Medphysics Planning
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Transcript of Medphysics Planning
Treatment Planning:
Volume Definition: Beam Selection
References:
Radiation Therapy Planning, Bentel
Treatment Planning in Radiation Oncology, Khan and Potish
ICRU 50, 62
ICRU definitions
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
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
Volume Definition: Imaging ModalitiesCT, US, MRI, PET, Nuc Med,
SpectfMRI, Optical?, ……..Addition of margins
Coordinate Systems:Patient: internal reference pointImaging: simulator isocentre/noneTreatment: isocentre
Virtual Simulation: - ImmobilizationCTCoordinate systemStructure DelineationIsocentre localizationBeam placement/definition
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
Advantages:Improved volume definitionPatient data collected in digital
form for dose calculationSpeed
Conventional SimulationImmobilizationDiagnostic energy X-rays replace
Megavoltage beamsLower patient dose, better images, real-time
fluoroExternal coordinate system same as treatment
coordinate system
Volume DefinitionExternal reference
palpation,visual radio-opaque markers
Internal reference bony landmarks, other anatomical transfer from CT contrast agents, internal markers
Lateral Field
Nodes outlined
With solder
Problems:External contours must be
obtained for dose distribution calculation
Time consumingVolume definition is difficult
Advantages:Organ motion can be visualized on
fluoroCo-localization of simulation/treatment
geometriesTreatment geometry problems can be
avoided
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
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?)
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
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
Numbers of Beams
KISSConformal RTAbility to escalate doseHigher demands on setup accuracy
Wedges: modify primary beam profile so as to produce isodose lines at angle wrt to surface
Open beam 45 degree wedge
15 degree 30 degree 45 degree 60 degree
Different wedges available for Varian 600C
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.
Use of wedges I: To correct for patient contour
Variation at level of isocentre:40% 5%
Use of wedges II: To correct for beam attenuation when usingmultiple fields
Example: 3 field plan, variation in treated volume:30% 5%
Example: wedged pair, dose variation in treated volume:50% 5%
CompensatorsMissing tissue: corrects for patient surface to give uniform
dose to a surface perpendicular to central axis of beam.
Compensation Plane
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
Compensators
Dose: corrects to give uniform dose to an arbitrary surface in patient
Compensation Surface
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
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
Example: neck: compensate to give uniform dosealong midplane throughout treatment field
Uncompensated Compensated15-20% <5%