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%