Isodose lines
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Isodose lines represent points of equal absorbed radiation dose on a dose distribution map. They are depicted as curves on isodose charts showing the volumetric and planar variations in absorbed dose. Factors influencing isodose curves include beam quality, field size, source-to-skin distance, beam modifiers like wedges or bolus, and depth. Isodose curves are used in radiation therapy treatment planning to evaluate dose distributions and ensure tumor coverage while sparing surrounding healthy tissues. They provide critical information about the radiation dose profile essential for safe and effective treatment.
Isodose lines
- 1. Isodose lines 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
- 4. PERCENT DEPTH DOSE Quotient expressed as percentage, of the absorbed dose at any depth d to the absorbed dose at dmax 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
- 7. 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
- 9. 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
- 11. PENUMBRA • Dose transitions near the boarders of the field • Region 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
- 12. For cobalt-60 teletherapy: P ~ 1cm For linacs : P~ 3mm
- 15. 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
- 17. 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 • Final response A/B is independent of fluctuation in the output
- 18. Sources of isodose chart Atlases of premeasured isodose charts Generated by calculations using different algorithms Manufacturers of radiation generators
- 19. Parameters of isodose curves Beam quality Source size, SSD, SDD Penumbra Collimation and flattening filter Field size
- 20. Beam Quality • Depth 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
- 22. 4 MV 6MV 15MV
- 23. 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
- 24. 5*5 cm2 10*10 cm2 15*15cm2
- 26. SSD affects the PDD and the depth of the isodose curves PDD icreases with SSD
- 27. 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
- 28. 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
- 29. 15o Wedge
- 30. 30o Wedge
- 31. 45o Wedge
- 32. 60o Wedge
- 33. Wedge 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. Angle 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
- 34. 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
- 37. Bolus • A tissue equivalent material used to reduce the depth of the maximum dose (Dmax) • 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
- 40. 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
- 42. Parallel Opposed Fields • Advantages – The simplicity and reproducibility of setup – Homogeneous dose to the tumour – Less chances of geometrical miss • Disadvantage – Excessive dose to normal tissues and critical organs above and below the tumour
- 43. 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
- 44. 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 • Practical feasibility • Setup accuracy • Reproducibility of the treatment technique
- 49. Isodose lines of Electrons
- 51. Isodose curves • Scattering of electrons determines shapes of isodose curves –Expansion –Lateral constriction • Larger field size required at surface
- 55. “Charged Particle Therapy” to better preserve healthy tissues and organs at risk
- 57. 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
- 58. Thank u…