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Isodose Curves

Adhikari Abish, MD ResidentNAMS/BPKMCH

2013.01.11, Bharatpur

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Definition

Isodose curves are the lines joining the points of equal Percentage Depth Dose (PDD), therefore a means of mapping variation in the dose as a function of depth and transverse distance from the central axis of the beam.

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Definition

Isodose curves are the lines joining the points of equal Percentage Depth Dose (PDD), therefore a means of mapping variation in the dose as a function of depth and transverse distance from the central axis of the beam.

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Depends upon

• Beam energy (quality)• Field size• SSD• Attenuating factors • Source geometry/collimation

510 x 10 cm field. Cobalt-60

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Measurement of Isodose Curves

Dose measurement in a water phantom:

A. Moving probe (central axis depth dose, beam profile)

B. Fixed reference probe (position fixed, located in field)

C. Relative dose = A/B

A

B

IBA - Wellhofer Blue Phantom

Homogenous PhantomFlat SurfacePerpendicular beam

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Isodose Curve Depends on Beam Quality

200 kVp, bulging penumbra due to

greater scatter of low energy photons.

Co-60, penumbra primarily due to source size (geometric penumbra)

4 MV, small penumbra, due to

small photon scatter and short electron

range

10 MV, penumbra greater than that of 4 MV, due to greater electron range

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Isodose Curve Depends upon the Field Size

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

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Isodose Curve Effected by Wedge Filter

Normalized to dmax with wedge Normalized to dmax without wedge (open field)

Wedge Angle : ICRU #24: The angle through which an isodose curve is titled at the central ray of a beam at a specified depth. (10 cm)

wedge angle

wedge angled

= 10

cm

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Wedge Field Techniques

9080

70

60

50

180

160

140

120

Orthogonal pair – open field Orthogonal pair – wedged field

80

90

70

60

Uniform dose

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Wedge Field Techniques (hinge angle and wedge angle)

φ

In clinical reality, this relationship isn’t true in general, due to surface curvature.

Computer treatment plan is needed to find the optimal wedge angle

angle hinge

290

angle wedge

θ

80

90

70

60

80

90

70

60

Isodose Distribution – parallel opposed fields

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THANK YOU