Dose Distribution Measurement (part 1)

PART I
Dose Distribution Measurement
TOPIC 3
EXTERNAL PHOTON BEAMS THERAPY
128/1/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak

PART 1
3.0 Introduction Of Treatment Planning
3.1 Dose Distribution Measurement
3.1.1 Beam Profile
3.1.2 Isodose curve
3.1.2.1 Properties of Isodose Charts
3.1.2.2 Measurement of Isodose Curves
3.1.2.3 Parameters Affecting Isodose Curves
3.1.2.4.1 Beam Quality
3.1.2.4.2 SSD
3.1.2.4.3 Collimation and Flattening Filter
3.1.2.4.4 Field Size
3.1.2.45. Wedge Filter
2

3.2 Central Axis Depth Doses In Water
3.2.1 Central Axis Depth Doses In Water SSD Technique
3.2.1.1 Percentage Depth Dose (PDD)
3.2.1.1.1 Effect of Beam Quality & Depth
3.2.1.1.2 Effect of Field Size & Shape
3.2.1.1.3 Effect of SSD
3.2.2 Central Axis Depth Doses In Water SAD Technique
3.2.2.1 Tissue Air Ratio (TAR)
3.2.2.1.1 Effect of Distance
3.2.2.1.2 Effect of energy, depth, and field size
3.2.2.1.3 TAR and SAD
3.2.2.1.4 BSF
3.2.2.2 Tissue Phantom Ratio (TPR)
3.2.2.3 Tissue Maximum Ratio (TMR)
3.2.2.4 Scatter Air Ratio (SAR)
PART 2
28/1/2018 3

PART 3
3.3 Treatment Technique
3.3.1 Single Field Technique
3.3.2 Parallel Opposed Field
3.3.3 Multiple Field
3.3.4 Rotation Therapy
3.3.5 Wedge Field

3.0 Introduction of treatment planning

TREATMENT PLANNING
Positioning
Immobilization
Localization
Field Selection
Dose distribution
Calculations
Verification
Execution
3.0 Introduction of Treatment Planning

• Water phantom:
• closely approximates the radiation
absorption and scattering properties of
muscle and other soft tissues;
• universally available with reproducible
radiation properties
water phantom
1 Phantom

• Solid dry phantoms:
• same effective atomic number, number of electrons per gram and mass density
• E.g: Lucite (perspex), polystyrene
• Solid Water Phantom:
• epoxy resin--based solid substitute for water
• E.g: acrylic
1 Phantom

28/1/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak 9

• Alderson Rando Phantom:
• incorporates materials to simulate various
body tissues---muscle, bone, lung, and air
cavities
Alderson Rando phantom
1 Phantom

2 Treatment Planning

13
1
• Treatment planning can be described as the iterative process
whereby the treatment strategy of the radiation oncologist is
quantified as a set of treatment instructions including a
description of the expected dose distribution in the patient
2
• Planning is based on predictions of dosage delivered to the
individual patient by the proposed arrangement of radiation
beams

14
3
• External photon beam radiotherapy is usually carried out
with more than one radiation beam in order to achieve a
uniform dose distribution inside the target volume and an as low
as possible a dose in healthy tissues surrounding the target
4
• ICRU Report No. 50 recommends a target dose uniformity within
+7% and –5% of the dose delivered to a well defined prescription
point within the target

The main parameters in external beam dose delivery with
photon beams are:
Depth of treatment
Fields size
Source-skin distance (SSD) in SSD
setups.
Number of beams used in dose
delivery to the patient.
Photon beam energy.
Source-axis distance (SAD) in SAD
setup
Treatment time for orthovoltage and
teletherapy machines.
Number of monitor units (MUs) for
linacs.

3.1 Measurement of Dose Distribution
3.1.1 Beam Profile
3.1.2 Isodose curve
3.1.3 PDD

Dose distribution &
scatter analysis
Off axis ratios &
beam profiles
Central axis depth dose in
water
Isodose distribution
Beam flatness
Isodose
correctionSource to
surface
distance
Source to
axis
distance
Beam
symmetry
Percentage
depth dose
Tissue air
ratio
Relationship
Tissue
phantom ratio
Tissue maximum ratio
Scatter air ratio
Scatter maximum ratio

3.1.1 Beam Profile
3.1.1.3 Penumbra
3.1.1.1Beam Flatness (F)
3.1.1.2 Beam symmetry (S)

3.1.1 Beam Profile
A beam profile is
measured at multiple
points on a plane
perpendicular to the
central beam axis.
Measurement is usually
performed in a water
phantom using a
cylindrical ionisation
chamber
Beam profile may
also be determined
by film dosimetry or
other dosimeters,
particularly TLD or
silicon diodes which
have a small
detection area.
A beam profile can
be one dimensional
(along one axis) or
two dimensional
(measuring in the x
and y axes).

3.1.1 Beam Profile3.1.1 Beam Profile

3.1.1 Beam Profile

Beam Profiles
(dose variation across the field at a specified depth)
3.1.1 Beam Profile
• usually specified at 10 cm
• within 3% over 80% of the field
flatness
• usually specified at 10 cm
• within 2% over 80% of the fieldsymmetry
• dose falls off rapidly at the beam edge,
between a dose of 20-80% of the central
beam axis
penumbra region
• dose is minimal (under 20% of the central
beam dose)umbra region

• Is assessed by finding the maximum Dmax and
Dmin dose point values on the beam profile
within the central 80% of
F=100 x Dmax-Dmin
Dmax+Dmin
the beam width
•
Standard linac specification generally require that F be less than 3% when
measured in a water phantom at a depth of 10 cm and an SSD of 100cm for
the largest field size available
3.1.1.1Beam Flatness (F)

• Determined at Dmax which represents the most
sensitive depth
A typical symmetry specification is that any
two dose points on a beam profile, equidistant
from the central axis point, are within 2 % of
each other
•

• Alternately area under the dmax beam profiles
on each side (left and right) of central axis
extending to the 50% dose level (normalized
to 100% at the central axis point)
determined.
S = 100 x arealeft-arearight
arealeft + arearight
are
•

• Penumbra is the unsharp edge of the radiation beam created mainly by the
finite source size
• Penumbra could be classified as
– Geometric Penumbra which depends on
• Source Size
• Source to Diaphragm distance
• Source to skin distance
– Radiological Penumbra which is
• Geometric penumbra + Scatter
Part VIII.3.5 Determination of Dose to a Patient-I Slide 28
3.1.1.3 Penumbra

The Geometric Penumbra
Slide 29
SAD Field size at surface
3.1.1.3 Penumbra

Physical Penumbra / Radiological
penumbra
• Radiation beam penumbra
– Lateral scatter increases the radiation beam penumbra
– Lower energy, more side scatter and hence larger
penumbra
– Density of the scattering medium affects the penumbra
• Issues : larger the penumbra, greater the dose to normal tissues
3.1.1.3 Penumbra

Scattered radiation
Scatter
3.1.1.3 Penumbra

3.1.2 Isodose curve
3.1.2.4.2 Source Size, SSD, and SDD
3.1.2.4.5 Wedge Filter

Beams of ionising
radiation have
characteristic
process of energy
deposition, in order
to represent
volumetric & planar
variations in
absorbed dose are
depicted by isodose
curves
3.2 Isodose curve3.2 Isodose curve

 Isodose curves
– Lines joining the points of equal percentage depth
dose(PDD)
– Curves are usually drawn at regular intervals of
absorbed dose & expressed as a percentage of dose
• Isodose charts
– Family of isodose curves
– PDD values are normalised at Dmax or reference
depth
3.2 Isodose curve

3.2 Isodose curve
36
1
• Isodose curves are lines that join points of equal dose
(drawn at interval absorbed dose & Expressed as a
percentage of the dose at a reference point).
2
• They offer a planar representation of the dose distribution and
easily show the behaviour of one beam or a combination of
beams with different shielding, wedges, bolus, etc.

Isodose Distribution
Isodose is calculated by using 2 normalization:
– For SSD set ups, all isodose values are normalized
to 100 at point P on the central beam axis
– For SAD set-ups, the isodose values are
normalized to 100 at the isocentre
•
• The isodose charts for an SSD set up are
of PDD values
plots
• For SAD set up, plots are either TAR or TMR
values
3.2 Isodose curve

3.2 Isodose curve

Part VIII.3.7 Operational Considerations – Planning of physical
treatment
Slide 39
Isodose curves and chart
2 3 4 5 6 7 8 9 10 10 10 9 8 7 6 5 4 3 2
2 3 4 5 6 7 8 9 9.5 9.5 9.5 9 8 7 6 5 4 3 2
2 3 4 5 6 7 8 9 9 9 9 9 8 7 6 5 4 3 2
2 3 4 5 6 7 8 8.5 8.5 8.5 8.5 8.5 8 7 6 5 4 3 2
2 3 4 5 6 7 8 8 8 8 8 8 8 7 6 5 4 3 2
2 3 4 5 6 7 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7 6 5 4 3 2
2 3 4 5 6 7 7 7 7 7 7 7 7 7 6 5 4 3 2
2 3 4 5 6 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6 5 4 3 2
2 3 4 5 6 6 6 6 6 6 6 6 6 6 6 5 4 3 2
2 3 4 5 5.5 5.5 5.5 5.5 5.5 5.5 5.5 5.5 5.5 5.5 5.5 5 4 3 2
2 3 4 5 5 5 5 5 5 5 5 5 5 5 5 5 4 3 2
2 3 4 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4 3 2
2 3 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 3 2
2 3 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3 2
2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 2
2 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2
2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
Isodose curve or line; line joining points of equal dose value
a representative diagram
A typical isodose chart
90
80
70
60
50
40
30
20

3.2 Isodose curve
40
3
• Isodose curves can be measured in water directly or can be
calculated from PDD and beam profile data
4
• A set of isodose curves is valid for a given treatment machine,
beam energy, SSD and field size

3.2 Isodose curve
41
5
• While isodose curves can be made to display the actual dose in
grays, it is more common to present them normalized to 100%
at a fixed point
6
• Two such common point normalizations are as follows:

3.2 Isodose curve

2.Dose greatest on the CA at any depth and dec
rease toward the edges of the
beam with the exception
of LINAC beams with horns at shallow depths
1.Horns created by flattening filter which is designed to
overcompensate near the
urface in order to obtain flat isodose curves at depth
(10 cm)

3. The penumbra region:
The dose rate decreases rapidly as a function of lateral distance
from the beam axis.
The width of geometric penumbra depends on source size(d), distance from
the source (SD), and source-to-collimator diaphragm distance (SDD).
The falloff near the edge of the beam is caused by geometric
penumbra and reduced side scatter
This falloff can be described using physical (total) penumbra:
Physical penumbra is defined as lateral distance between two specified isodose curves at a specified dept
h (e.g., lateral distance between 90% and 20% isodose lines at the depth of Dmax).

4. Outside the geometric limits of the beam
and the penumbra
• The dose variation is the result of side
scatter from the field and both leakage
and scatter from the collimator system.
 The dose in this region is generally
low results from radiation transmitted
through the collimator and head
shielding

1. Measurement in water phantom
Ionization chamberWater phantom

2. Measurement in solid water phantom

3.1.2.3.2 Source Size and SSD

3.2.4 Parameters Affecting Isodose Curves
Beam
quality
Source
Size and
SSD
Collimation
and
flattening
filter
Field size
Parameters
affecting
the single-
beam
isodose

Depth
The depth of a
given isodose
curve increases
with beam
quality
Beam energy
Beam energy
also influences
isodose curve
shape near the
field border
Lateral scatter
Greater lateral scatter
associated with lower-energy
beams causes the isodose curves
outside the field to bulge out
In other words, the absorbed
dose in the medium outside the
primary beam is greater for low
energy beams than for those of
higher energy

3.1.2.3.1 Beam Quality3.1.2.3.1 Beam Quality

One disadvantage of the
orthovoltage beams is increased
scattered dose to tissue outside the
treatment region.
For megavoltage beams, the scatter
outside the field is minimized as a
result of forward scattering and
becomes more a function of
collimation than energy.

3.1.2.3.2 SSD

3.1.2.3.2 SSD
SSD affect the isodose curves by the geometric
penumbra.
The SSD affects the PDD and the depth of the
isodose curves.
The dose variation across the field border is a
complex function of geometric penumbra, lateral
scatter, and collimation.
SSD

Collimation
If no filter used, the result is conical
field distribution
Makes field uniform across whole
width, with the same intensity
Flattening filter thickest in the middle
and tapers to sides
Filter causes change in beam quality
Different photon spectrum at edge than
in middle, harder in middle
1. Flattening filter
2.Collimator blocks

• Field size is the width and length of the radiation beam at SSD or SAD
• Field size at any depth is usually defined by the 50% width of the profile at
that depth

Field size (dosimetrical):
lateral distance between the 50%
isodose lines at a reference depth
Field size (geometrical) :
field defining light is made to coincide
with 50% isodose lines of the radiation
beam projected on a plane
perpendicular to the beam axis and at
standard SSD or SAD

Field size
< 6 cm
TPS should be
mandatory for
small field size.
Bell shape
Relative
large
penumbra
region

Wedge Filters
Figure A:normalized to Dmax,
B:normalized to Dmax without wedge
the degree of tilt being
dependent on the slope
of the wedge filter
Wedge shaped: cause
the isodose curve
tilted from their
normal position
The most
commonly used
beam-
modifying
device

Mounted on a (transparent
plastic) tray
Arranged at a distance of at
least 15 cm from the skin
surface, avoid destroying
the skin-sparing effect of
the megavoltage beam

10 cm
wedge angle
Wedge angle
the angle through which an isodose curve is titled at the
central ray of a beam at a specified depth (10 cm)
The angle between the isodose curve and the normal to the
central axis
The angle of isodose tilt decreases with increasing depth in
the phantom due to presence of scattered radiation

WEDGE FACTOR
The ratio of doses
with and without the
wedge, at a point in
phantom along the
central axis of the
beam
Measured at a
suitable depth beyond
dmax
(5 to 10 cm) either in
air or in phantom
Sometimes
incorporated into the
isodose curves
The presence of a wedge filter decreases the output of the machine.

Hinge angle,φ
It is the angle between central axes of two
beams passing through the wedge
Relationship b/w φ & θ
Wedge angle,θ= 90 – φ/2

Dose Distribution Measurement (part 1)

Dose Distribution Measurement (part 1)

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Dose Distribution Measurement (part 1)

Similar to Dose Distribution Measurement (part 1) (20)

More from Nik Noor Ashikin Nik Ab Razak

More from Nik Noor Ashikin Nik Ab Razak (12)

Recently uploaded

Recently uploaded (20)

Dose Distribution Measurement (part 1)