 1.0031 1.35
Table 5: Correction factor for electron beams for three energies
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6. Master in Medical Physics ICTP 2015-2016
The table 6 contains the reading measure for electron beam 6MeV, 9MeV and 12MeV,
delivery 50MU in the console of linear accelerator at reference condition the ionization
chamber positioning at central axis.
N◦ of measure Measure [pC] Energy of beam [MeV]
1 323 6
2 323 6
3 323 6
Average 323
1 321 9
2 321 9
3 321 9
Average 321
1 315 12
2 315 12
3 315 12
Average 315
Table 6: Lecture Measure in referennce condition for electron beams
Now with al parameter obtained and using the equation 6 we can calculate the
absorbed dose in water for each one of energy used during the practice.
Energy (MeV) field (cm2) SSD (cm) absorbed Dose in water (cGy)
6 10x10 100 40.51
9 10x10 100 39.81
12 10x10 100 38.71
Table 7: result of absorbed dose in water for electron beams in reference condition
V. Conclusion
• The result of the absorbed dose in photon beam and electron beams were mea-
sured in reference condition, this result of absorbed dose in water was not carried
to the maximum dose for lack PDD value for each energy used during the practice,
for this reason can not compair the result measurement with the dose delivered
from the machine.
References
[1] IAEA Absorbed Dose Determination in External Beam Radiotherapy,Viena 2000
[2] E.B. Podgorsak, Radiation Oncology Physics: A Handbook for Teachers and Students,
Vienna, International Atomic Energy Agency, 2005.
[3] Klein EE, Hanley J, Bayouth J, et al Task Group 142 report: quality assurance of
medical accelerators, Med Phys. 2009
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