Dosimetric Evaluation of High Energy Electron Beams Applied in Radiotherapy

Dosimetric Evaluation of
High Energy Electron
Beams Applied in
Radiotherapy
aymanstohy@yahoo.com

Preparation by
DR. AYMAN G. STOHY
MEDICAL PHYSICIST
Showing what has been done in the
Master's Thesis,

Supervisors
 Prof. Dr. M. A. Abouzeid
 Prof. Dr. M. A. Elleithy
 Dr. T. A. Dawod

Aim of the work
1. Investigate the physical properties
of electron beams at different beam
energy.
2. Evaluate the accuracy of dose calculated
by Treatment Planning System (TPS)
and measured for different field
configurations.

INTRODUCTION

Electron beam therapy has become
an important component in
radiotherapy.
 It used for treating superficial
tumors (less than 5 cm depth).

Such as the treatment of:
 Skin and lip cancers.
 Chest wall irradiation for breast
cancer.
 in the treatment of head and neck
cancers.

Electron Beam
Therapy

Percent depth dose
 PDD is the dose along the beam
central axis normalized to 100%
at the depth of maximum dose (dmax).
 Electron energy can be specified as
the most probable electron energy
Ep,0 at the surface of the water
phantom.
• Determined from this equation:
22.098.10025.0 2
0,  ppp RRE

Beam profile
The dose at any point in a plane perpendicular to the
beam direction.
 Beam flatness (F ) :obtained
from equation:
 Beam symmetry (S):
Obtained from the profile in
depth of maximum dose:
rightleft
rightleft
areaarea
areaarea
S


100
minmax
minmax
100
DD
DD
Flatness




 The penumbra is
defined as the distance
between the 80% and
20% dose points on a
transverse beam profile
measured 10 cm deep in
the water phantom.

Effective SSD
 The distance between
the beam virtual source
and the surface of the
water phantom, determined
from equation:
f = effective SSD, I0 = dose
with zero gap, and Ig = dose
with gap g between the
standard SSD point and the
phantom surface.
2
max
max









df
gdf
I
I
g
o

Electron applicators and
cutouts
 Electron beam applicators are used to
collimate the beam, the electron field is
defined at distance as small as 5 cm from
the patient.
 Cutouts are used to shape the radiation
field to the tumor, hence minimizing the
dose to the surrounding tissues.

Output factors
The radiation output is a function of field size
 Output is measured at
standard SSD with a small
volume ionization chamber
at dmax on the central axis
of the field.
 Output factors are
defined as the ratio of the
dose for any field to the
dose for 10x10 cm2 field at
dmax.
 The output factors for
cutouts depend on the
electron energy, applicator
and the area of the cutout.

IAEA (International Atomic Energy Agency) protocol
 Determination of water absorbed dose Dw,Q in
quality beam radiation Q by used ionization
chamber from eq. ;
where MQ is the reading of the dosimeter
ND,W,Q0 is the calibration factor
kQ Q0 is the beam quality correction factor
00,,, QQQwDQQw kNMD 

Surface dose (Dose build-up):
 The build-up effect with
electron increases when
beam penetrates the
surface until a build-up
reach maximum dose.
Mean energy gradually
decreases with depth.

Treatment planning system
 A computer consists of hardware and software
components of the system.
 Description of dose calculation models used in the
planning system and generate beam shapes to maximize
tumor control and minimize normal tissue complications.
 The pencil-beam algorithm used for Fermi–Eyges pencil-
beam theory to calculate dose in patients.
 Enter patient data and machine parameters into the
system and comparing treatment plans calculated for
standard phantom.

MATERIALS
AND
METHODS

The radiation facilities used in
this study is

Linear Accelerator, Elekta Precise
 It is used as a
source of electron
energies

 Open applicator field
sizes of 6x6, 10x10,
14x14, and 20x20 cm2

PTW-UNIDOS Electrometer
 It used to
measure dose ( the
absolute and
relative dose )

Ionization Chambers
 Markus Ionization
Chamber
 0.125 cc ion chamber with
build up cap

Therapy Beam Analyzer
 A computer system
used for measuring
dose distribution and
radiation analysis in
radiotherapy.

Water phantom
 It is made of
prespex in form of
cubic tank.

The treatment planning
system
FOCUS Plan
2D radiation treatment
planning system
Precise Plan
3D treatment planning
system

Low Temperature Melting Alloy
Blocks
 Cerrobend shielding
block.
 It used to make of cutout
field.

 Used a water phantom system with the dual
0.125 cc ionization chamber and reference
detector.
 The water surface is set to the standard
treatment distance 100 cm, giving a 5 cm
stand off between the end of applicator and
the water surface, the gantry set at 0°
position.
 Used applicators 6x6 to 20x20 cm2 at 6 to15
MeV at SSD = 100 cm.

This work consists of 7 steps
1. Physical parameters of the electron
beams.
2. Percent Depth Dose and Beam Profile.
3. Beam Output Correction Factors.
4. Effective SSD.
5. Relative output factors.
6. Surface dose (Dose build-up).
7. Effect of gantry angle on output dose
rate.

RESULTS
AND
DISCUSSION

Physical parameters of the
electron beams

The physical parameters of the electron beam
for each applicator at different energy:

The central axis depth–dose curve of
electron beams depends on many
factors such as: the beam energy,
field size, SSD, collimation, depth of
penetration, and angle of beam
incidence.

Percent Depth Dose
(PDD)

PDD curves of energy 10 MeV
at SSD=100 cm for different
applicators.
Central axis PDD curves for
all energies from linear
accelerator. All curves are
normalized to 100% at dmax
0
20
40
60
80
100
0 20 40 60 80 100
Depth (mm)
Dose(%)
6 MeV
8 MeV
10 MeV
12 MeV
15 MeV
10 MeV
0
20
40
60
80
100
0 10 20 30 40 50 60 70
Depth (mm)
Dose(%)
6x6
10x10
14x14
20x20

(a) Measured and Calculated open field depth-doses for all
applicators , at SSD=100cm, 10 MeV.
(b) The differences between measured and calculated doses.
(a)
0
20
40
60
80
100
0 20 40 60 80
Depth (mm)
Dose(%)
calc.6x6
meas.6x6
calc.10x10
meas.10x10
calc.14x14
meas.14x14
calc. 20x20
meas.20x20
(b)
-5
-3
-1
1
3
5
0 20 40 60
Depth (mm)
Difference(%)

The mean difference and SD between measured and calculated depth-
doses for all energies and for all field sizes .
Appl. Energy
(MeV)
Precise Plan Focus Plan
Mean ±SD Mean ±SD
6x6
6 0.48 1.43 -0.52 1.16
8 0.55 1.30 0.42 0.84
10 0.92 1.58 0.35 1.02
12 0.73 1.17 0.15 0.65
15 0.97 1.33 0.11 0.53
10x10
6 0.36 1.61 0.26 1.03
8 0.22 1.09 0.27 0.71
10 0.23 1.24 0.20 0.65
12 0.42 0.99 0.04 0.50
15 0.73 1.04 0.21 0.60
14x14
6 0.28 0.80 0.23 0.96
8 0.16 0.82 0.39 1.03
10 0.02 0.76 0.34 0.79
12 0.14 0.65 0.20 0.69
15 0.28 0.57 0.17 0.64
20x20
6 0.50 1.39 0.20 1.03
8 0.58 1.36 0.31 0.77
10 0.71 1.42 0.35 0.77
12 0.73 1.15 0.29 0.65

The deviation between measured and
calculated values for smaller applicator of
size 6x6 cm2, is larger than the large
applicators.
The differences between measured and
calculated central axis percent depth
doses for all field sizes and at all energies
are found to be within 2%.

Dose profile

Calculated and measured beam
profiles for 10-MeV for Precise
Plan
Calculated and measured beam
profiles for 10-MeV for Focus
Plan
0
20
40
60
80
100
-120 -90 -60 -30 0 30 60 90 120
Off axis distance (mm)
Relativedose
0
20
40
60
80
100
-150 -100 -50 0 50 100 150
Off axis distance (mm)
Relativedose

The differences between the
calculated beam profile and
measured data found to be
less than ±2%.

The beam profile scans for
applicator 10x10 cm2 at different
depths by using energy 10 MeV.
applicator 20x20 cm2 at different
depths by using energy 10 MeV.
applicator 10x10 cm2 at
different depths by using
energy 15 MeV.
The beam profile scans for applicator
20x20 cm2 at different depths by
using energy 15 MeV.

The flatness, symmetry and penumbra (left and right)
for energy 10 MeV, applicators 10x10 and 20x20 cm2.
Energy
Appl.
(cm2)
Depth
(cm)
Pen. Left Pen. Right Flatness
(%)
Symmetry
(%)(mm) (mm)
10 MeV
10x10
0 7.3 7.8 1.97 101.17
0.5 11.25 11.03 2.72 101.31
1 15.03 14.61 3.94 101.26
1.5 19.33 19.05 5.02 101.29
2 23.61 23.13 6.53 101.34
2.5 28.15 27.74 8.42 100.97
20x20
0 5.71 6.11 1.13 100.2
0.5 9.35 9.5 1.41 100.34
1 13.22 12.9 1.97 100.3
1.5 16.72 16.77 2.4 100.45
2 21 20.75 3.24 100.72
2.5 24.74 24.79 3.92 100.66

The flatness, symmetry and penumbra (left and right)
for energy 15 MeV, applicators 10x10 and 20x20 cm2.
Energy
Appl.
(cm2)
Depth
(cm)
Pen. Left Pen. Right Flatness
(%)
Symmetry
(%)(mm) (mm)
15 MeV
10x10
0 6.64 6.83 1.59 101.31
0.5 8.8 8.02 2.31 101.52
1 10.99 10.98 2.88 101.42
1.5 14.22 13.98 3.33 101.44
2 17.58 17.01 4.05 101.29
2.5 20.46 19.89 5.13 101.06
20x20
0 4.96 5.17 1.22 100.59
0.5 7.19 7.18 1.2 10.54
1 9.32 9.92 1.45 100.73
1.5 12.28 12.43 1.86 100.68
2 15.16 14.97 2.03 100.94
2.5 17.72 18.01 2.40 100.67

 The results showed that for applicators
10x10 and 20x20cm2 at energy 10 MeV
the flatness increased from 1.97 at
water surface to 8.43%.
 At energy 15 MeV the flatness
increased from 1.59 at water surface to
5.13%.

The penumbra linearly with the
depths for energy 10 &15 MeV
10 MeV 15 MeV

The flatness, symmetry and penumbra (left and right) for 6
MeV and applicator 10x10 cm2 at extended SSD.
SSD
(cm) Profile
Felid
(50%)
(cm)
Pen. Left
(mm)
Pen. Right
(mm)
Flatness
(%)
Symmetry
(%)
100
X 10.51 9.69 9.40 3.35 106.35
Y 10.16 9.81 9.21 1.77 101.33
105
X 11.19 14.70 13.90 3.65 104.33
Y 11.05 14.31 14.42 3.23 102.75
110
X 11.77 19.10 18.72 4.94 103.62
Y 11.76 19.42 19.23 5.09 101.28
115
X 12.46 22.60 21.25 7.45 101.88
Y 12.32 23.50 23.27 7.30 101.75
120
X 13.02 27.73 25.54 9.70 103.31
Y 13.05 28.63 27.85 9.79 103.29

Profiles for 6 MeV electron beam and 10 × 10 cm2
field size at extended SSD.
The effect of extended
SSD on transverse
beam profiles found
that loss of flatness
and increase in the
penumbra.

Beam Output Correction
Factor
1. At fixed SSD
2. At extended SSD

1. The output factor at fixed SSD, dmax, in the
electron beam

2. The output factor at extended SSD, dmax, in
the electron beam

Output factor should be checked annually for
the accuracy, and a commissioning process
would also verify these factors.

Effective SSD

Variation of extended SSD for different energies of
applicator sizes (cm2) 6x6 (a), 10x10(b),14x14 (c) and
20x20(d)

From these figures, found SSDeff from the
slope of straight line by Khan equation:
max
1
d
slope
f 

SSDeff values in cm from Elekta Precise with the variation of
applicator and energy by using Markus chamber.
Energy
Appl. (cm2)
Electron Beam Energy (MeV)
6 8 10 12 15
6x6 65.4 72.4 74.4 75.4 77
10x10 87.3 91 94.1 96.4 96.7
14x14 93.1 93.6 96.9 96.9 97
20x20 97.7 99.3 102 102.7 102.9

 The effective SSD increased from 65.4 cm for
energy 6 MeV to maximum of 102.9 cm for
energy 15 MeV.
 SSDeff increases by increasing applicator size
and energy.

Correction of output
 The dose distribution values of the electron
beam can be calculated from the following
equation:
at extended SSD and dmax.
2
max
max
100 









 
dgSSD
dSSD
DD
eff
eff
SSDcalc

Comparison between measured and
calculated values:

 Found that small deviation for the mean value
of relative ranged from 0.34 to 0.83, and relative
SD range of 1.5 to 3.7%,the deviation of SSDeff
was ≤4 % for combination of energy and field
size.

The relative output
factor
1. For fixed SSD using cutouts
2. For extended SSD

1. The ROFcut for 10 x 10 cm2 applicator at SSD = 100 cm
(a) Measured ROFcut for all
energies electron beam.
(b) Calculated ROFcut for all
energies electron beam.
(c) Difference between measured
and Calculated ROFcut.

2.The relative output for 10 MeV electron beam for different
field sizes was plotted against extended SSD.
The relative output at
extended SSD decreased
more rapidly.

Surface dose
(Dose build-up)

Central axis depth dose curves for electron beams of 6 to 15 MeV
measured in prespex sheet by using applicator 10x10 cm2.

The build-up effect of electrons
varies with electron energy.
The electron scattering is strongly
energy independent and decrease
when the energy increase.

Effect of gantry angle on output
dose rate:
 Rotate gantry angle by 5 degree for left and right
side until 25º using linac, comparison between
the measured and calculated on (TPS) we found
that the relative output for rotate gantry angle,
by using Precise Plan and Focus Plan ,the %
Difference between calculated and measured less
than 2%.

Comparison between measured and calculated relative output
for rotate gantry angle by steps 5º, applicator 10x10 cm2 for all
energies at SSD=100cm for Precise plan.

The % difference between measured and calculated
from precise plan for different energy with rotate
gantry angle by steps 5º.

Comparison between calculated from precise plan and measured
relative output for rotate gantry angle by steps 5º, use applicator
10x10 cm2 for energies 6,8,10,12 and 15 MeV.
6 MeV
0.93
0.95
0.97
0.99
1.01
-30 -20 -10 0 10 20 30
Gantry angle (o
)
RelativeOutput
Meas.
Calc.
8 MeV
0.95
0.97
0.99
1.01
-30 -20 -10 0 10 20 30
Gantry Angle (o
)
RelativeOutput
Meas
.
Calc.
10 MeV
0.96
0.98
1.00
-30 -20 -10 0 10 20 30
Gantry Angle (o
)
RelativeOutput
Meas.
Calc.
12 MeV
0.95
0.97
0.99
1.01
-30 -20 -10 0 10 20 30
Gantry Angle (o
)
RelativeOutput
Meas.
Calc.

15 MeV
0.95
0.97
0.99
1.01
-30 -20 -10 0 10 20 30
Gantry Angle (o
)
RelativeOutput
Meas.
Calc.

Conclusion

From the previous measurements we
can conclude that:

1. The central axis depth–dose curve of electron
beams depends on many factors such as: the
beam energy, field size, SSD, collimation, depth
of penetration, and angle of beam incidence.
2. In central axis percent depth dose the deviation
between measured and calculated values for
smaller applicator 6x6 cm2, is larger than the
large applicators, the differences between
measured and calculated for all sizes and at all
energies are found to be within 2%.

3. The differences between the calculated beam
profiles and measured data found to be less than
±2%.
4. The flatness, symmetry, and penumbra depends
on energy, field size, and depth.
5. The OUF should checked annually for accuracy,
and a commissioning process would also verify
these factors.
6. The SSDeff, depends on energy and field size. It is
necessary to measure SSDeff for each field insert
and energy.

7. It is recommended to enter SSDeff values in the
treatment planning system.
8. The relative output at extended SSD decreased
more rapidly.
9. The build-up effect of electrons varies with
electron energy. The electron scattering is
strongly energy independent and decreases when
the energy increases.

THANK YOU
Ayman
Gomaa Stohy

Dosimetric Evaluation of High Energy Electron Beams Applied in Radiotherapy

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (20)

En vedette

En vedette (14)

Similaire à Dosimetric Evaluation of High Energy Electron Beams Applied in Radiotherapy

Similaire à Dosimetric Evaluation of High Energy Electron Beams Applied in Radiotherapy (20)

Dernier

Dernier (20)

Dosimetric Evaluation of High Energy Electron Beams Applied in Radiotherapy