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

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TREATMENT PLANNING II PATIENT DATA, CORRECTIONS,
AND SET-UP
?????????The Physics of Radiation Therapy.
Faiz M. Khan
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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.

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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

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Body Contours
ACQUISITION OF PATIENT DATA

• 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

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Some important points for contour making
ACQUISITION OF PATIENT DATA

• 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.

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Internal Structures

• Transverse Tomography
• Computed Tomography
• Magnetic Resonance Imaging
• Ultrasound

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Internal Structures
Transverse Tomography

• provide cross-sectional information of internal
  structures in relation to the external contour
• poor contrast and spatial resolution

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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.

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Internal Structures
Computed Tomography

• CT numbers
• related to attenuation coefficients
• Hounsfield numbers
• CT numbers normalized

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Internal Structures
Computed Tomography

• CT numbers
• it is possible to infer electron density
  (electrons cm-3)

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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

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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

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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

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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

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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.

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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

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TREATMENT SIMULATION
TREATMENT SIMULATION

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

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TREATMENT VERIFICATION
TREATMENT VERIFICATION

• Port Films
• Electronic Portal Imaging (EPI)
• Cone Beam CT
• MV-CT ( Tomotherapy )

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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

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TREATMENT VERIFICATION
Port Films
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TREATMENT VERIFICATION
Port Films

• Limitations of port film
• Viewing is delayed because of the time required
  for processing
• Its impractical to do port films before each
  treatment
• Film image is of poor quality especially for
  photon energies greater than 6MV

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TREATMENT VERIFICATION

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

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

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• 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)

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• liquid filled ionization chamber matrices
• 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

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• 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.

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CORRECTIONS
CORRECTIONS FOR CONTOUR IRREGULARITIES

• Effective Source-to-Surface Distance Method
• Tissue-air (or Tissue-maximum) Ratio Method
• lsodose Shift Method

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CORRECTIONS
Effective SSD Method
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CORRECTIONS
TAR Method

• ratio depend on only of the depth and the field
  size at that depth

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CORRECTIONS
lsodose Shift Method

• Sliding the isodose chart up or down, depending
  on whether there is tissue excess or deficit
  along that line, by an amount kh where k is a
  factor less than 1

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CORRECTIONS
CORRECTIONS FOR TISSUE INHOMOGENEITIES

• 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

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CORRECTIONS
CORRECTIONS FOR TISSUE INHOMOGENEITIES

• 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.

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CORRECTIONS
CORRECTIONS FOR TISSUE INHOMOGENEITIES

• 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

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CORRECTIONS
Corrections for Beam Attenuation and Scattering

• TAR method
• d' d1 ?1 d2 d3
• d is the actual depth of P from the surface

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CORRECTIONS
Corrections for Beam Attenuation and Scattering

• 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

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CORRECTIONS
Corrections for Beam Attenuation and Scattering

• 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.

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CORRECTIONS
Corrections for Beam Attenuation and Scattering

• Equivalent Tissue-air Ratio Method
• correctly predicted the effect of scattering
  structures depends on their geometric arrangement
  with respect to point P

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CORRECTIONS
Corrections for Beam Attenuation and Scattering

• 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 ?' scaled field size dimension

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CORRECTIONS
Corrections for Beam Attenuation and Scattering

• lsodose Shift Method
• manually correcting isodose charts for the
  presence of inhomogeneity

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CORRECTIONS
Corrections for Beam Attenuation and Scattering

• 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

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CORRECTIONS
Corrections for Beam Attenuation and Scattering

• Typical Correction Factors
• Their results (Tang et al. )
• the TAR method overestimates 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)

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CORRECTIONS
Absorbed Dose within an Inhomogeneity

• Bone Mineral

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CORRECTIONS
Absorbed Dose within an Inhomogeneity

• Bone-tissue Interface
• Soft Tissue in Bone

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CORRECTIONS
Absorbed Dose within an Inhomogeneity

• Bone-tissue Interface
• Soft Tissue Surrounding Bone

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CORRECTIONS
Absorbed Dose within an Inhomogeneity

• 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

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CORRECTIONS
Absorbed Dose within an Inhomogeneity

• Bone-tissue Interface
• Soft Tissue Surrounding Bone

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CORRECTIONS
Absorbed Dose within an Inhomogeneity

• Bone-tissue Interface
• parallel-opposed beams

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CORRECTIONS
Absorbed Dose within an Inhomogeneity

• Bone-tissue Interface
• parallel-opposed beams

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CORRECTIONS
Absorbed Dose within an Inhomogeneity

• 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

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CORRECTIONS
Absorbed Dose within an Inhomogeneity

• 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 (
  lt 6 x 6 cm ) and higher energies ( gt6 MV )

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CORRECTIONS
Absorbed Dose within an Inhomogeneity

• Air Cavity
• The most important effect of air cavities in
  megavoltage beam dosimetry is the partial loss of
  electronic equilibrium at the cavity surface
• 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)

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TISSUE COMPENSATION

• To preserve the skin-sparing properties of the
  megavoltage photon beams, the compensator is
  placed a suitable distance (gt20 cm) away from the
  patient's skin

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??

• 1. ??????????????????
• (1) CT (2) PET (3) MRI (4) Ultrasound
• 2. ??????????????????????
• (1) CT (2) Simulator (3) MRI (4)
  ultrasound
• 3. ??????????????????????????????
• (1) CT (2) Simulator (3) MRI (4)
  ultrasound
• 4. ????,?? Hounsfield number ???
• (1) 0 (2) -1000 (3) 1000 (4) 500
• 5. ????????????????
• (1) CT (2) Simulator (3) MRI (4) CT-SIM
• 6. ??????????? TREATMENT VERIFICATION ?
• (1) Port Films (2) Electronic Portal
  Imaging (3) Cone Beam CT (4) MRI

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??

• 6. ???? TREATMENT VERIFICATION ???????????????
• (1) Port Films (2) Electronic Portal
  Imaging (3) Cone Beam CT (4) MRI
• 7. ??????????????????
• (1) Effective SSD (2) TAR method
• (3) lsodose Shift Method
• (4) Power Law Tissue-air Ratio Method

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PDD - Dependence on Source-Surface Distance

• PDD increases with SSD
• the Mayneord F Factor ( without considering
  changes in scattering )