Isodose lines

Importance of Isodose lines in Radiotherapy and Clinical Physics practise

1. Iso dose l i nes 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 d max 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

6. 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

7. 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

8. PEN UMBRA Dose transitions near the boarders of the field R egion 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

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

10. 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

11. 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 F inal response A/B is independent of fluctuation in the output

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

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

14. Beam Quality D epth 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

15. 4 MV 6MV 15MV

16. 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

17. 5*5 cm 2 10*10 cm 2 15*15cm 2

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

19. 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

20. 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

21. 15 o Wedge

22. 30 o Wedge

23. 4 5 o Wedge

24. 60 o Wedge

25. W edge 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 . A ngle 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

26. 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

27. B o lus A tissue equivalent material used to reduce the depth of the maximum dose ( D max ) 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

28. 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

29. Parallel Opposed Fields Advantages The simplicity and reproducibility of setup Homogeneous dose to the tumour Less chances of geometrical miss Disadvantage E xcessive dose to normal tissues and critical organs above and below the tumour

30. 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

31. 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 P ractical feasibility Setup accuracy Reproducibility of the treatment technique

32. Isodose lines of Electrons

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

34. Proton isoeffective dose

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

36. 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

37. T h a n k u …