medphysics planning
DESCRIPTION
TRANSCRIPT
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Treatment Planning:
Volume Definition: Beam Selection
References:
Radiation Therapy Planning, Bentel
Treatment Planning in Radiation Oncology, Khan and Potish
ICRU 50, 62
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ICRU definitions
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GTV - palpable or visible extent of tumorCTV - GTV + subclinical microscopic diseasePTV - geometric concept designed to cover
CTVTreated Volume - volume enclosed by dose
level appropriate to treat diseaseIrradiated Volume - volume that receives
significant dose
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Internal Margin: variations in size and shape of CTV during treatment
Set-up Margin: uncertainties in patient positioning and alignment
PRV: planning organ at risk volume includes margins on critical structures
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Volume Definition: Imaging ModalitiesCT, US, MRI, PET, Nuc Med,
SpectfMRI, Optical?, ……..Addition of margins
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Coordinate Systems:Patient: internal reference pointImaging: simulator isocentre/noneTreatment: isocentre
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Virtual Simulation: - ImmobilizationCTCoordinate systemStructure DelineationIsocentre localizationBeam placement/definition
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Problems:Images are static and organ motion is not
evidentCorrelation of imager/patient/treatment
coordinate systems is non-trivial - DRRsResolution of data set is limited by slice
thickness - structure definition/DRRImaging modality - image fusion
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Advantages:Improved volume definitionPatient data collected in digital
form for dose calculationSpeed
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Conventional SimulationImmobilizationDiagnostic energy X-rays replace
Megavoltage beamsLower patient dose, better images, real-time
fluoroExternal coordinate system same as treatment
coordinate system
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Volume DefinitionExternal reference
palpation,visual radio-opaque markers
Internal reference bony landmarks, other anatomical transfer from CT contrast agents, internal markers
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Lateral Field
Nodes outlined
With solder
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Problems:External contours must be
obtained for dose distribution calculation
Time consumingVolume definition is difficult
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Advantages:Organ motion can be visualized on
fluoroCo-localization of simulation/treatment
geometriesTreatment geometry problems can be
avoided
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Treatment Planning Objectives (Goals, Desires, Constraints, etc…)
Deliver a uniform dose to PTVDeliver as little dose as possible to OARKeep integral dose lowReduce number of high dose ‘hot spots’
outside PTVKISS
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Treatment Parameters Degrees of Freedom (with apologies to True Statisticians)
Number of treatment beamsIndividual beam energyRelative beam weightingShieldingPrimary beam profile modifiersPatient modifiers (bolus, and other?)
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Treatment Optimization
Selection of treatment parameters that best conforms to planning objectives
Manual: based on experience - time consuming - artform?
Automated - forward calculation - compensation
Automated - inverse planning - optimization algorithms
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Patient Modifiers:
Bolus: tissue equivalent materialPlaced directly on patients’ surfacePurpose is usually to reduce skin sparingCan be used to ‘block up’ complex surface
to simplify dose distribution
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Numbers of Beams
KISSConformal RTAbility to escalate doseHigher demands on setup accuracy
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Wedges: modify primary beam profile so as to produce isodose lines at angle wrt to surface
Open beam 45 degree wedge
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15 degree 30 degree 45 degree 60 degree
Different wedges available for Varian 600C
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Types of wedges
Physical: a wedge shaped piece of metal (steel or lead) machined to shape the primary beam profile. Must be physically placed in head of machine. Limited selection of wedge angles.
Universal: a physical wedge with very high wedge angle permanently in head of machine. Different effective wedge angles are obtained by combining open and wedged beams for different fractions of treatment.
Dynamic: one field jaw sweeps across field during treatment so that integrated dose-distribution matches that of physical wedge.
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Use of wedges I: To correct for patient contour
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Variation at level of isocentre:40% 5%
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Use of wedges II: To correct for beam attenuation when usingmultiple fields
Example: 3 field plan, variation in treated volume:30% 5%
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Example: wedged pair, dose variation in treated volume:50% 5%
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CompensatorsMissing tissue: corrects for patient surface to give uniform
dose to a surface perpendicular to central axis of beam.
Compensation Plane
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Primary Beam Profile Modulation
Physical: Attenuators, compensators. Thickness is calculated using attenuation coefficient of compensator material
Ip* = EXP (- tp )
Dynamic MLC: similar to dynamic wedge, MLC leaves are moved during treatment to affect required distribution
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Compensators
Dose: corrects to give uniform dose to an arbitrary surface in patient
Compensation Surface
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Forward Algorithm: Additional Complications
Dose compensators: compensation surface is not a constant SAD - will require additional ISF factor
Primary beam profile is not flat (horns, penumbra). How/should one correct for beam profile?
Introduction of shielding gives differential scatter loss across field - integrate scatter dose point by point
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Compensators: Inverse Algorithm
Optimization problemNeed a good forward dose calculation algorithmDivide beam into many smaller ‘pencil’ beamsAdjust pencil beam weights iteratively to achieve
uniform dose on compensation planeUsually flat plane, solution exists
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Example: neck: compensate to give uniform dosealong midplane throughout treatment field
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Uncompensated Compensated15-20% <5%
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