1. Isodose curves represent the dose distribution from radiation beams and are lines connecting points of equal percentage depth dose. They are used to depict the volumetric and planar variations in absorbed dose. 2. The parameters that affect the shape of isodose curves include beam quality, source size, SSD, SDD, field size, and beam modifiers like wedges and flattening filters. Lower beam energy results in greater lateral scatter and more bulging curves. 3. Multiple radiation fields can be combined using appropriate beam weights, sizes, angles and modifiers to deliver a more uniform dose to the tumor while sparing surrounding tissues. Parameters like setup accuracy and plan practicality are also considered.

1. ISODOSE CURVES
RADIATION ONCOLOGY
DR.PAUL GEORGE
RADIATION ONCOLOGY
REGIONAL CANCER CENTER
TRIVANDRUM

2. INTRODUCTION
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
CURVES .

3. 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
original interaction.
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
Depth.

4. SKIN SPARING EFFECT

5. PDD
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.

6. ISODOSE CURVES
DEFINITION:
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
normalized:
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).

7. Isodose chart Co60 10*10cm
SSD =80cm SAD

8. BEAM PROFILE
The dose variation across the field at a specified
depth.

9. • 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
the field
Created by the flattening filter
Under-compensate near the surface in order to obtain flat
isodose curves at greater depths (10 cm)

11. PENUMBRA
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.
Geometric Penumbra:
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 8020%)

12. Geometric penumbra : due to finite dimensions
of the source

13. Geometric penumbra:
W=D× SSD─SDD
SDD
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
diaphragm distance.

14. 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.

15. 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
precision

16. Two ion chambers
Detector A–To move in the
tank of water to sample the
dose rate
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
in output.

17. 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

18. Parameters of isodose curves
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

19. 19
Isodose Curve Depends on Beam Quality
200 kVp,Dmax at
patient surface.
PDD at 10cm 35%
Sharp beam edge.
Bulging penumbra
due to greater scatter
of low energy
photons.
Co-60,Dmax0.5cm
PDD at 10=56%
penumbra
primarily due to
source size
(geometric
penumbra)
6 MV,Dmax 1.5cm
PDD at 10=67%
small
penumbra,due to
small photon
scatter and short
electron range
20MV,Dmax 4.5
PDDat 10=80%.
Penumbra greater
than that of 4 MV,
due to greater
electron range

20. 1.BEAM QUALITY
BEAM QUALITY
The depth of a given isodose curve increases with
beam quality.
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.

21. 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
isodose curves.
The dose variation across the field border is a
complex function of geometric penumbra, lateral
scatter, and collimation.

23. 23
Field size is determined based on dosimetric coverage, not
geometric coverage.

24. 3.FIELD SIZE
One of the most important parameters in
treatment planning
Field size smaller than 6 cm
• Relative large penumbra region
• Bell shape
Thus TPS should be mandatory for small field
size.

25. 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
scattered radiation.
• for smaller field very
small area flatness over
field

26. 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.

29. Wedge Filters
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

30. WEDGE SYSTEMS
Individualized wedge system
• A separate wedge for each
beam width
• To minimize the loss of
beam output
• 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

31. 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.

32. NORMALISED TO Dmax NORMALISED TO ‘Dmax without wedge’

33. Wedge angle
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
Axis

34. Wedge angle
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= 90º-φ/2
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

36. Combination of radiation fields
Isodose Distribution – parallel opposed open fields
BEAM WEIGHED 100 AT Dmax BEAM WEIGHED 100 at Isocenter

37. Parallel opposed fields
Advantages
• The simplicity and reproducibility of setup
• Homogeneous dose to the tumor
• Less chances of geometrical miss
Disadvantage
• The excessive dose to normal tissues and critical
organs above and below the tumor

38. MULTIPLE FIELDS
•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.

39. 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
beam arrangement.
The acceptability of a treatment
plan depends not
only on the dose distribution but also on
•The practical feasibility
•Setup accuracy
•Reproducibility of the treatment
technique

40. ROTATION THERAPY

41. 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.

42. ELECTRONS
• 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

43. ELECTRONS PHOTONS

44. 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
increases.
However unlike photon beams ,
the percent of surface
dose for electron beam increases
with energy

45. 45
Electron Beams
For low energy electron beams,
isodose curves bulging out for
all dose levels
Isodose curves

46. 46
Electron beam cont…
Isodose curves
But bulge out for low dose levels
For high energy electron
beams, isodose curves
constrict for high dose levels

47. 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 8020%)
4.Therapeutic housing/source housing: lateral scatter from the
medium and leakage from the head of the machine

48. 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.

49. THANK YOU
PAUL GEORGE