Treatment Planning Ii Patient Data, Corrections, And Set Up

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Treatment Planning Ii Patient Data, Corrections, And Set Up

  1. 1. TREATMENT PLANNING II: PATIENT DATA, CORRECTIONS, AND SET-UP The Physics of Radiation Therapy. Faiz M. Khan
  2. 2. TREATMENT PLANNING II <ul><li>Basic depth-dose data and isodose curves are usually measured in a cubic water phantom, beams incident normally on the flat surface at specified distance </li></ul><ul><li>The patient's body, however, is neither homogeneous nor flat in surface contour. </li></ul><ul><li>correction for contour curvature , and tissue inhomogeneities and patient positioning. </li></ul>
  3. 3. ACQUISITION OF PATIENT DATA <ul><li>Accurate patient dosimetry is only possible when sufficiently accurate patient data are available </li></ul><ul><li>body contour, outline, and density of relevant internal structures, location, and extent of the target volume </li></ul>
  4. 4. Body Contours <ul><li>Acquisition of body contours and internal structures is best accomplished by imaging </li></ul><ul><ul><li>CT and MRI …. </li></ul></ul><ul><ul><li>Scans are performed with the patient positioned the same way as for actual treatment </li></ul></ul><ul><ul><li>lead wire </li></ul></ul><ul><ul><ul><li>measure antero/posterior and/or lateral diameters of the contour </li></ul></ul></ul><ul><ul><li>Optical and ultrasonic </li></ul></ul>ACQUISITION OF PATIENT DATA
  5. 5. Some important points for contour making <ul><li>same position as used in the actual treatment. </li></ul><ul><li>Horizontal line representing the tabletop </li></ul><ul><li>Important bony landmarks must be indicated on the contour. </li></ul><ul><li>Checks of body contour during the treatment course </li></ul><ul><li>If body thickness varies significantly , contours should be determined in more than one plane. </li></ul>ACQUISITION OF PATIENT DATA
  6. 6. Internal Structures <ul><li>Transverse Tomography </li></ul><ul><li>Computed Tomography </li></ul><ul><li>Magnetic Resonance Imaging </li></ul><ul><li>Ultrasound </li></ul>
  7. 7. Internal Structures <ul><li>provide cross-sectional information of internal structures in relation to the external contour </li></ul><ul><li>poor contrast and spatial resolution </li></ul>Transverse Tomography
  8. 8. Internal Structures <ul><li>the distribution of attenuation coefficients within the layer </li></ul><ul><li>an image can be reconstructed that represents various structures with different attenuation properties. </li></ul>Computed Tomography
  9. 9. Internal Structures <ul><li>CT numbers </li></ul><ul><ul><li>related to attenuation coefficients </li></ul></ul><ul><li>Hounsfield numbers </li></ul><ul><ul><li>CT numbers normalized </li></ul></ul>Computed Tomography
  10. 10. Internal Structures <ul><li>CT numbers </li></ul><ul><ul><li>it is possible to infer electron density (electrons cm -3 ) </li></ul></ul>Computed Tomography
  11. 11. Internal Structures <ul><li>The CT information is useful in two aspects of treatment planning: </li></ul><ul><ul><li>delineation of target volume and the surrounding structures in relation to the external contour </li></ul></ul><ul><ul><li>providing quantitative data (in the form of CT numbers) for tissue heterogeneity corrections </li></ul></ul>Computed Tomography
  12. 12. Internal Structures <ul><li>MRI has developed, in parallel to CT </li></ul><ul><li>advantages over CT </li></ul><ul><ul><li>scan directly in axial, sagittal, coronal, or oblique planes </li></ul></ul><ul><ul><li>not involving the use of ionizing radiation </li></ul></ul><ul><ul><li>higher contrast </li></ul></ul><ul><ul><li>Better imaging of soft tissue tumor </li></ul></ul>Magnetic Resonance Imaging
  13. 13. Internal Structures <ul><li>Disadvantages compared with CT </li></ul><ul><ul><li>inability to image bone or calcifications </li></ul></ul><ul><ul><li>longer scan acquisition time </li></ul></ul><ul><ul><li>technical difficulties due to small hole of the magnet and </li></ul></ul><ul><ul><li>magnetic interference with metallic objects </li></ul></ul>Magnetic Resonance Imaging
  14. 14. Internal Structures <ul><li>Ultrasound can provide useful information in localizing many malignancy-prone structures in the lower pelvis, retroperitoneum, upper abdomen, breast, and chest wall </li></ul>Ultrasound
  15. 15. TREATMENT SIMULATION <ul><li>uses a diagnostic x-ray tube but duplicates a radiation treatment unit in terms of its geometrical, mechanical, and optical properties. </li></ul>TREATMENT SIMULATION
  16. 16. TREATMENT SIMULATION <ul><li>By radiographic visualization of </li></ul><ul><ul><li>internal organs, </li></ul></ul><ul><ul><li>correct positioning of fields </li></ul></ul><ul><ul><li>and shielding blocks </li></ul></ul><ul><ul><li>can be obtained in relation to external landmarks </li></ul></ul><ul><li>fluoroscopic capability by dynamic visualization </li></ul>TREATMENT SIMULATION
  17. 17. TREATMENT SIMULATION <ul><li>An exciting development in the area of simulation is that of converting a CT scanner into a simulator </li></ul><ul><ul><li>CT-SIM </li></ul></ul>TREATMENT SIMULATION
  18. 18. TREATMENT VERIFICATION <ul><li>Port Films </li></ul><ul><li>Electronic Portal Imaging (EPI) </li></ul><ul><li>Cone Beam CT </li></ul><ul><li>MV-CT ( Tomotherapy ) </li></ul>TREATMENT VERIFICATION
  19. 19. TREATMENT VERIFICATION <ul><li>The primary purpose of port filming is to verify the treatment volume under actual conditions of treatment </li></ul><ul><li>the image quality with the megavoltage x-ray beam is poorer than with the diagnostic or the simulator film </li></ul>Port Films
  20. 20. TREATMENT VERIFICATION Port Films
  21. 21. TREATMENT VERIFICATION <ul><li>Limitations of port film </li></ul><ul><ul><li>Viewing is delayed because of the time required for processing </li></ul></ul><ul><ul><li>It’s impractical to do port films before each treatment </li></ul></ul><ul><ul><li>Film image is of poor quality especially for photon energies greater than 6MV </li></ul></ul>Port Films
  22. 22. <ul><li>Electronic portal imaging device ( EPID ) </li></ul><ul><ul><li>Mount on the linac </li></ul></ul><ul><ul><li>Real-time, digital feedback to the user. </li></ul></ul>TREATMENT VERIFICATION
  23. 23. <ul><li>Portal imaging devices </li></ul><ul><ul><li>fluoroscopy-based systems </li></ul></ul><ul><ul><li>liquid filled ionization chamber matrices </li></ul></ul><ul><ul><li>amorphous silicon based system </li></ul></ul>
  24. 24. <ul><li>Fluoroscopy-based systems </li></ul><ul><ul><li>The detector quantum efficiency ( DQE ) of these systems is limited by electronic noise in the camera system and poor optical coupling between the light emitter and the camera system (only 0.01% of the emitted photons reach the camera) </li></ul></ul>
  25. 25. <ul><li>liquid filled ionization chamber matrices </li></ul><ul><ul><li>The maximum spatial resolution is 2.3 mm x 2.9 mm , increasing to 2.3 mm x 4.5 mm depending on acquisition mode </li></ul></ul>
  26. 26. <ul><li>amorphous silicon based system </li></ul><ul><ul><li>less excess dose to be delivered to the patient per portal image and yet yielding a superior image quality, resolution of 0.784 x 0.784 mm 2 . </li></ul></ul>
  27. 27. CORRECTIONS <ul><li>Effective Source-to-Surface Distance Method </li></ul><ul><li>Tissue-air (or Tissue-maximum) Ratio Method </li></ul><ul><li>lsodose Shift Method </li></ul>CORRECTIONS FOR CONTOUR IRREGULARITIES
  28. 28. CORRECTIONS Effective SSD Method
  29. 29. CORRECTIONS <ul><li>ratio depend on only of the depth and the field size at that depth </li></ul>TAR Method
  30. 30. CORRECTIONS <ul><li>Sliding the isodose chart up or down, depending on whether there is tissue excess or deficit along that line, by an amount k ×h where k is a factor less than 1 </li></ul>lsodose Shift Method
  31. 31. CORRECTIONS <ul><li>The presence of inhomogeneities will produce changes in the dose distribution, depending on the amount and type of material present and on the quality of radiation </li></ul>CORRECTIONS FOR TISSUE INHOMOGENEITIES
  32. 32. CORRECTIONS <ul><li>The effects of tissue inhomogeneities </li></ul><ul><ul><li>changes in the absorption of the primary beam and the associated pattern of scattered photons </li></ul></ul><ul><ul><ul><li>primary beam : points that lie beyond the inhomogeneity, </li></ul></ul></ul><ul><ul><ul><li>Scattered : points near the inhomogeneity </li></ul></ul></ul><ul><ul><li>changes in the secondary electron fluence </li></ul></ul><ul><ul><ul><li>tissues within the inhomogeneity and at the boundaries. </li></ul></ul></ul>CORRECTIONS FOR TISSUE INHOMOGENEITIES
  33. 33. CORRECTIONS <ul><li>Corrections for Beam Attenuation and Scattering </li></ul><ul><ul><li>TAR method, Power law TAR method , Equivalent TAR method, Isodose shift method, Typical correction factors </li></ul></ul><ul><li>Absorbed Dose within an Inhomogeneity </li></ul>CORRECTIONS FOR TISSUE INHOMOGENEITIES
  34. 34. CORRECTIONS <ul><li>TAR method </li></ul><ul><ul><li>d' = d1 + ρ 1 d2 + d3 </li></ul></ul><ul><ul><li>d is the actual depth of P from the surface </li></ul></ul>Corrections for Beam Attenuation and Scattering
  35. 35. CORRECTIONS <ul><li>Power Law Tissue-air Ratio Method </li></ul><ul><ul><li>correction factor does depend on the location of the inhomogeneity relative to point P but not relative to the surface or in the build-up region </li></ul></ul>Corrections for Beam Attenuation and Scattering
  36. 36. CORRECTIONS <ul><li>Power Law Tissue-air Ratio Method </li></ul><ul><ul><li>A more general form, provided by Sontag and Cunningham </li></ul></ul><ul><ul><li>allows for correction of the dose to points within an inhomogeneity as well as below it . </li></ul></ul>Corrections for Beam Attenuation and Scattering
  37. 37. CORRECTIONS <ul><li>Equivalent Tissue-air Ratio Method </li></ul><ul><ul><li>correctly predicted the effect of scattering structures depends on their geometric arrangement with respect to point P </li></ul></ul>Corrections for Beam Attenuation and Scattering
  38. 38. CORRECTIONS <ul><li>Equivalent Tissue-air Ratio Method </li></ul><ul><ul><li>d' is the water equivalent depth, d is the actual depth, r is the beam dimension at depth d, </li></ul></ul><ul><ul><li>r' = r × ρ ' = scaled field size dimension </li></ul></ul>Corrections for Beam Attenuation and Scattering
  39. 39. CORRECTIONS <ul><li>lsodose Shift Method </li></ul><ul><ul><li>manually correcting isodose charts for the presence of inhomogeneity </li></ul></ul>Corrections for Beam Attenuation and Scattering
  40. 40. CORRECTIONS <ul><li>Typical Correction Factors </li></ul><ul><ul><li>None of the methods discussed above can claim an accuracy of ± 5% for all irradiation conditions encountered in radiotherapy </li></ul></ul><ul><ul><li>Tang et al. have compared a few commonly used methods against measured data using a heterogeneous phantom containing layers of polystyrene and cork </li></ul></ul>Corrections for Beam Attenuation and Scattering
  41. 41. CORRECTIONS <ul><li>Typical Correction Factors </li></ul><ul><ul><li>Their results (Tang et al. ) </li></ul></ul><ul><ul><ul><li>the TAR method overestimates the dose for all energies </li></ul></ul></ul><ul><ul><ul><li>the ETAR is best suited for the lower-energy beams (≦6 MV) </li></ul></ul></ul><ul><ul><ul><li>the generalized Batho method is the best in the high-energ range (≧10 MV) </li></ul></ul></ul>Corrections for Beam Attenuation and Scattering
  42. 42. CORRECTIONS <ul><li>Bone Mineral </li></ul>Absorbed Dose within an Inhomogeneity
  43. 43. CORRECTIONS <ul><li>Bone-tissue Interface </li></ul><ul><ul><li>Soft Tissue in Bone </li></ul></ul>Absorbed Dose within an Inhomogeneity
  44. 44. CORRECTIONS <ul><li>Bone-tissue Interface </li></ul><ul><ul><li>Soft Tissue Surrounding Bone </li></ul></ul>Absorbed Dose within an Inhomogeneity
  45. 45. CORRECTIONS <ul><li>Bone-tissue Interface </li></ul><ul><ul><li>Soft Tissue Surrounding Bone </li></ul></ul><ul><ul><li>forward scatter </li></ul></ul><ul><ul><ul><li>For energies up to 10 MV, the dose at the interface is initially less than the dose in a homogeneous soft tissue medium but then builds up to a dose that is slightly greater than that in the homogeneous case. </li></ul></ul></ul><ul><ul><ul><li>For higher energies, there is an enhancement of dose at the interface because of the increased electron fluence in bone due to pair production </li></ul></ul></ul>Absorbed Dose within an Inhomogeneity
  46. 46. CORRECTIONS <ul><li>Bone-tissue Interface </li></ul><ul><ul><li>Soft Tissue Surrounding Bone </li></ul></ul>Absorbed Dose within an Inhomogeneity
  47. 47. CORRECTIONS <ul><li>Bone-tissue Interface </li></ul><ul><ul><li>parallel-opposed beams </li></ul></ul>Absorbed Dose within an Inhomogeneity
  48. 48. CORRECTIONS <ul><li>Bone-tissue Interface </li></ul><ul><ul><li>parallel-opposed beams </li></ul></ul>Absorbed Dose within an Inhomogeneity
  49. 49. CORRECTIONS <ul><li>Lung Tissue </li></ul><ul><ul><li>Dose within the lung tissue is primarily governed by its density </li></ul></ul><ul><ul><li>But in the first layers of soft tissue beyond a large thickness of lung, there is some loss of secondary electrons </li></ul></ul>Absorbed Dose within an Inhomogeneity
  50. 50. CORRECTIONS <ul><li>Lung Tissue </li></ul><ul><ul><li>problem of loss of lateral electronic equilibrium when a high-energy photon beam traverses the lung </li></ul></ul><ul><ul><ul><li>dose profile to become less sharp </li></ul></ul></ul><ul><ul><li>The effect is significant for small field sizes ( < 6 x 6 cm ) and higher energies ( >6 MV ) </li></ul></ul>Absorbed Dose within an Inhomogeneity
  51. 51. CORRECTIONS <ul><li>Air Cavity </li></ul><ul><ul><li>The most important effect of air cavities in megavoltage beam dosimetry is the partial loss of electronic equilibrium at the cavity surface </li></ul></ul><ul><ul><li>The most significant decrease in dose occurs at the surface beyond the-cavity, for large cavities (4 cm deep) and the smallest field (4 x 4 cm) </li></ul></ul>Absorbed Dose within an Inhomogeneity
  52. 52. <ul><li>To preserve the skin-sparing properties of the megavoltage photon beams, the compensator is placed a suitable distance ( > 20 cm) away from the patient's skin </li></ul>TISSUE COMPENSATION
  53. 55. <ul><li>PDD increases with SSD </li></ul><ul><ul><li>the Mayneord F Factor ( without considering changes in scattering ) </li></ul></ul>PDD - Dependence on Source-Surface Distance

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