IMRT for beginners

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IMRT for beginners

• Dr Nikolay Nedev
• Department of Radiation Oncology
• Palmerston North - New Zealand

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Intensity Modulation

• We have been doing it for years! Seriously?

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Wedges
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Segments
Y
Y
100MU
100MU
100MU
200MU
Y
Y
RT segment Field-in-field
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Forward Planned IMRT Segment Based
-10
-10 -10
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Forward Planned IMRTCompensator Based
-10 -20
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Than what is new?

• The paradigm of planning is changing
• The complexity of the intense maps is much higher
• The way these maps are generated
• The way treatment is delivered
• The amount of QA and plan verification

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Two basic ways to deliver the radiotherapySTEP
AND SHOOT

• Different apertures deliver part of the dose in
  sequential manner
• Very similar to delivering of multiple
  field-in-field segments
• The dose gets build up in small steps
• The dose is delivered only when the MLCs are
  stationary

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STEP AND SHOOT
Segmental MLC - IMRT
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DYNAMIC IMRT

• a.k.a Sliding window
• a.k.a DMLC-IMRT
• Leaves are in motion during the radiotherapy
  delivery
• Only on Varian machines
• More difficult to verify
• Requires
• Very precise leaf calibration
• Constant and high dose rate on the Linac

Picture Variantm web site
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Sliding window
IJROBP, Vol51, No.4, pp 880-914, 2001
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Dynamic MLC IMRT vs. Static MLC IMRT
IJROBP, Vol51, No.4, pp 880-914, 2001
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Inverse Planning

• Inverse Planning
• Specify the dose aim
• PTV organs at risk
• DVH parameters
• Total dose,
• Minimum dose,
• Max dose
• Let the computer do the rest

Forward planning Specify the beams Angles Beam
Modificators Wedges Compensators Beam
weight Calculate and assess the dose
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Determine the beam angles

• The more angles the greater the chance of
  achieving a good plan
• More gentry angles longer delivery time
• Usually 3-5-7 evenly spaced beams
• Do not use opposing beams

72 degrees apart
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Intensity levels and min field size

• More intensity levels better conformity
• Smaller min. field size - better conformity
• Longer time to deliver the treatment
• More dose leakage
• More time the patient to move
• Potentially more time for tumor recovery

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What does the computer do?

• Get a combination of beam intensities that
• Cover the PTV
• Limit the dose to OAR
• Using the priority system
• Try to fulfill the DVH-requirements

There is never the guarantee that the proposed
plan is the best plan possible
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Dose fluency map
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Segmentation

• Translation of the dose intensities of the ideal
  plan on the language of the Linac
• Infinity number of segments become finite number
  of segments Linac could deliver
• The infinity number of intensity levels become
  finite number of intensity delivered by the
  physical segments
• The dose distribution may change dramatically

Final dose intensity map
Segmentation 1
Segmentation 2
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Get the priorities right

• Tumor (GTV) needs to be covered 1
• CTV needs to be covered 1
• PTV needs to be covered 2
• Cord/optic nerve dose lt50Gy 2
• Both Parotid volume 50 24Gy 3
• Skin 10
• Deal with overlaps

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The problem with overlaps

• PTV 66Gy
• Optic nerve 54Gy
• Overlap area 54-56 Gy

PTV
Overlap area
optic nerve
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Plan assessment Myths about IMRT plans
Why use IMRT

• Deliver as conformal treatment as possible
• Spare the organs at risk
• Dose escalation
• Improve the dose distribution

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As conformal as possible

• The greatest challenge with IMRT
• Increases the doctors responsibility for
  contouring
• Especially in CTV outlining
• The technique would not bail out the poor
  contouring
• Lots of wisdom and not so much sophistication in
  the traditional bone landmark based portal
  determination
• Learn the profession again

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Spare the organs at risk

• The most powerful reason for IMRT introduction
• IMRT really shines in cases where the PTV wraps
  around organ at risk
• IMRT will not make you treatment more precise
  (compared to 3D-RT)

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IMRT improves the homogeneity

• Partially true
• Especially in breast cancer cases
• Partially wrong
• Especially in head and neck cases

1/3
2/3
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Inhomogeneity effects are not clear

• The idea as the dose to the tumor as the only
  tumorocidal event is probably wrong
• Other factors are
• Dose to the surrounding cells
• Bystander effect
• Dose to the feeding blood vessels
• Ischemic events
• Both the bystander effect and the effect of the
  vessels is difficult to quantify
• Increased cell kill in low dose areas, when
  surrounding areas receive high but non-lethal dose

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Then again - why IMRT?

• It provides the best tools for achieving all
  these goals in the desired combination and with
  as little trade-offs as possible
• Dose conformity
• Dose escalation
• Plan quality improvement
• Its easier and quicker to deliver
• Easier to plan especially compared to good
  quality 3D-planning
• Easier to deliver
• All modern machines support automatic field
  sequencing
• The RT does need to enter in the room to change
  anything

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Plan assessmentSlice-by-slice
Segmented Plan finite number of intensity
steps finite numbers of segments
Non-segmented Plan infinite number of intensity
steps infinite numbers of segments
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Plan assessmentDVH
Prescribed dose
OAR tolerance
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ICRU50 vs. IMRT

• ICRU50
• Point in the center of tumor
• Minimal coverage gt 95
• Hot spots lt107

IMRT Prescribe at 100 Use isodose plots Use
DVH
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The DVH
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DVH simple version

• Prescribe _at_100
• Cover PTV with 90
• Most of the volume needs to receive between 95
  and 107
• Hot spots should be kept under 110
• Small areas are allowed to receive gt110

ICRU 50
95 100 107
IMRT
90 100 110
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IMRT introduction - summary

• IMRT produces great plans
• The demands it imposes are much greater
• Machine commissioning
• Immobilization
• Verification
• Anatomy knowledge - less forgiving technique
• More difficult to visualize
• Similar to instrument flight in a dark night
• You have to trust your instruments more than you
  senses

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• This presentation is done with close co-operation
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