Electron dosimetry

1. Electron beam dosimetry
Presenter : Hirakjyoti bezbaruah
(Intern medical Physicist)
Moderator: Bhaveshwar Yadav
(Medical physicist cum RSO)
Dr. B. Borooah cancer institute.

2. CONTENTS
• INTRODUCTION OF ELECTRON BEAM
• DOSIMERTRY EQUIPMENT
• EFFECTIVE POINT OF MEASUREMENT
• ABSORBED DOSE MEASUREMENT AND ITS DIFFERENT
FACTOR
• PDI TO PDD

3. INTRODUCTION OF ELECTRON BEAM
The most clinically useful energy range for electrons is 6 to 20 Mev. The electron beam are used
for treating superficial tumours (less than 5 cm deep)
Principal applications of electron beam are
The treatment of skin and lip cancers.
Chest wall irradiation of breast cancer.
Administering boost dose to nodes.
The treatment of head and neck cancers.

4. Parameters for describing beam quality of electron beam:
•Percentage depth dose (PDD).
•Practical range(Rp), Half- value depth (R50), Therapeutic range
(R80), R90, Maximum range (Rmax)
•Most probable energy (Ep)0 and mean energy (E0) of electron
beam on the surface and any in depth inside the water
phantom.

5. PDD(Percentage depth dose)
• Dose build up region is broader with
higher energy.
• Skin dose increases with increasing beam
Energy ( which is the reverse for photon
beam ).
• PDD increases with energy increases.
• There is rapid dose fall-off beyond the
maximum dose.(by which we can save
OAR and normal tissue irradiation).
• The Rapid dose fall decreases with
energy increases .

6. • R50 , R80 and R90 are defined as the depths
on the electron PDD curves at which PDD
beyond the depth of dose maximum attain
values of 50% , 80% and 90% respectively.
• Practical range is the depth at which tangent
drawn at steepest section of the electron
depth Dose curve intersects with the
extrapolation line of the bremsstrahlung tail
.
Half-value depth , therapeutic depth and R90

7. Most probable energy (Ep)0 on the phantom surface:
• The most probable energy on the phantom surface is defined by the position of spectral peak.
• (Ep)0 is related to the practical range Rp of the electron beam through following polynomial equation
Most probable energy at a certain depth z inside the water:
• The most probable energy of the spectrum decreases linearly with the depth. This can be
express by the following relationship
where z is the depth.

8. Mean electron energy on the phantom surface:
• Mean electron energy of the electron beam on the phantom surface is slightly
smaller than the most probable energy on the phantom surface as a result of
asymmetrical shape of the electron spectrum . The mean electron energy is
related to the half value depth (R50).
Mean electron energy at a certain depth inside the phantom:

9. Flatness of the electron beam : Separation between the 90% and 50%
dose points on either side of the beam profile for all available electron
beam at Zmax energy should not exceed 10 mm .
Symmetry of the electron beam: Maximum ratio of absorbed doses at
symmetrical points from the central beam axis more than 1 cm inside
the 90% isodose contour for all electron beam energy should not
exceed 105% of the absorbed dose on the axis of the beam at the same
depth.
Off axis ratio : It is the ratio of the dose at any point in a plane
perpendicular to the beam direction to the dose on the central axis in
that plane.
Electron beam profiles and off axis ratio:

10. Dosimetry equipment
Types of ionization chambers used
1. cylindrical( thimble) ion chamber.
2. parallel plate chamber .
Cylindrical ion chamber :

11. • It is most commonly used in e-, photons, proton beam dosimetry application .
• Its operable voltage is very high ( i.e : +/- 200 to +/- 500 ) and independent of
beam direction .
• The sensitive volume of the cylindrical ion chamber is 0.1 -1cc(0.6 cc).
• The dimension of sensitive volume are 0.2 to 0.7 cm (radius) and 0.4 to 2.5 cm
(length).
• The diameter of the ion chamber is not be large enough .(because perturbation
correction factor decreases with increasing chamber diameter and decreasing
electron density ).
• the wall material (i.e cathode ,of ion chamber is graphite material to reduced
perturbation .(it is treated as wall less chamber, since mass stopping power of
graphite is similar to air) .wall thickness is less than 0.1 gm/cm2
• Anode material of the ion chamber is Aluminium .
Cylindrical thimble ion-chamber :

12. Parallel –plate (also called end window or plane parallel ):
• Plane parallel chamber are the recommended type for all beam quantities .
• The reference point for plane-parallel chambers is taken to be on the inner surface of the
entrance window at the centre of the window .

13. Figure – comparison of effective point of measurement in photon beam vs electron beam .

15. Parallel-plate chamber:
Front window is made of plastic , in PTW chamber manual the effective water equivalent depth is 1.13 mm
where as physical depth is 1 mm , which depth should be considered to move the chamber at effective point of
measurements.

16. Dosimetry Equipment :
Phantoms
water solid

17. • water is always recommended in the IAEA Codes of practice as the phantom material for
the calibration of high energy electron beam .
• The phantom should be extended to at least 5 cm beyond all four sides of the largest
field size employed at the depth of the measurement
• There should also be margin of at least 5 gm/cm square beyond the maximum depth of
measurement .
• Solid phantoms are also used for easy handling and set up . But their use is strongly
discouraged for reference measurement . Its main reason is scaling of depth .
• In a horizontal electron beam , the window of the phantom should be of plastic and of
thickness between 0.2 to 0.5 cm .
• PMMA and polystyrene are normally used in phantom and it`s density are 1.19 gm/cm3
and 1.06 gm/cm3
Phantoms:

18. The main reason of avoiding plastic phantom for higher energy electron beam
The depth in plastic phantoms Zpl ( gm/ cm2) = z X ρpl (gm/cubic cm )
Where , Z depth in plastic phantom in cm.
ρpl is the density of the plastic .
The density of the plastic should be measured for the batch of plastic in use rather
than using a nominal value for the plastic type . Measurement made in a plastic
phantom at depth Zpl relate to the depth in water will be
Cpl is the depth scalling factor
scalling of dosimeter reading in water,
hpl is the fluence scalling factor

20. Absorbed dose measurement:
• Dw,Q denotes absorbed dose in water at reference depth Zref .
• MQ denotes the reading of dosimeter corrected for the influence quantity
temperature, pressure , electrometer calibration , polarity effect and ion
recombination effect .
• ND,W,Q0 denotes calibration factor in terms of absorbed dose to water for the
dosimeter at the reference beam quality Q0 .
• KQ,Q0 denotes the factor for correction for the difference between the
response of an ionization chamber in the reference beam quality Q0 used for
calibration the chamber and in the actual user beam quality Q.

21. KQ,Q0( Chamber specific )
PQ and PQ0 denotes perturbation correction factor in the case of user
beam and reference beam.
(Sw,air)Q and (Sw,air)Q0 denotes the stopping power ratio medium to air in
user beam and reference bream .
(Wair)Q and (Wair)Q0 denotes mean energy expended in air per ion pair
formation for user beam and reference beam( its value is taken 33.97 j/c
for high energy electron beam in dry air medium ) .

22. PQ (perturbation factor )
• The overall perturbation factor(PQ) includes all departures from the behaviour of
an ideal Bragg-Gray detector .
PQ = Pcav.Pdis.Pwall.Pcel ( for cylindrical chamber type)
PQ = Pcav.PWall ( for parallel chamber type)
Pcav (cavity correction): corrects the perturbation of the electron fluence due to
scattering differences between the air cavity and the medium.
• Pwall corrects differences in the photon mass energy absorption coefficient and
electron stopping powers of the chamber wall material and the medium.
• Pdis (displacement correction): accounts for the fact that a cylindrical chamber
cavity with its center at Zref ,the electron fluence at a point which is closer to the
radiation source than Zref and it is derived from the inner radius of the cavity
(rcyl) .

23. Where rcyl is in mm .
Pcel corrects for the lack of air equivalance of the central electrode . The
correction for this effect is negligible for graphite central electrode .
Stopping power ratio (Sw,air) : the stopping power accounts the scattering of
charged particle inside the cavity . It depends on the R50 for electron beam .
Sw,air = 1.253 - 0.1487 (R50)0.214
R50 is in gm/cm square.

25. Percentage depth ionization(PDI) to the Percentage depth dose(PDD)
Since air is present in the cavity, we get exposure ( no of charged particle per unit
mass ) at that region . And energy deposited in the air by collisional loss of kinetic
energy of the electrons
This is the conversion factor from exposure to dose in air under the condition of
charge particle equilibrium .
continued…

26. absorbed dose in water medium (Dw):
DW = Dair x Sw,air x P
Dair denotes the dose deposited in the air .
Sw,air denotes the stopping power ratio water to air
P denotes the perturbation factor for the chamber wall .
By this way we can get PDD from PDI:

28. ND,w,Q0 (Chamber calibration factor in terms of absorbed dose to water for a dosimeter at a
reference beam quality Q0) :
ND,air formalism (W/e) is assumed to be constant for electrons and therefore ND,air
depends only on the mass of air ( v. Air ) inside the cavity .
continued…

29. Kair denotes Air kerma.
Km correction for non-air equivalent of wall material .
Katt correction for attenuation of the beam in the cavity.
Kcel correction for the lack of air equivalent of the central
electrode of the chamber .
g is the average fraction of an electron energy lost to radiative process (i.e
bremsstrahlung ) .
** since major part of the kinetic energy of electron in low Z material(ex:
air,water,soft tissue) is expanded by inelastic collision (ionization and excitation)
with atomic electrons . Only a small part is expanded in the radiative collision with
atomic nuclei (bremsstrahlung) .

30. MQ (Reading of the Dosimeter)
• Dosimeter reading depends on some quantities ( temperature-
pressure,electrometer calibration , polarity effect and ion recombination )
KTP is the temperature- pressure correction factor .
Kelec is the electrometer correction factor .
Kpol is the polarity correction factor .
KS is the recombination correction factor .

31. KTP( TEMPERATURE PRESURE CORRECTION FACTOR)
• The mass of the air in the chamber cavity varies with atmospheric pressure and
temperature , therefore correction factor KTP is applied to convert the cavity air
mass to the reference conditions .
KTP =
• P and T are the cavity air pressure and temperature in the water near the ion
chamber at the time of measurement .
• P0 and T0 are the reference values at the time of calibration (101.3 kilo Pascal
and 20 degree Celsius) .
• The measurement is done in the relative humidity lies between 20% to 80% .
CONTINUED…

32. • Before taking measurement , we give warm up for the chamber to reach thermal
equilibrium with its surroundings .
• The temperature of the air in a chamber cavity should be taken to be that of the
phantom which should be measured , this is not necessarily the same as the
temperature of the surrounding air .
• The pressure is not corrected to see level and including latitude corrections for a
mercury barometer .

33. Ks( Saturation correction factor )
• In an ion chamber cavity some electrons and positive ions recombine before
completely collected . Therefore, a correction factor is required to correct for this
lack of 100% charge collection . This correction factor has two components
(i) an initial recombination .
(ii) a general or volume recombination .
An initial recombination: it is independent of dose rate and results from the
recombination of ions formed by a single ionizing particle track . This can be used
for continuous radiation ( gamma ray beams)
its value is 0.1% for cylindrical chambers
its value is 0.1 to 0.2 % for parallel plate chambers .
continued…

34. • general or volume recombination: it is obtained from the recombination of ions
formed by separate ionizing particle tracks . Its magnitude depend on the density
of ionizing particles in the cavity and therefore on the dose rate .
Both initial recombination and volume recombination depend on the
geometry of the chamber and on the applied collection voltage .

35. Kpol( polarisation correction factor)
• Polarity effect : the reading of the ionization chamber may change when
polarizing potentials of opposite polarity are applied to it . Polarity effects for
electron beam is significant , especially for parallel plate ion chamber .
polarity effect is given by following expression
M+ is the electrometer reading when positive charge is collected
M- is the electrometer reading when negative charge is collected
M is the reading obtained with the polarity used at the chamber calibration

36. • After reversing polarity , adequate time must be allowed before taking the next
reading so that the ion chamber's reading can reach equilibrium. Depending on
the chamber type and polarity , some chambers may take several ( up to 20 min)
before stable operating condition is reached.
• Stable conditions can also be accomplished by irradiating the chamber to 3-5
Gray .

38. Absorbed Dose measurement at dmax
At first we measure dose at Zref ,Then we convert it to at dmax.
Zref = 0.6 x R50 - 0.1 ( gm/ cm square)
The main reasons of dose measurement at Zref are
• There is exist CPE ( charge particle equilibrium ) .
• At Zref , D( absorbed dose) = Kcol ( collisional loss of kinetic energy)
Absorbed dose at dmax, Dw,Q = Dw,Q ( at Zref ) X 100 Gray/MU
PDD ( at Zref )

39. REFERENCES
• F.M KHAN The Physics of Radiation Therapy
• TRS-277
• TRS-398
• TG-51

40. THANK YOU…….