Importance of Isodose lines in Radiotherapy and Clinical Physics practise

1. Isodose lines
definition, influencing factors,
clinical application
LIBIN SCARIA
Resident Medical Physicist

2. 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. Dose distribution

4. PERCENT DEPTH DOSE
Quotient expressed as percentage, of the absorbed dose at any
depth d to the absorbed dose at dmax along the central axis of the
beam

5.  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

7. 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

9. Beam profile
• Falloff of the beam
– By the reduced side scatter
– Physical penumbra width
• High dose or ‘horns’ near the surface in the
periphery of the field
• Outside the geometric limits of the beam and the
penumbra, the dose variation is the result of
– side scatter from the field
– both leakage and scatter from the collimator system

11. PENUMBRA
• Dose transitions near the boarders of the
field
• Region at the radiation beam over which
the dose rate changes rapidly as a
function of distance from the central axis
Geometric penumbra
Transmission penumbra
Physical penumbra
Scatter penumbra

12. For cobalt-60 teletherapy: P ~ 1cm
For linacs : P~ 3mm

15. Measurement of isodose
curves
• Ion chambers
• Solid state detectors
– TLD,Silicon diodes,
• Radiographic films
• Computer driven devices
 Ion chamber is the most reliable method due
to its relatively flat energy response and
precision

17. Beam Analysing System
3D water phantom- IBA Wellhofer Blue Phantom
• Two ion chambers :
– Detector A: to move in the
tank of water to sample the
dose rate
– Detector B: fixed at some point
in the field to monitor the
beam intensity with time
• Final response A/B is
independent of fluctuation
in the output

18. Sources of isodose chart
Atlases of premeasured isodose
charts
Generated by calculations using
different algorithms
Manufacturers of radiation
generators

19. Parameters of isodose curves
Beam quality
Source size, SSD, SDD
Penumbra
Collimation and flattening filter
Field size

20. Beam Quality
• 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
– becomes more a function of collimation than
energy

22. 4 MV 6MV 15MV

23. Field size
 One of the most important parameters in
treatment planning
 PDD increases as field size increases
 Field size dependence of PDD is less pronounced
for higher energy than for lower energies
 Field size smaller than 6 cm
Relative large penumbra region
Bell shape
TPS should be mandatory for small field size

24. 5*5 cm2 10*10 cm2 15*15cm2

26. SSD affects the PDD and the depth of the isodose curves
PDD icreases with SSD

27. Beam fattening filter
• Intensity is more at central axis and
decreases as we move away
• Non-uniform dose at any given depth
• FF is used to uniform it
• Usually made up of Al or Brass

28. Wedge filters
 Wedge shaped absorber causes a progressive
decrease in intensity across the beam
 Results in tilt of the isodose curve & degree of
the tilt depends upon the slope of the wedge
filter
 Material: tungsten, brass. Lead or steel

29. 15o Wedge

30. 30o Wedge

31. 45o Wedge

32. 60o Wedge

33. Wedge isodose angle (θ) is the complement of the
angle through which the isodose curve is tilted with respect to
the central ray of the beam at any specified depth
This depth is important because the
angle will decrease with increasing
depth.
Angle of isodose tilt to decrease with
increasing depth in the phantom.
The choice of the reference depth
varies:
10 centimeters
1/2 - 2/3rd of the beam width
At the 50% isodose curve

34. 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

37. Bolus
• A tissue equivalent material
used to reduce the depth of
the maximum dose (Dmax)
• In megavoltage radiation
bolus is primarily used to
bring up the buildup zone
near the skin in treating
superficial lesions.
• The thickness is usually 0.5
cm to 1.5 cm

40. Combined Open Field Technique
• Criteria:
– The dose distribution within the tumor volume is
reasonably uniform (±5%).
– The maximum dose to the tissue in the beam is not
excessive (not more than 110% of the prescribed
dose)
– Normal critical structures in the beam do not
receive doses near or beyond tolerance

42. Parallel Opposed Fields
• Advantages
– The simplicity and reproducibility of setup
– Homogeneous dose to the tumour
– Less chances of geometrical miss
• Disadvantage
– Excessive dose to normal tissues and critical
organs above and below the tumour

43. Multiple fields
 To deliver maximum dose to the tumour
and minimum dose to the surrounding
tissues
• 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

44. Multiple fields
 Certain beam angles are prohibited
• Presence of critical organs in those directions
• 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
• Practical feasibility
• Setup accuracy
• Reproducibility of the treatment technique

49. Isodose lines of Electrons

51. Isodose curves
• Scattering of electrons
determines shapes of
isodose curves
–Expansion
–Lateral constriction
• Larger field size
required at surface

54. Proton isoeffective dose

55. “Charged Particle Therapy” to better preserve healthy tissues and organs at risk

57. Medicine
is
a science of
uncertainity
and
an art of
probability
Dr. William Osler
A canadian Physician and one of the four founding
professors of John Hopkins Hospital, USA

58. Thank u…