In 2000 IAEA published another International Code of Practice. “Absorbed Dose Determination in External Beam Radiotherapy” (Technical Report Series No. 398) Recommending procedures to obtain the absorbed dose in water from measurements made with an ionisation chamber in external beam radiotherapy (EBRT).

1. IAEA TRS - 398 Formalism
Output calibration procedures for
High Energy Photon and Electron Beams
Vinay Desai
M.Sc Radiation Physics
KIDWAI MEMORIAL INSTITUTE OF ONCOLOGY
Bengaluru

2. IAEA International Codes of Practice
TRS-277
(1987)
TRS-381
(1997)
Protocols based on primary standards of air kerma
Absorbed dose Determination in
Photon and Electron Beams: An
International Code of Practice
To obtain the absorbed dose in
water from measurements
made with an ionisation
chamber in external beam
radiotherapy
The Use of Plane-Parallel
Ionisation Chambers in High
Energy Electron and Photon
Beams: An International Code of
Practice
To further update TRS - 277
and complement it with
respect to the area of plane
parallel ionisation chambers

3. Absorbed dose to water from high energy photon and electron beams can be
obtained using the expression,
Dw (peff) = M ND,air (S/)w
air pu pcel
Where,
ND,air = Nk (1- g) km katt = absorbed dose to air calibration factor for the
chamber.
Nk = air kerma calibration factor for the chamber.
km = correction for non-air equivalence of wall material.
katt = correction for attenuation of the beam in the cavity.
Formalism of Air Kerma Based Protocols

4. Basis of Absorbed Dose to Water Based Formalism
i. The new trend of calibrating ionisation chambers directly in a
water phantom in terms of absorbed dose to water was
introduced.
ii. Many PSDLs already provide calibrations in terms of absorbed dose
to water at the radiation quality of Co-60 gamma rays.
iii. The development of primary standards of absorbed dose to water
for high energy photon and electron beams, and improvements in
radiation dosimetry concepts, offer the possibility of reducing the
uncertainty in the dosimetry of radiotherapy beams.

5. IAEA TRS-398
• In 2000
• IAEA published another International Code of Practice.
• “Absorbed Dose Determination in External Beam
Radiotherapy” (Technical Report Series No. 398)
• Recommending procedures to obtain the absorbed dose in
water from measurements made with an ionisation
chamber in external beam radiotherapy (EBRT).

7. Salient Features of TRS - 398
• Based on standards of absorbed dose to water.
• endorsed by WHO, PAHO and ESTRO(European Society of
Therapeutic Radiology and Oncology ).
• Fulfils the need for a systematic and internationally
unified approach to the calibration of ionisation chambers
in terms of absorbed dose to water and to the use of
these detectors in determining the absorbed dose to
water for the radiation beams used in RT.
• Provides methodology for the determination of absorbed
dose to water in the low, medium and high energy
photon beams, electron beams, proton beams and heavy
ion beams used for EBRT.

8. Advantages of Absorbed Dose to Water Based Formalism
Reduced Uncertainty:
 Measurements based on calibration in air in terms of air kerma (Nk)
require chamber dependent conversion factors to determine
absorbed dose to water.
 These conversion factors do not account for differences between
individual chambers of a particular type.
A more robust system of primary standards:
 Primary standards of absorbed dose to water are based on a
number of different physical principles. There are no common
assumptions or estimated correction factors.
 Therefore, good agreement among these standards gives much
greater confidence in their accuracy.
Use of a simple formalism:
 No use of several coefficients, perturbation and other correction
factors, unlike Nk based formalism.

9. Chamber to chamber variations in the ratio of ND,w/NK for
chambers of a given type
 Not accounted for by air kerma (Nk)based formalism
• Figure, shows chamber to
chamber variations,
demonstrated for a given
chamber type by the lack of
constancy in the ND,w/NK ratio
at 60Co, for a large number of
cylindrical ionization chambers
commonly used in radiotherapy
dosimetry.
• For a given chamber type, chamber to chamber differences of up to 0.8% have also
been reported by the BIPM.(Bureau International des Poids et Mesures )
• The ratio of 60Co calibration factors ND,w /NK is a useful indicator of the uniformity
within a given type of chamber.

10. Primary Standards for Dw

11. ND,w BASED FORMALISM
The absorbed dose to water at the reference depth zref in water for a
reference beam of quality Qo and in the absence of the ionisation chamber
is given by
Dw,Qo = MQo ND,w,Qo .........(1)
where,
• MQ0 = dosimeter reading under reference conditions
(Practical conditions - same as standards lab)
• ND,w,Qo= absorbed dose to water calibration factor of the dosimeter
obtained from standards laboratory

12. • However, other than reference beam quality/ (Beam quality Q used
is other than the reference quality Q0 ),
Dw,Q = MQ ND,w,Qo kQ,Qo .......(2)
• kQ,Qo = beam quality correction factor (BQCF)
• kQ,Qo = corrects for the effects of the difference between the
reference beam quality Qo

13. Beam Quality Correction Factor kQ,Qo
• Defined as,
......(3
• For 60Co as calibration quality(Q0), kQ,Qo = kQ
• Ideally, BQCF should be measured directly for each
chamber at the beam quality of that of the user.

14.  When no experimental data are available,
 Correction factors are calculated theoretically,
 kQ,Qo can be derived comparing Dw,Q = MQ ND,w,Qo kQ,Qo with ND,Air formalism,
......(4)
• which is valid for all types of high energy beams,
• includes ratios, at the qualities Q and Qo, of Spencer–Attix water/air
stopping-power ratios, Sw,air, of the mean energy expended in air per ion
pair formed, Wair, and of the perturbation factors PQ.
• PQ includes pwall, pcav, pcel and pdis
(Contd...)

15. Beam Quality Correction Factor
In therapeutic electron and photon beams, (Wair)Q  (Wair)Qo
......(5)
• pQ = (pcavpcelpdispwall)Q
• = overall perturbation factor for an ionisation chamber for in-phantom
measurements at beam quality Q.
•
• pwall = corrects for non-medium equivalence of the chamber wall.
• pdis = corrects for replacing a volume of water with the detector cavity
when the reference point of the chamber is taken to be at the chamber
centre - alternative to peff.
• pcel = corrects for the effect of the central electrode during in-phantom
measurements.
• pcav = corrects for effects related to the air cavity, predominantly the
in-scattering of electrons.

16. Mean values of kQ at various photon beam qualities for
NE 2561and NE 2611 ion chambers
open circles - NE 2561 filled circles- NE 2611
line - fit to the expt. data triangle - calculated values
normalised to 0.568
(TPR20,10 of 60Co)

17. Relation Between NK and ND,w
• The connection between the NK - ND,air formalism and the ND,w
formalism is established for high energy beams by the
relationship
• where,
Qo = reference quality (60Co -rays), and
pQo = overall perturbation factor
• pcel refers exclusively to in-phantom measurements

18. General Practical Considerations
 Chamber sleeve : Material - PMMA, Wall thickness  1.0 mm
Air gap (chamber & sleeve) : 0.1- 0.3 mm
 sleeve should not be left in water longer than is necessary to carry out the
measurements
 The use of a thin rubber sheath is not recommended,

19.  Verify stability of the dosimeter system using a check source.
 Enough time should be allowed for the dosimeter to reach thermal
equilibrium.
 Mains powered electrometers should be switched on at least two hours
before use to allow stabilisation
 Pre-irradiate the ionisation chamber with 2 - 5 Gy to achieve charge
equilibrium in the different materials
 Operate the measuring system under stable conditions whenever the
polarity or polarising voltage are modified
 Measure the leakage current before and after irradiation(< 0.1%)
Chamber wall
Central electrode
P.D.
Insulator

20. Correction for Influence Quantities

21. Evaluation of Influence Quantities
Atmospheric variations :
• 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.
 conversion of the cavity air mass to the reference conditions,
(generally 101.3 kPa and 20°C)
 No correction for humidity, if ND,w is referred to a relative humidity
(RH) of 50% and is used in 20 - 80% of RH.
 If ND,w is referred to dry air, apply kh = 0. 997 (Qo = 60Co)

22. Polarity effect (kpol) :
• The effect on a chamber reading of using polarizing potentials of
opposite polarity must always be checked.
• 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, the effect may be
significant.
• True reading is taken to be the mean of the absolute values of
readings taken at both polarities.
• For routine use of a single potential and polarity.
• Where,
• M = electrometer reading obtained with the polarity used routinely (+ or - )

23. Ion Recombination(ks):
• 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, is independent of the dose
rate.
• 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%.

24. • For pulsed beams, it is recommended in this Code of Practice that
the correction factor ks be derived using the two voltage method,
• This method assumes a linear dependence of 1/M on 1/V.
• The recombination correction factor ks at the normal operating voltage V1 is
obtained from,
• Where,
• M1 = electrometer reading at polarising voltage V1 (Normal Voltage).
• M2 = electrometer reading at polarising voltage V2 (Lower Voltage) .
• (M1 and M2 are corrected for kpol at their respective voltages).
• a0, a1 and a2 = quadratic fit co-efficients for pulsed and scanned beams
• V1/V2 = 3.

25. For continuous radiation (60Co gamma rays),
Ion Recombination(ks): Contd...

26. Influence quantity Reference value/characteristics
Phantom material Water
Chamber type Cylindrical or plane parallel (PP)
Measurement depth, zref 5 or 10 g/cm2
Reference point For cylindrical chambers, on the
of the chamber central axis at the centre of the cavity
volume. For pp chambers, on inner
surface of the window at its centre
Position of the reference At the measurement depth zref
point of the chamber
SSD or SCD 80/100 cm
Field size 10 cm × 10 cm
60C0 -Rays :Reference Dosimetry

27. 5 cm (depth)
80 cm (SSD)
10 x 10 cm2
Electro-
meter
Water Phantom
Ion chamber
Experimental Set-up : SSD
Water
60C0 -Rays :Reference Dosimetry

28. • The absorbed dose to water at zref in water, in the user 60Co beam
and in the absence of the chamber ,
Dw (zref ) = MND,w Gy/min
• where,
• M = reading of the dosimeter corrected for temperature and pressure,
electrometer calibration, polarity effect, ion recombination and timer error.
• M = MunckTPkeleckpolks/(t  t) & t = time of irradiation (min)
• Absorbed dose at zmax :
• For SSD Set-up, Dw (zmax ) = Dw (zref )x 100/PDD(zref)
• For SAD Set-up, Dw (zmax ) = Dw (zref )/TMR(zref)
60C0 -Rays :Reference Dosimetry

29. High Energy X-rays:Reference Dosimetry

30. Choice of beam quality index
 For high energy photons produced by clinical accelerators the
beam quality Q is specified by the tissue phantom ratio TPR20,10.
 This is the ratio of the absorbed doses at depths of 20 and 10
cm in a water phantom, measured with a constant SCD of 100
cm and a field size of 10 cm × 10 cm at the plane of the
chamber.
 The most important characteristic of
the beam quality index TPR20,10 is its
independence of the electron
contamination in the incident beam.

31. High Energy X-rays: Measurement of QI (TPR20
10)
Influence quantity Reference value/characteristics
Phantom material Water
Chamber type Cylindrical or plane parallel (PP)
Measurement depths 20 and 10 g/cm2
Reference point of For cylindrical chambers, on the
the chamber central axis at the centre of the
cavity volume. For PP chambers,
on the inner surface of the window
at its centre
Position of the reference At the measurement depths
point of the chamber
SCD 100 cm
Field size at SCD 10 cm × 10 cm

32. Experimental Set-up for QI
WaterChamber
= 100 cm

33. High Energy X-rays: Reference Dosimetry
Influence quantity Reference value/characteristics
Phantom material Water
Chamber type Cylindrical
Measurement depth zref For TPR20
10 < 0. 7, 10 (or 5) g/cm2
For TPR20
10 = 0. 7, 10 g/cm2
Reference point of On the central axis at the centre of the
the chamber cavity volume
Position of the reference At the measurement depth zref
point of the chamber
SSD/SCD 100 cm
Field size 10 cm × 10 cm

34. 10 cm (depth)
100 cm (SSD)
10 x 10 cm2
Electro-
meter
Water Phantom
Ion chamber
Experimental Set-up : SSD
Water
High Energy X-rays:Reference Dosimetry

35. High Energy X-rays:Reference Dosimetry
Absorbed dose to water at the reference depth zref
Dw,Q(zref) = MQ ND,w kQ Gy/MU
Where,
• MQ = MunckTPkeleckpolks = Corrected Electrometer reading
• Absorbed Dose to water at zmax
 Dw,Q(zmax) = 100 Dw,Q(zref)/PDD (zref) Gy/MU - SSD
 Dw,Q(zmax) = 100 Dw,Q(zref)/TMR (zref) Gy/MU - SAD

36. Calculated Values of KQ For HE Photon Beams
High Energy X-rays:Reference Dosimetry

37. Calculated values of kQ for various cylindrical ionisation chambers
High Energy X-rays:Reference Dosimetry

38. High Energy Electrons:
Determination of Beam Quality (R50)

39. Choice of beam quality index:
• Beam quality index is the half-value depth in water R50.
• This is the depth in water (in g/cm2) at which the absorbed dose
is 50% of its value at the absorbed dose maximum, measured
with a constant SSD of 100 cm.
• Field size at the phantom surface of at least ,
 10 cm × 10 cm for R50 ≤ 7 g/cm2 (Eo ˂ 16 MeV) and
 20 cm × 20 cm for R50 > 7 g/cm2 (Eo ˃16 MeV).

40. High Energy Electrons: Determination of BQ (R50)
Influence quantity Reference value/characteristics
Phantom material water - R50  4 g/cm2 (Eo  10 MeV)
water or plastic - R50 < 4 g/cm2
Chamber type PP or cylindrical - R50  4 g/cm2
Plane parallel (PP) - R50 < 4 g/cm2
Reference point of PP - on the inner surface of the
the chamber window at its centre
Cylindrical - on the central axis at the
centre of the cavity volume
Position of the reference PP - at the point of interest
point of the chamber Cylindrical : 0.5 rcyl deeper than the
point of interest
SSD 100 cm
Field size 10 cm × 10 cm - R50  7 g/cm2
at phantom surface 20 cm × 20 cm - R50 > 7 g/cm2

41. • When using an ionisation chamber, the measured quantity is R50,ion. The
R50 is obtained using,
• When using detectors other than ion chambers (e. g. diode, diamond,
etc.) the measured quantity is R50
High Energy Electrons: Determination of BQ (R50)

42. High Energy Electrons: Reference Dosimetry
Influence quantity Reference value/characteristic
Phantom material water - R50  4 g/cm2 (Eo  10 MeV)
water or plastic - R50 < 4 g/cm2
Chamber type PP or cylindrical - R50  4 g/cm2
Plane parallel (PP) - R50 < 4 g/cm2
Measurement depth zref = (0.6 R50 - 0.1) g/cm2
Reference point of PP - on the inner surface of the
the chamber window at its centre
Cylindrical - on the central axis at the
centre of the cavity volume
Position of the reference PP - at zref
point of the chamber Cylindrical : 0.5 rcyl deeper than zref
SSD 100 cm
Field size 10 cm × 10 cm or that used for
at phantom surface normalisation of output factors

43. zref
= (0.6 R50 - 0.1)
100 cm (SSD)
10 x 10 cm2
Electro-
meter
Water Phantom
PP chamber
Experimental Set-up : SSD
Water
High Energy Electrons:Reference Dosimetry

44. • Absorbed dose to water at the reference depth zref
Dw,Q(zref) = MQ ND,w kQ Gy/MU
• MQ = MunckTPkeleckpolks = Corrected Electrometer reading
Absorbed Dose to water at zmax,
• Dw,Q(zmax) = 100 Dw,Q(zref)/PDD (zref) Gy/MU - SSD
High Energy Electrons:Reference Dosimetry

45. High Energy Electrons:Calculated KQ Values

46. Calculated KQ values for PP chambers calibrated in 60Co
High Energy Electrons:Reference Dosimetry

47. Calculated KQ values for Cylindrical chambers calibrated in 60Co
High Energy Electrons:Reference Dosimetry

48. High Energy Electrons:Use of Plastic Phantoms
• The use of plastic phantom is strongly discouraged, as in general they
are responsible for the largest discrepancies in the determinations of
absorbed dose in electron beams.
• Nevertheless, when accurate chamber positioning in water is not
possible, or when no waterproof chamber is available, their use is
permitted.
• Plastic phantoms may only be used at beam qualities R50 < 4 g/cm2 (E0 <
10 MeV).
-------------------------------------------------------------------------------------------------
Depth scaling zw = zpl cpl g/cm2 (zpl in g/cm2)
--------------------------------------------------------------------------------------------------
BQI R50,ion = R50,ion,pl cpl g/cm2 (R50,ion,pl in g/cm2)
--------------------------------------------------------------------------------------------------
Reference Depth zref,pl = zref/cpl g/cm2 (zref in g/cm2)
--------------------------------------------------------------------------------------------------
Dw MQ = MQ,pl hpl
zw depth in water, cpl is a depth scaling factor, fluence scaling factor hpl

49. High Energy Electrons:Use of Plastic Phantoms
Values of Depth Scaling Factor cpl, Fluence Scaling Factor hpl and Nominal
Density ppl for Certain Plastics

51. Thank you.
Vinay Desai
M.Sc Radiation Physics
Radiation Physics Department
KIDWAI MEMORIAL INSTITUTE OF ONCOLOGY
Bengaluru
vinaydesaimsc@gmail.comPpt Reference:SD Sharma