TREATMENT PLANNING II: PATIENT DATA, CORRECTIONS, AND SET-UP

1. TREATMENT PLANNING II: PATIENT DATA, CORRECTIONS, AND SET-UP 講者：蕭安成參考資料：The Physics of Radiation Therapy. Faiz M. Khan

2. TREATMENT PLANNING II • 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 • The patient's body, however, is neither homogeneous nor flat in surface contour. • correction for contour curvature, and tissue inhomogeneities and patient positioning.

3. ACQUISITION OF PATIENT DATA • Accurate patient dosimetry is only possible when sufficiently accurate patient data are available • body contour, outline, and density of relevant internal structures, location, and extent of the target volume

4. ACQUISITION OF PATIENT DATA Body Contours • Acquisition of body contours and internal structures is best accomplished by imaging • CT and MRI …. • Scans are performed with the patient positioned the same way as for actual treatment • lead wire • measure antero/posterior and/or lateral diameters of the contour • Optical and ultrasonic

5. ACQUISITION OF PATIENT DATA Some important points for contour making • same position as used in the actual treatment. • Horizontal line representing the tabletop • Important bony landmarks must be indicated on the contour. • Checks of body contour during the treatment course • If body thickness varies significantly , contours should be determined in more than one plane.

6. Internal Structures • Transverse Tomography • Computed Tomography • Magnetic Resonance Imaging • Ultrasound

7. Internal Structures Transverse Tomography • provide cross-sectional information of internal structures in relation to the external contour • poor contrast and spatial resolution

8. Internal Structures Computed Tomography • the distribution of attenuation coefficients within the layer • an image can be reconstructed that represents various structures with different attenuation properties.

9. Internal Structures Computed Tomography • CT numbers • related to attenuation coefficients • Hounsfield numbers • CT numbers normalized

10. Internal Structures Computed Tomography • CT numbers • it is possible to infer electron density (electrons cm-3)

11. Internal Structures Computed Tomography • The CT information is useful in two aspects of treatment planning: • delineation of target volume and the surrounding structures in relation to the external contour • providing quantitative data (in the form of CT numbers) for tissue heterogeneity corrections

12. Internal Structures Magnetic Resonance Imaging • MRI has developed, in parallel to CT • advantages over CT • scan directly in axial, sagittal, coronal, or oblique planes • not involving the use of ionizing radiation • higher contrast • Better imaging of soft tissue tumor

13. Internal Structures Magnetic Resonance Imaging • Disadvantages compared with CT • inability to image bone or calcifications • longer scan acquisition time • technical difficulties due to small hole of the magnet and • magnetic interference with metallic objects

14. Internal Structures Ultrasound • Ultrasound can provide useful information in localizing many malignancy-prone structures in the lower pelvis, retroperitoneum, upper abdomen, breast, and chest wall

15. TREATMENT SIMULATION TREATMENT SIMULATION • uses a diagnostic x-ray tube but duplicates a radiation treatment unit in terms of its geometrical, mechanical, and optical properties.

16. TREATMENT SIMULATION TREATMENT SIMULATION • By radiographic visualization of • internal organs, • correct positioning of fields • and shielding blocks • can be obtained in relation to external landmarks • fluoroscopic capability by dynamic visualization

17. TREATMENT SIMULATION TREATMENT SIMULATION • An exciting development in the area of simulation is that of converting a CT scanner into a simulator • CT-SIM

18. TREATMENT VERIFICATION TREATMENT VERIFICATION • Port Films • Electronic Portal Imaging (EPI) • Cone Beam CT • MV-CT ( Tomotherapy )

19. TREATMENT VERIFICATION Port Films • The primary purpose of port filming is to verify the treatment volume under actual conditions of treatment • the image quality with the megavoltage x-ray beam is poorer than with the diagnostic or the simulator film

20. TREATMENT VERIFICATION Port Films

21. TREATMENT VERIFICATION Port Films • Limitations of port film • Viewing is delayed because of the time required for processing • It’s impractical to do port films before each treatment • Film image is of poor quality especially for photon energies greater than 6MV

22. TREATMENT VERIFICATION • Electronic portal imaging device ( EPID ) • Mount on the linac • Real-time, digital feedback to the user.

23. Portal imaging devices • fluoroscopy-based systems • liquid filled ionization chamber matrices • amorphous silicon based system

24. Fluoroscopy-based systems • 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)

25. liquid filled ionization chamber matrices • The maximum spatial resolution is 2.3 mm x 2.9mm, increasing to 2.3 mm x 4.5 mm depending on acquisition mode

26. amorphous silicon based system • 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 mm2.

27. CORRECTIONS FOR CONTOUR IRREGULARITIES CORRECTIONS • Effective Source-to-Surface Distance Method • Tissue-air (or Tissue-maximum) Ratio Method • lsodose Shift Method

28. Effective SSD Method CORRECTIONS

29. TAR Method CORRECTIONS • ratio depend on only of the depth and the field size at that depth

30. lsodose Shift Method CORRECTIONS • 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

31. CORRECTIONS FOR TISSUE INHOMOGENEITIES CORRECTIONS • 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

32. CORRECTIONS FOR TISSUE INHOMOGENEITIES CORRECTIONS • The effects of tissue inhomogeneities • changes in the absorption of the primary beam and the associated pattern of scattered photons • primary beam : points that lie beyond the inhomogeneity, • Scattered : points near the inhomogeneity • changes in the secondary electron fluence • tissues within the inhomogeneity and at the boundaries.

33. CORRECTIONS FOR TISSUE INHOMOGENEITIES CORRECTIONS • Corrections for Beam Attenuation and Scattering • TAR method, Power law TAR method , Equivalent TAR method, Isodose shift method, Typical correction factors • Absorbed Dose within an Inhomogeneity

34. Corrections for Beam Attenuation and Scattering CORRECTIONS • TAR method • d' = d1 + ρ1 d2 + d3 • d is the actual depth of P from the surface

35. Corrections for Beam Attenuation and Scattering CORRECTIONS • Power Law Tissue-air Ratio Method • 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

36. Corrections for Beam Attenuation and Scattering CORRECTIONS • Power Law Tissue-air Ratio Method • A more general form, provided by Sontag and Cunningham • allows for correction of the dose to points within an inhomogeneity as well as below it.

37. Corrections for Beam Attenuation and Scattering CORRECTIONS • Equivalent Tissue-air Ratio Method • correctly predicted the effect of scattering structures depends on their geometric arrangement with respect to point P

38. Corrections for Beam Attenuation and Scattering CORRECTIONS • Equivalent Tissue-air Ratio Method • d' is the water equivalent depth, d is the actual depth, r is the beam dimension at depth d, • r' = r × ρ' = scaled field size dimension

39. Corrections for Beam Attenuation and Scattering CORRECTIONS • lsodose Shift Method • manually correcting isodose charts for the presence of inhomogeneity

40. Corrections for Beam Attenuation and Scattering CORRECTIONS • Typical Correction Factors • None of the methods discussed above can claim an accuracy of ± 5% for all irradiation conditions encountered in radiotherapy • Tang et al. have compared a few commonly used methods against measured data using a heterogeneous phantom containing layers of polystyrene and cork

41. Corrections for Beam Attenuation and Scattering CORRECTIONS • Typical Correction Factors • Their results (Tang et al. ) • the TAR methodoverestimates the dose for all energies • the ETAR is best suited for the lower-energy beams (≦6 MV) • the generalized Batho method is the best in the high-energ range (≧10 MV)

42. Absorbed Dose within an Inhomogeneity CORRECTIONS • Bone Mineral

43. Absorbed Dose within an Inhomogeneity CORRECTIONS • Bone-tissue Interface • Soft Tissue in Bone

44. Absorbed Dose within an Inhomogeneity CORRECTIONS • Bone-tissue Interface • Soft Tissue Surrounding Bone

45. Absorbed Dose within an Inhomogeneity CORRECTIONS • Bone-tissue Interface • Soft Tissue Surrounding Bone • forward scatter • 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. • For higher energies, there is an enhancement of dose at the interface because of the increased electron fluence in bone due to pair production

46. Absorbed Dose within an Inhomogeneity CORRECTIONS • Bone-tissue Interface • Soft Tissue Surrounding Bone

47. Absorbed Dose within an Inhomogeneity CORRECTIONS • Bone-tissue Interface • parallel-opposed beams

48. Absorbed Dose within an Inhomogeneity CORRECTIONS • Bone-tissue Interface • parallel-opposed beams

49. Absorbed Dose within an Inhomogeneity CORRECTIONS • Lung Tissue • Dose within the lung tissue is primarily governed by its density • But in the first layers of soft tissue beyond a large thickness of lung, there is some loss of secondary electrons

50. Absorbed Dose within an Inhomogeneity CORRECTIONS • Lung Tissue • problem of loss of lateral electronic equilibrium when a high-energy photon beam traverses the lung • dose profile to become less sharp • The effect is significant for small field sizes ( ＜ 6 x 6 cm ) and higher energies ( >6 MV )