Isodose Curves Adhikari Abish, MD Resident NAMS/BPKMCH 2013.01.11, Bharatpur 1
May 25, 2015
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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