Isodose curves depict absorbed dose distributions and variations in volume and planes. They join points of equal dose. Isodose charts show the variation in dose as a function of depth and transverse distance from the central beam axis. Factors like beam energy, field size, and distance affect isodose curve shape through penumbra and dose deposition. Multiple beams are often needed to adequately treat tumors while sparing surrounding tissues. Beam arrangements, weights, and modifiers must be optimized for each plan.

1. Treatment Planning I
ISODOSE DISTRIBUTIONS
DR.KIRAN KUMAR BR

2. • Introduction
-Beams of ionizing radiations have characteristic
process of energy deposition, hence the expected
dose distribution can be estimated.
-In order to represent volumetric and planar
variations absorbed dose, distributions are
depicted by means of ISODOSE CURVES.

3. • Isodose curves
-They are lines joining the points of equal dose.
(Percentage Depth Dose-PDD)
-They are drawn at regular intervals of absorbed dose and
expressed as a percentage of the dose at a reference point.

4. • Isodose Chart
- It is a family of isodose curves for a given beam.
- It is usually drawn at equal increments of PDD representing
the variation in dose as a function of depth and transverse
distance from the central axis.
- It can be normalized either at the point of maximum dose on
the central axis (SSD) or at a fixed distance along the
central axis in the irradiated medium (SAD).

5. Isodose Chart
A: SSD type, 60Co beam, SSD = 80 cm, field size = 10 × 10 cm at
surface.
B: SAD type, 60Co beam, SAD = 100 cm, depth of isocenter = 10 cm,
field size at isocenter = 10 × 10 cm.

9. Geometric Penumbra
• The penumbra width increases with increase in
source diameter, SSD, and depth but decreases
with an increase in SDD.
• The geometric penumbra, however, is independent
of field size as long as the movement of the
diaphragm is in one plane, that is, SDD stays
constant with increase in field size

10. • General properties for isodose charts:
-The dose at any depth is greatest on the central axis of the beam and
gradually decreases toward the edges of the beams.
-The dose rate decreases rapidly as a function of lateral distance from the
beam axis in the penumbra region.
-The decrease in dose rate is also due to Physical penumbra width which
is defined as the lateral distance between two specified isodose curves
at a specified depth.
-Therapeutic housing/source housing: lateral scatter from the medium and
leakage from the head of the machine.

11. Depth Dose Profile-
Showing variation of dose across the field. 60Co beam, SSD = 80 cm,
depth = 10 cm, field size at surface = 10 × 10 cm.
Dotted line indicates geometric field boundary at a 10-cm depth.

12. Cross-Sectional Isodose Distribution
Isodose values are normalized to 100% at the center of the field.

13. • Measurement of Isodose Curves
Detectors:
• Ion chamber (most reliable method, due to its relatively
flat energy response and precision)
• Solid state detectors
• Radiographic films
Medium:
• Water Phantom

14. • The Ion Chamber used for Isodose
Measurements:
-It can be made in regions of high dose gradient.
-The sensitive volume of the chamber should be less
than 15 mm long and have an inside diameter of 5
mm or less.

15. Automatic devices for measuring Isodose curves

16. • Sources of Isodose Charts
-Atlases of premeasured isodose charts.
-It can be generated by calculations using
various algorithms for treatment planning.

17. Parameters affecting the isodose Curves
• Beam quality
• The penumbra effect:
1.Source size
2.SSD
3.Source-diaphragm distance
• Collimation and flattening filter
• Field size

18. Beam Quality vs. Isodose Curves
• The central axis depth dose distribution depends on the
beam energy. As a result, the depth of a given isodose
curve increases with beam quality.
• As Beam energy increase, lateral scatter decreases,
Isodose curve shape near the field borders.
• As Beam energy decreases, physical penumbra increases

19. Isodose Distributions for Different Quality Radiations
A: 200 kVp, SSD = 50 cm, HVL = 1 mm Cu, field size = 10 × 10 cm.
B: 60Co, SSD = 80 cm, field size = 10 × 10 cm.
C: 4-MV x-rays, SSD = 100 cm, field size = 10 × 10 cm.
D: 10-MV x-rays, SSD = 100 cm, field size = 10 × 10 cm.

21. The Penumbra Effect
-Source size, SSD and SDD affect the shape of isodose
curves by virtue of the geometric penumbra.
-The SSD affects PDD and the depth of the isodose curves.
-As such, the field sharpness at depth is not simply
determined by the source or focal spot size.

22. • Field Size
Adequate dosimetric coverage of the tumor requires a
determination of appropriate field size.
The treatment planning with isodose curves should be
mandatory for small field sizes in which a relatively large
part of the field is in the penumbra region..
One needs to calculate dose at several off-axis points or use a
beam-flattening compensator.

23. Collimation & 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 be different for the peripheral
areas compared with the central part of the beam being
cone shaped.
• Beam flatness is usually specified at a 10-cm depth with
the maximum limits set at the depth of maximum

26. Wedge Filter: beam-modifying device
• It’s a wedge-shaped absorber that causes a progressive
decrease in the intensity across the beam, resulting in a tilt
of the isodose curves from their normal position.
• It’s usually made of a dense material, such as lead or steel,
and is mounted on a tray.

33. Wedge Isodose Angle
• The angle through which an isodose curve is titled at the
central ray of a beam at a specified depth.
• The wedge angle is the angle between the isodose curve
and the normal to the central axis.
• The current recommendation is to use a single reference
depth of 10 cm for wedge angle specification

34. Wedge systems
(A) an individualized wedge for a specific field width in which the thin
end of the wedge is always aligned with the field border
(B) a universal wedge in which the center of the wedge filter is fixed at
the beam axis and the field can be opened to any width.

35. Wedge angle
q= 90 -f/2
 Where q is the wedge angle,
 F is the hinge angle, which the angle between the central axis of the
2 beams

37. Effect of Wedges on Beam Quality
• Wedge systems causes attenuation of the lower-energy
photons causing beam hardening
• It may be assumed to be the same as for the
corresponding open beams.
• The error caused by this assumption is minimized if the
wedge transmission factor has been measured at a
reference depth close to the point of interest.

38. Wedge Transmission Factor
• The presence of wedge filters decrease the output of the
machine.
• Wedge Transmission Factor is the ratio of doses with and
without the wedge.
• It should be measured in phantom at a suitable depth
beyond the depth of maximum dose (e.g. 10 cm).
• A common approach is to normalize the isodose curves
relative to the central axis Dmax. With this approach, the
output of the beam must be corrected using the wedge
factor.

39. Combination of Radiation Fields
• A single photon beam is seldom used (e.g. internal
mammary nodes, the spinal cord)
• For treatment of most tumors a combination of two or
more beams is required for an acceptable distribution of
dose within the tumor and the surrounding normal tissue.

40. 1. Parallel Opposed Fields
Advantages:
1. Simplicity
2. Reproducibility of set-up
3. Homogeneous dose to the tumor
4. Less chances of geometrical miss
Disadvantage:
1. The excessive dose to normal tissue and critical organs
above and below the tumor

41. Isodose Distribution – parallel opposed field
A: Each beam is given a weight of 100 at the depth of Dmax.
B: Isocentric plan with each beam weighted 100 at the isocenter.

42. Factors affecting Parallel Opposed Fields
A.Patient thickness vs dose uniformity
• Uniformity of the dose is dependent on the patient
thickness, beam flatness and beam energy.
• As the patient thickness or the beam energy decreases, the
central axis maximum dose near the surface increases
relative to the midpoint dose. This effect is called tissue
lateral effect

43. Depth dose curves for parallel opposed field normalized to
midpoint value. Patient thickness = 25 cm, field size = 10
× 10 cm, SSD = 100 cm.

44. • The curves for cobalt-60 and 4 MV show that for a patient
of this thickness parallel opposed beams would give rise to
an excessively higher dose to the subcutaneous tissues
compared with the tumor dose at the midpoint.
• As the energy is increased to 10 MV, the distribution
becomes almost uniform and at 25 MV it shows significant
sparing of the superficial tissues relative to the midline
structures.

45. B.Edge Effect (Lateral Tissue Damage)
• When treating with multiple beams, the question arises whether one
should treat one field per day or all fields per day.
• For parallel opposed beams,treating with one field per day produces
greater biologic damage to normal subcutaneous tissue than treating
with two fields per day, despite the fact that the total dose is the same.
• Apparently, the biologic effect in the normal tissue is greater if it
receives alternating high- and low-dose fractions compared with the
equal but medium-size dose fractions resulting from treating both
fields daily. This phenomenon has been called the edge effect, or the
tissue lateral damage .
• The problem becomes more severe when larger thicknesses are treated
with one field per day using a lower-energy beam (e.g.,6 MV).

46. C.Integral dose
It is the Total absorbed dose in the treated volume expressed
by gram-rad or Kg-gray.
It is used provide qualitative guidelines for treatment
planning for selecting beam energy, field sizes, and
multiplicity of fields.

47. Multiple Fields
It is used when we need 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.

48. Strategies:
– (a) using fields of appropriate size
– (b) increasing the number of fields
– (c) selecting appropriate beam directions
– (d) adjusting beam weights
– (e) using appropriate beam energy
– (f) using beam modifiers

49. Multiple Fields
A: Two opposing pairs at right angles.
B: Two opposing pairs at 120 degrees.
C: Three fields: one anterior and two posterior oblique, at 45 degrees with
the vertical.

50. Multiple field Plans
A: Three-field isocentric
technique. Each beam
delivers 100 units of dose at
the isocenter; 4 MV, field size
= 8 × 8 cm at isocenter,
SAD = 100 cm.
B: Four-field isocentric
technique. Each beam
delivers 100 units of dose at
the isocenter; 10 MV, field
size = 8 × 8 cm at
isocenter, SAD = 100 cm.
C: Four-field SSD technique
in which all beams are
weighted 100 units at their
respective points of Dmax; 10
MV, field size = 8 × 8 cm
at surface, SSD = 100 cm.

51. Thank you.