Isodose curves

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Presentation on Isodose Curves.

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Isodose curves

  1. 1. Isodose CurvesAdhikari Abish, MD Resident NAMS/BPKMCH 2013.01.11, Bharatpur 1
  2. 2. DefinitionIsodose curves are the lines joiningthe points of equal PercentageDepth Dose (PDD), therefore ameans of mapping variation in thedose as a function of depth andtransverse distance from the centralaxis of the beam. 2
  3. 3. DefinitionIsodose curves are the lines joiningthe points of equal PercentageDepth Dose (PDD), therefore ameans of mapping variation in thedose as a function of depth andtransverse distance from the centralaxis of the beam. 3
  4. 4. Depends upon• Beam energy (quality)• Field size• SSD• Attenuating factors• Source geometry/collimation 4
  5. 5. 10 x 10 cm field. Cobalt-60 5
  6. 6. Measurement of Isodose Curves A B Homogenous PhantomDose measurement in a water phantom: Flat SurfaceA. Moving probe (central axis depth dose, beam profile) Perpendicular beamB. Fixed reference probe (position fixed, located in field)C. Relative dose = A/B IBA - Wellhofer Blue Phantom 6
  7. 7. Isodose Curve Depends on Beam Quality 200 kVp, bulging Co-60, penumbra penumbra due to primarily due to sourcegreater scatter of low size (geometric energy photons. penumbra)4 MV, small penumbra, 10 MV, penumbra due to small photon greater than that of 4 scatter and short MV, due to greater electron range electron range 7
  8. 8. Isodose Curve Depends upon the Field SizeField size is determined based on dosimetric coverage, notgeometric coverage. 8
  9. 9. Isodose Curve Effected by Wedge Filter Normalized to dmax without wedge Normalized to dmax with wedge (open field)d = 10 cm wedge wedge angle angle 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) 9
  10. 10. Wedge Field Techniques 180 160 140 90 90 80 70 Uniform120 80 dose 60 70 50 60 Orthogonal pair – open field Orthogonal pair – wedged field 10
  11. 11. Wedge Field Techniques (hinge angle and wedge angle) hinge angle θ wedge angle  90 2 φIn clinical reality, this relationship isn’ttrue in general, due to surfacecurvature.Computer treatment plan is needed tofind the optimal wedge angle 11
  12. 12. Isodose Distribution – parallel opposed fields
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  14. 14. THANK YOU 15

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