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  2. 2. INTRODUCTION Beams of ionising radiation have characteristic processes of energy deposition, hence the Expected dose distribution can be estimated. In order to represent volumetric and planar variations in absorbed dose, distributions are depicted by means of ISODOSE CURVES .
  3. 3. DOSE BUILD UP As high energy photons enter patient, high speed electrons are ejected from surface and subsequent layers These electrons deposit energy a significant distance from original interaction. Hence, electron fluence and dose increases with depth Until a maximum is reached. Photon fluence continuously decreases with depth, hence production of electrons decrease with depth. net effect - beyond a certain depth, dose decreases with Depth.
  5. 5. PDD The quantity percentage depth dose may be defined as- the quotient, expressed as a percentage, of the absorbed dose at any depth 'd‘ to the absorbed dose at a fixed reference depth 'd0' ,along the central axis of the beam.
  6. 6. ISODOSE CURVES DEFINITION: Isodose curves are the lines joining the points of equal Percentage Depth Dose (PDD). The curves are usually drawn at regular intervals of absorbed dose and expressed as a percentage of the dose at a reference point. ISODOSE CHARTS : It consists of a family of isodose curves. The depth dose values of the curves are normalized: 1) At the point of maximum dose on the central axis (Dmax) 2) At a fixed distance along the central axis in the irradiated medium (SAD).
  7. 7. Isodose chart Co60 10*10cm SSD =80cm SAD
  8. 8. BEAM PROFILE The dose variation across the field at a specified depth.
  9. 9. • Field size: the lateral distance between the 50% isodose lines at a reference depth. • Beam alignment: the field-defining light is made to coincide with the 50% isodose lines of the radiation beam projected on a plane perpendicular to the beam axis and at standard SSD or SAD • High dose or ‘horns’ near the surface in the periphery of the field Created by the flattening filter Under-compensate near the surface in order to obtain flat isodose curves at greater depths (10 cm)
  10. 10. PENUMBRA Dose transition near the borders of the field. The region at the of a radiation beam over which the dose rate changes rapidly as a function of distance from the central axis. Geometric Penumbra: Transmission Penumbra: variable transmission of beam through non divergent collimator angle. Physical Penumbra: the lateral distance between two specified isodose curves at a specified depth. ( lateral distance between 90%10% or 8020%)
  11. 11. Geometric penumbra : due to finite dimensions of the source
  12. 12. Geometric penumbra: W=D× SSD─SDD SDD Co-60 teletherapy High energy linac SDD = 40 cm SDD= 45.5cm SSD = 80 cm SSD= 100cm D = 1 cm D = 0.3cm W 1cm W 0.36cm The width of geometric penumbra depends on source size, distance from the source, and source-to diaphragm distance.
  13. 13. Falloff of the beam 1.By the geometric penumbra 2.By the reduced side scatter 3.Physical penumbra width Outside the geometric limits of the beam and the penumbra, the dose variation is the result of side scatter from the field and both leakage and scatter from the collimator system.
  14. 14. Measurement of isodose curves 1. Ion Chambers 2. Solid state detectors 3. Radiographic Films 4. Computer driven devices Ion chamber is the most reliable method, because of its relatively flat energy response and precision
  15. 15. Two ion chambers Detector A–To move in the tank of water to sample the dose rate Monitor B–fixed at some point in the field to monitor the beam intensity with time The final response A/B is independent of fluctuations in output.
  16. 16. Sources of ISODOSE CHART 1.Atlasas of premeasured isodose charts 2.It can be generated by calculations using various algorithms for treatment planning 3.Mnufacturers of radiation generaters
  17. 17. Parameters of isodose curves The parameters that affect the single beam isodose distribution are: 1.Beam quality 2.Source size, SSD, and SDD -the penumbra Effect 3.Collimation and flattening filter 4.Field size
  18. 18. 19 Isodose Curve Depends on Beam Quality 200 kVp,Dmax at patient surface. PDD at 10cm 35% Sharp beam edge. Bulging penumbra due to greater scatter of low energy photons. Co-60,Dmax0.5cm PDD at 10=56% penumbra primarily due to source size (geometric penumbra) 6 MV,Dmax 1.5cm PDD at 10=67% small penumbra,due to small photon scatter and short electron range 20MV,Dmax 4.5 PDDat 10=80%. Penumbra greater than that of 4 MV, due to greater electron range
  19. 19. 1.BEAM QUALITY BEAM QUALITY The 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 and becomes more a function of collimation than energy.
  20. 20. 2.Source Size, SSD, and SDD THE PENUMBRA EFFECT Source size, SSD, and SDD affect the isodose curves by the geometric penumbra. The SSD affects the PDD and the depth of the isodose curves. The dose variation across the field border is a complex function of geometric penumbra, lateral scatter, and collimation.
  21. 21. 23 Field size is determined based on dosimetric coverage, not geometric coverage.
  22. 22. 3.FIELD SIZE One of the most important parameters in treatment planning Field size smaller than 6 cm • Relative large penumbra region • Bell shape Thus TPS should be mandatory for small field size.
  23. 23. FIELD SIZE CONT… Compare 5 cm × 5 cm field with 10 cm × 10 cm field for 60Co •central axis depth dose larger for larger field size •increase in amount of scattered radiation. • for smaller field very small area flatness over field
  24. 24. 4. Collimation and Flattening Filter Collimation: the collimator block + the flattening filter + absorbers + scatterers The flattening filter has the greatest influence in determining the shape of the isodose curves. • The photon spectrum may different for the peripheral areas compared with the central part of the beam. • THE CHANGE IN QUALITY across the beam causes the flatness to change with depth.
  25. 25. Wedge Filters A beam modifying device, which causes a progressive decrease in intensity across the beam, resulting in tilting the isodose curves to thinner side. Material: tungsten, brass. Lead or steel
  26. 26. WEDGE SYSTEMS Individualized wedge system • A separate wedge for each beam width • To minimize the loss of beam output • To align the thin end of the wedge with the border of the light field • Used in Co60 Universal wedge system • A single wedge for all beam widths • Fixed centrally in the beam • Used in Linac
  27. 27. Advanced Wedge Systems Omni wedge (Elekta) • There is only one universal wedge (60 degree) attached above the jaws. • To control the wedge angle, an appropriate combination of open and wedged fields are used. Dynamic wedge (Varian) • One side of jaws move in (or close) while beam is on. • Wedge angle is determined by controlling the speed of the moving jaw.
  28. 28. NORMALISED TO Dmax NORMALISED TO ‘Dmax without wedge’
  29. 29. Wedge angle The angle through which an isodose curve is titled at the central ray of a beam at a specified depth10cm/ 50% isodose curves The angle between the isodose curve and the normal to the central Axis
  30. 30. Wedge angle The wedge should be such that the isodose curves from each field are parallel to the bisector of the hinge angle. When the isodoses are combined, the resultant distribution is uniform. θ= 90º-φ/2 θ = the wedge angle φ= the hinge angle S = the separation or the distance between the thick ends of the wedge filters as projected on the surface
  31. 31. Combination of radiation fields Isodose Distribution – parallel opposed open fields BEAM WEIGHED 100 AT Dmax BEAM WEIGHED 100 at Isocenter
  32. 32. Parallel opposed fields Advantages • The simplicity and reproducibility of setup • Homogeneous dose to the tumor • Less chances of geometrical miss Disadvantage • The excessive dose to normal tissues and critical organs above and below the tumor
  33. 33. MULTIPLE FIELDS •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. To deliver maximum dose to the tumor and minimum dose to the surrounding. Tissues dose uniformity with the tumor volume and sparing of critical organs are important considerations in judging a plan.
  34. 34. Certain beam angles are prohibited due to the presence of critical organs in those directions The 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 •The practical feasibility •Setup accuracy •Reproducibility of the treatment technique
  36. 36. ROTATION THERAPY CONTIN…. • The beam moves continuously about the patient, or the patient is rotated while the beam is held fixed. • For small and deep-seated tumors, not for Too large. • Beam should be aimed a suitable distance beyond the tumour area and is called PAST POINTING. • The maximum dose for 360 degree rotation occurs at the isocenter and for partial arcs it is displaced towards the irradiated sector.
  37. 37. ELECTRONS • Delivers a reasonably uniform dose from the surface to a specific depth, after which dose falls of rapidly , eventually to near zero value. DEPTH DOSE CURVE
  39. 39. Most useful treatment depth , therapeutic range of electrons is given by the depth of 90% of the isodose curves………. The PDD increases as the energy increases. However unlike photon beams , the percent of surface dose for electron beam increases with energy
  40. 40. 45 Electron Beams For low energy electron beams, isodose curves bulging out for all dose levels Isodose curves
  41. 41. 46 Electron beam cont… Isodose curves But bulge out for low dose levels For high energy electron beams, isodose curves constrict for high dose levels
  42. 42. TAKE HOME POINTS 1.The dose at any depth is greatest on the central axis of the beam and gradually decreases toward the edges of the beams. ( horns in some LINACs  overcompensate ). 2.The dose rate decreases rapidly as a function of lateral distance from the beam axis in the penumbra region. ( geometric penumbra with side scatter ↓) 3.It could be defined a physical penumbra as the lateral distance between two specified isodose curves at a specified depth. ( lateral distance between 90%10% or 8020%) 4.Therapeutic housing/source housing: lateral scatter from the medium and leakage from the head of the machine
  43. 43. HOME POINTS CONT….. 5. The parameters that affect the single beam isodose distribution are:1.Beam quality 2.Source size, SSD, and SDD the penumbra Effect 3.Collimation and flattening filter 4.Field size. 6. Wedge Angle is The angle through which an isodose curve is titled at the central ray of a beam at a specified depth 10cm/50% isodose curves. 7. Isodose curves are different for Co60 , photon , & Electrons 8. Therapeutic range of electrons is given by the depth of 90% of the isodose curves.