Intensity Modulated RadioTherapy · 2017-06-24 · Intensity Modulated RadioTherapy A clinical...
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Intensity Modulated RadioTherapy
Intensity Modulated RadioTherapy
A clinical application of a small accelerator
A.A. van ‘t Veld PhDI. Hoveijn PhD
part 1
part2
CERN/KVI accelerator school, Zeegse, June 1st
2005
University Medical Center
Groningen
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AimsAims
• Introduction to radiotherapy• The place of IMRT in radiotherapy• The place of linac design in IMRT
• Realization of IMRT• IMRT clinical examples
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Mechanism of radiotherapyMechanism of radiotherapy
treatment beamCharged particles
Treatment beam
Photons
Ionization tracks
Damage to DNA
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Introduction: a patient’s viewIntroduction: a patient’s view
Localisator SimulatorCT
Linac: multiple fractions
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Behind the screensBehind the screens
Localisator SimulatorTreatmentplanning
CT
Linac: multiple fractions
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Treatment planning: step 1Treatment planning: step 1
Brain
Target
Eyes
Parotids
• Definition of targets and critical structures
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Treatment planning: step 1Treatment planning: step 1
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Treatment planning: step 2Treatment planning: step 2
• Design of treatment technique– Number of beams and their directions– Modality: photons/electrons and energy
• Design of multiple beams– Beam shape
• Design of treatment plan– Weighting of beam contribution
• Example– Nasopharynx + bilateral neck nodes– Max. 85% dose (50 Gy) to spinal cord– 10 beams (4 shown)
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BTW: why so many beams?
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By the way: why so many beams??By the way: why so many beams??
Single beam Multiple beams
100%
60%
160%
40%
100%
80%
115%
90%
Very high dose to normal tissueInhomogeneous target dose
Accepted dose to normal tissueHomogeneous target dose
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Treatment beams in Beam’s-Eye-View*Treatment beams in Beam’s-Eye-View*
Lateral Posterior*Projection of the patient as seen by the linac target
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Treatment beams in Beam’s-Eye-ViewTreatment beams in Beam’s-Eye-View
Lateral oblique Caudal
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Treatment planning: step 3Treatment planning: step 3
• Calculate dose distribution
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Treatment planning: step 4Treatment planning: step 4
• Evaluate dose distribution• Adapt treatment plan (step 2) until acceptable
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Treatment planning: step 4Treatment planning: step 4
95%
Conformality to target50% 70% 80% 90% 95% 100%
Acceptable spinal cord dose
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Treatment plan = linac recipeTreatment plan = linac recipe
treatment beam
• Photon or electron beams
• 6 degrees of freedom ...• … all around the
isocentre
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Treatment planning: a step further neededTreatment planning: a step further needed
95%
Conformality to target50% 70% 80% 90% 95% 100%
Acceptable spinal cord dose
No sparing of parotid gland
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Radiotherapy develops towards ...Radiotherapy develops towards ...
• Progress in radiotherapy depends on three areas in conjunction:
– Medical knowledge
– Dose calculation
– Technical facilities
Medicine
Physics
Engineering
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Radiotherapy develops towards ...Radiotherapy develops towards ...
• … more conformal treatment
Beams must conform to target
Beam’s-Eye-Viewwith simple dose
calculation
1983
High dose must conform to target
Dose calculation with physics of linac
model
1995
MLC: Multi-Leaf Collimator allows more and better shaped beams
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Radiotherapy develops towards ...Radiotherapy develops towards ...
Intensity modulated radiotherapy (IMRT)
Multi-Leaf Collimator
allows modulation of
a beam
Homogeneousfluence/beam
• … IMRT
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Radiotherapy develops towards IMRTRadiotherapy develops towards IMRT
Beams must conform to target
1985
Beam’s-Eye-Viewwith simple dose
calculation
1995
High dose must conform to target
Dose calculation with physics of linac
model
2000
Dose distributionmust conform to
constraints
Math. optimization & linac construction
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The place of IMRT in radiotherapyThe place of IMRT in radiotherapy
Spinal cord: < 30 GyTarget 2: > 50 GyTarget 1: > 60 Gy
IJORBP 51, 880 (2001)
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The place of linac design in IMRTThe place of linac design in IMRT
• Linear accelerator
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Linac constructionLinac construction
• Linear accelerator– Vacuum technology– High-voltage circuits– RF wave guide– (Fine) mechanics– Electronic control
circuits
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Linac constructionLinac construction
• Linear accelerator– Vacuum technology– High-voltage circuits– EM wave guide– (Fine) mechanics– Electronic control
circuits– Rotatable over 360°
• In-house maintenance & support
– 5 engineers– automatic checks
(servo)– weekly & monthly
checks
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Wave guide and flight tubeWave guide and flight tube
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Bending magnet, electron windowBending magnet, electron window
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Filter carrousel and monitor ion chamberFilter carrousel and monitor ion chamber
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Housing of 40
leafs of MLC with motors
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Multi-Leaf Collimator (Elekta BM)Multi-Leaf Collimator (Elekta BM)
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MLC motorsMLC motors
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• Better isocentric stability: 1 mm• Fast beam startup: coil sets magnetron plunger • Beam starting stability: slitless without servos• Multi-leafs: fast change beam shape• Multi-leaf specification: leakage, travel range, speed• Leaf calibration: field junction dose• Geometry and materials: must be well-known for dose
calculation in treatment planning system
The place of linac design in IMRTThe place of linac design in IMRT33
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Improved mechanics and gantry motorsImproved mechanics and gantry motors
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Dose calculation and linac geometryDose calculation and linac geometry
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focusopening of prim. collimatorflattening filter
• Construction of Elekta Slilinac model for calculation of a 6 MV-X 3x3cm2
photon beam – BEAM
Monte Carlo model
monitor chamberbackscatter plateMLC (not visible)backup coll.
lower (X) coll.
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Dose calculationDose calculation
• Construction of Elekta Sli linac model
• Photon exiting linac head
• Characteristics of exiting photons is determined by linac geometry
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1000 histories
(= electrons on target)
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• Linac model in treatment planning system should yield:
– 2D energy fluence distribution ..– .. i.e. the number of particles ..– .. times their energy ..– .. for all relevant linac parts ..– .. at actual field shape ..
Dose calculation and linac geometryDose calculation and linac geometry
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Flatnessfilter
Collimator Collimator
Focus
Monitorchamber
• Treatment planning system should distinguish– Head scatter contribution from linac head components– Based on dose-point’s-eye-view backprojection– MU calculation including backscatter into monitor
chamber
Dose calculation and linac geometryDose calculation and linac geometry
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Flatnessfilter
Collimator Collimator
Dosepoint(x,y,z)
Focus
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• Tomotherapy: CT-scan and linac in one• CyberKnife: linac on a robotic arm
Two other possibilities for IMRTTwo other possibilities for IMRT
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Tomotherapy at John Hopkins CyberKnife
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• IMRT offers new possibilities to better spare critical organs and/or enhance tumor dose
• The design of the linear accelerator plays a critical role in:
– facilitating new treatment techniques– treatment accuracy– dose calculation
• Maintenance and quality control of RT apparatus such as a linac is of eminent importance
Conclusions first partConclusions first part
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