REGIONAL CANCER CENTER
Beams of ionising radiation have characteristic
processes of energy deposition, hence the
Expected dose distribution can be estimated.
In order to represent volumetric and planar variations in
absorbed dose, distributions are depicted by means of ISODOSE
DOSE BUILD UP
As high energy photons enter patient, high speed
electrons are ejected from surface and subsequent layers
These electrons deposit energy a significant distance from
Hence, electron fluence and dose increases with depth
Until a maximum is reached.
Photon fluence continuously decreases with depth,
hence production of electrons decrease with depth.
net effect - beyond a certain depth, dose decreases with
The quantity percentage depth dose
may be defined as- the quotient,
expressed as a percentage, of the
absorbed dose at any depth 'd‘
to the absorbed dose at a fixed
reference depth 'd0' ,along
the central axis of the beam.
Isodose curves are the lines joining the points of
equal Percentage Depth Dose (PDD). The curves are usually
drawn at regular intervals of absorbed dose and expressed as a
percentage of the dose at a reference point.
ISODOSE CHARTS : It consists of a family of isodose curves.
The depth dose values of the curves are
1) At the point of maximum dose on the
central axis (Dmax)
2) At a fixed distance along the central axis in
the irradiated medium (SAD).
The dose variation across the field at a specified
• Field size: the lateral distance between the 50% isodose
lines at a reference depth.
• Beam alignment: the field-defining light is made to
coincide with the 50% isodose lines of the radiation
beam projected on a plane perpendicular to the beam
axis and at standard SSD or SAD
• High dose or ‘horns’ near the surface in the periphery of
Created by the flattening filter
Under-compensate near the surface in order to obtain flat
isodose curves at greater depths (10 cm)
Dose transition near the borders of the field.
The region at the of a radiation beam over
which the dose rate changes rapidly as a
function of distance from the central axis.
Transmission Penumbra: variable transmission
of beam through non divergent collimator angle.
Physical Penumbra: the lateral distance between
two specified isodose curves at a specified depth.
( lateral distance between 90%10% or 8020%)
Geometric penumbra : due to finite dimensions
of the source
Co-60 teletherapy High energy linac
SDD = 40 cm SDD= 45.5cm
SSD = 80 cm SSD= 100cm
D = 1 cm D = 0.3cm
W 1cm W 0.36cm
The width of geometric penumbra depends on
source size, distance from the source, and source-to
Falloff of the beam
1.By the geometric penumbra
2.By the reduced side scatter
3.Physical penumbra width
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.
Measurement of isodose curves
1. Ion Chambers
2. Solid state detectors
3. Radiographic Films
4. Computer driven devices
Ion chamber is the most reliable method,
because of its relatively flat energy response and
Two ion chambers
Detector A–To move in the
tank of water to sample the
Monitor B–fixed at some
point in the field to monitor
the beam intensity with time
The final response A/B is
independent of fluctuations
Sources of ISODOSE CHART
1.Atlasas of premeasured isodose charts
2.It can be generated by calculations using
various algorithms for treatment planning
3.Mnufacturers of radiation generaters
Parameters of isodose curves
The parameters that affect the single beam
isodose distribution are:
2.Source size, SSD, and SDD -the penumbra
3.Collimation and flattening filter
Isodose Curve Depends on Beam Quality
200 kVp,Dmax at
PDD at 10cm 35%
Sharp beam edge.
due to greater scatter
of low energy
PDD at 10=56%
primarily due to
6 MV,Dmax 1.5cm
PDD at 10=67%
scatter and short
than that of 4 MV,
due to greater
The depth of a given isodose curve increases with
Greater lateral scatter associated with lower-energy Beams.
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.
2.Source Size, SSD, and SDD
THE PENUMBRA EFFECT
Source size, SSD, and SDD affect the isodose curves
by the geometric penumbra.
The SSD affects the PDD and the depth of the
The dose variation across the field border is a
complex function of geometric penumbra, lateral
scatter, and collimation.
Field size is determined based on dosimetric coverage, not
One of the most important parameters in
Field size smaller than 6 cm
• Relative large penumbra region
• Bell shape
Thus TPS should be mandatory for small field
FIELD SIZE CONT…
Compare 5 cm × 5 cm
field with 10 cm × 10 cm
field for 60Co
•central axis depth dose
larger for larger field size
•increase in amount of
• for smaller field very
small area flatness over
4. Collimation and Flattening Filter
Collimation: the collimator block + the flattening
filter + absorbers + scatterers
The flattening filter has the greatest influence in determining the
shape of the isodose curves.
• The photon spectrum may different for the peripheral areas
compared with the central part of the beam.
• THE CHANGE IN QUALITY across the beam causes the flatness
to change with depth.
A beam modifying device, which
causes a progressive decrease
in intensity across the beam,
resulting in tilting the isodose
curves to thinner side.
Material: tungsten, brass. Lead or steel
Individualized wedge system
• A separate wedge for each
• To minimize the loss of
• To align the thin end of the
wedge with the border of
the light field
• Used in Co60
Universal wedge system
• A single wedge for all beam widths
• Fixed centrally in the beam
• Used in Linac
Advanced Wedge Systems
Omni wedge (Elekta)
• There is only one universal wedge
(60 degree) attached above the jaws.
• To control the wedge angle, an
appropriate combination of open
and wedged fields are used.
Dynamic wedge (Varian)
• One side of jaws move in
(or close) while beam is on.
• Wedge angle is determined by
controlling the speed of the moving jaw.
NORMALISED TO Dmax NORMALISED TO ‘Dmax without wedge’
The angle through which an isodose
curve is titled at the central ray of a
beam at a specified depth10cm/
50% isodose curves
The angle between the isodose
curve and the normal to the central
The wedge should be such that the
isodose curves from each
field are parallel to the bisector of
the hinge angle. When the
isodoses are combined, the
resultant distribution is uniform.
θ = the wedge angle
φ= the hinge angle
S = the separation or the
distance between the thick
ends of the wedge filters
as projected on the surface
Combination of radiation fields
Isodose Distribution – parallel opposed open fields
BEAM WEIGHED 100 AT Dmax BEAM WEIGHED 100 at Isocenter
Parallel opposed fields
• The simplicity and reproducibility of setup
• Homogeneous dose to the tumor
• Less chances of geometrical miss
• The excessive dose to normal tissues and critical
organs above and below the tumor
•Using fields of appropriate size
•Increasing the number of fields or portals
•Selecting appropriate beam directions
•Adjusting beam weights
•Using appropriate beam energy
•Using beam modifiers.
To deliver maximum dose to the tumor
and minimum dose to the surrounding.
Tissues dose uniformity with the tumor
volume and sparing of critical organs are
important considerations in judging a plan.
Certain beam angles are prohibited
due to the presence of critical
organs in those directions
The setup accuracy of a treatment
may be better with parallel opposed
The acceptability of a treatment
plan depends not
only on the dose distribution but also on
•The practical feasibility
•Reproducibility of the treatment
ROTATION THERAPY CONTIN….
• The beam moves continuously about the patient, or
the patient is rotated while the beam is held fixed.
• For small and deep-seated tumors, not for Too large.
• Beam should be aimed a suitable distance beyond the
tumour area and is called PAST POINTING.
• The maximum dose for 360 degree rotation occurs at
the isocenter and for partial arcs it is displaced towards
the irradiated sector.
• Delivers a reasonably uniform dose from the surface to a
specific depth, after which dose falls of rapidly , eventually to
near zero value.
DEPTH DOSE CURVE
Most useful treatment depth , therapeutic range of electrons is
given by the depth of 90% of the isodose curves……….
The PDD increases as the energy
However unlike photon beams ,
the percent of surface
dose for electron beam increases
For low energy electron beams,
isodose curves bulging out for
all dose levels
Electron beam cont…
But bulge out for low dose levels
For high energy electron
beams, isodose curves
constrict for high dose levels
TAKE HOME POINTS
1.The dose at any depth is greatest on the central axis of the
beam and gradually decreases toward the edges of the beams.
( horns in some LINACs overcompensate ).
2.The dose rate decreases rapidly as a function of lateral distance
from the beam axis in the penumbra region. ( geometric
penumbra with side scatter ↓)
3.It could be defined a physical penumbra as the lateral distance
between two specified isodose curves at a specified depth.
( lateral distance between 90%10% or 8020%)
4.Therapeutic housing/source housing: lateral scatter from the
medium and leakage from the head of the machine
HOME POINTS CONT…..
5. The parameters that affect the single beam
isodose distribution are:1.Beam quality 2.Source size, SSD, and
SDD the penumbra Effect 3.Collimation and flattening
filter 4.Field size.
6. Wedge Angle is The angle through which an isodose curve is
titled at the central ray of a beam at a specified depth
10cm/50% isodose curves.
7. Isodose curves are different for Co60 , photon , & Electrons
8. Therapeutic range of electrons is given by the depth of 90% of
the isodose curves.