ISODOSE CURVESRADIATION ONCOLOGY

DR.PAUL GEORGE RADIATION ONCOLOGY REGIONAL CANCER CENTER

TRIVANDRUM

INTRODUCTION

Beams of ionising radiation have characteristicprocesses of energy deposition, hence theExpected dose distribution can be estimated.

In order to represent volumetric and planar variations in absorbed dose, distributions are depicted by means of ISODOSECURVES .

DOSE BUILD UPAs high energy photons enter patient, high speedelectrons are ejected from surface and subsequent layers

These electrons deposit energy a significant distance fromoriginal interaction.Hence, electron fluence and dose increases with depthUntil a maximum is reached.

Photon fluence continuously decreases with depth,hence production of electrons decrease with depth.

net effect - beyond a certain depth, dose decreases withDepth.

SKIN SPARING EFFECT

PDD

The quantity percentage depth dose

may be defined as- the quotient,

expressed as a percentage, of the

absorbed dose at any depth 'd‘

to the absorbed dose at a fixed

reference depth 'd0' ,along

the central axis of the beam.

ISODOSE CURVESDEFINITION:

Isodose curves are the lines joining the points of equal Percentage Depth Dose (PDD). The curves are usually

drawn at regular intervals of absorbed dose and expressed as a percentage of the dose at a reference point.

ISODOSE CHARTS : It consists of a family of isodose curves.The depth dose values of the curves are normalized:

1) At the point of maximum dose on the central axis (Dmax)2) At a fixed distance along the central axis in the irradiated medium (SAD).

Isodose chart Co60 10*10cmSSD =80cm SAD

BEAM PROFILEThe dose variation across the field at a specifieddepth.

• Field size: the lateral distance between the 50% isodose lines at a reference depth.

• Beam alignment: the field-defining light is made to coincide with the 50% isodose lines of the radiation beam projected on a plane perpendicular to the beam axis and at standard SSD or SAD

• High dose or ‘horns’ near the surface in the periphery of the field

Created by the flattening filterUnder-compensate near the surface in order to obtain flat isodose curves at greater depths (10 cm)

PENUMBRA

Dose transition near the borders of the field.The region at the of a radiation beam overwhich the dose rate changes rapidly as afunction of distance from the central axis.

Geometric Penumbra:

Transmission Penumbra: variable transmissionof beam through non divergent collimator angle.

Physical Penumbra: the lateral distance between two specified isodose curves at a specified depth.( lateral distance between 90%10% or 8020%)

Geometric penumbra : due to finite dimensions of the source

Geometric penumbra:W=D× SSD─SDD SDD Co-60 teletherapy High energy linacSDD = 40 cm SDD= 45.5cmSSD = 80 cm SSD= 100cmD = 1 cm D = 0.3cm

W 1cm W 0.36cm

The width of geometric penumbra depends on source size, distance from the source, and source-todiaphragm distance.

Falloff of the beam1.By the geometric penumbra2.By the reduced side scatter3.Physical penumbra width

Outside the geometric limits of the beam and the penumbra, the dose variation is the result of side scatter from the field andboth leakage and scatter from

the collimator system.

Measurement of isodose curves1. Ion Chambers2. Solid state detectors3. Radiographic Films4. Computer driven devices

Ion chamber is the most reliable method, because of its relatively flat energy response andprecision

Two ion chambersDetector A–To move in the tank of water to sample the dose rateMonitor B–fixed at some point in the field to monitor the beam intensity with timeThe final response A/B is independent of fluctuations in output.

Sources of ISODOSE CHART1.Atlasas of premeasured isodose charts2.It can be generated by calculations usingvarious algorithms for treatment planning3.Mnufacturers of radiation generaters

Parameters of isodose curvesThe parameters that affect the single beamisodose distribution are:1.Beam quality2.Source size, SSD, and SDD -the penumbraEffect3.Collimation and flattening filter4.Field size

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Isodose Curve Depends on Beam Quality200 kVp,Dmax at

patient surface.

PDD at 10cm 35%

Sharp beam edge. Bulging penumbra

due to greater scatter of low energy

photons.

Co-60,Dmax0.5cm

PDD at 10=56%

penumbra primarily due to source size (geometric penumbra)

6 MV,Dmax 1.5cm

PDD at 10=67%

small penumbra,due to

small photon scatter and short

electron range

20MV,Dmax 4.5

PDDat 10=80%.

Penumbra greater than that of 4 MV, due to greater electron range

1.BEAM QUALITY

BEAM QUALITYThe depth of a given isodose curve increases withbeam quality.

Greater lateral scatter associated with lower-energy Beams.

For megavoltage beams, the scatter outside the field isminimized as a result of forward scattering and becomesmore a function of collimation than energy.

2.Source Size, SSD, and SDD

THE PENUMBRA EFFECTSource size, SSD, and SDD affect the isodose curvesby the geometric penumbra.The SSD affects the PDD and the depth of the isodose curves.The dose variation across the field border is a complex function of geometric penumbra, lateralscatter, and collimation.

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Field size is determined based on dosimetric coverage, not geometric coverage.

3.FIELD SIZEOne of the most important parameters intreatment planningField size smaller than 6 cm• Relative large penumbra region• Bell shapeThus TPS should be mandatory for small fieldsize.

FIELD SIZE CONT…

Compare 5 cm × 5 cm field with 10 cm × 10 cm field for 60Co•central axis depth dose larger for larger field size

•increase in amount of scattered radiation.

• for smaller field very small area flatness over field

4. Collimation and Flattening FilterCollimation: the collimator block + the flatteningfilter + absorbers + scatterers

The flattening filter has the greatest influence in determining the shape of the isodose curves.

• The photon spectrum may different for the peripheral areas compared with the central part of the beam.

• THE CHANGE IN QUALITY across the beam causes the flatness to change with depth.

Wedge FiltersA beam modifying device, whichcauses a progressive decreasein intensity across the beam,resulting in tilting the isodosecurves to thinner side.Material: tungsten, brass. Lead or steel

WEDGE SYSTEMS

Individualized wedge system• A separate wedge for each beam width• To minimize the loss of beam output• To align the thin end of the wedge with the border of the light field• Used in Co60Universal wedge system• A single wedge for all beam widths• Fixed centrally in the beam• Used in Linac

Advanced Wedge SystemsOmni wedge (Elekta)• There is only one universal wedge (60 degree) attached above the jaws. • To control the wedge angle, an appropriate combination of open and wedged fields are used.Dynamic wedge (Varian)• One side of jaws move in (or close) while beam is on.• Wedge angle is determined by controlling the speed of the moving jaw.

NORMALISED TO Dmax NORMALISED TO ‘Dmax without wedge’

Wedge angle

The angle through which an isodose curve is titled at the central ray of a beam at a specified depth10cm/50% isodose curves

The angle between the isodose curve and the normal to the central Axis

Wedge angle

The wedge should be such that the isodose curves from each field are parallel to the bisector of the hinge angle. When the isodoses are combined, the resultant distribution is uniform.

θ= 90º-φ/2θ = the wedge angleφ= the hinge angleS = the separation or the distance between the thick ends of the wedge filters as projected on the surface

Combination of radiation fieldsIsodose Distribution – parallel opposed open fields

BEAM WEIGHED 100 AT Dmax BEAM WEIGHED 100 at Isocenter

Parallel opposed fields

Advantages• The simplicity and reproducibility of setup• Homogeneous dose to the tumor• Less chances of geometrical missDisadvantage• The excessive dose to normal tissues and critical organs above and below the tumor

MULTIPLE FIELDS

•Using fields of appropriate size•Increasing the number of fields or portals•Selecting appropriate beam directions•Adjusting beam weights•Using appropriate beam energy•Using beam modifiers.

To deliver maximum dose to the tumor and minimum dose to the surrounding. Tissues dose uniformity with the tumorvolume and sparing of critical organs areimportant considerations in judging a plan.

Certain beam angles are prohibited due to the presence of critical organs in those directions

The setup accuracy of a treatment may be better with parallel opposed beam arrangement.

The acceptability of a treatment plan depends not only on the dose distribution but also on•The practical feasibility•Setup accuracy•Reproducibility of the treatment technique

ROTATION THERAPY

ROTATION THERAPY CONTIN….

• The beam moves continuously about the patient, or the patient is rotated while the beam is held fixed.

• For small and deep-seated tumors, not for Too large.

• Beam should be aimed a suitable distance beyond the tumour area and is called PAST POINTING.

• The maximum dose for 360 degree rotation occurs at the isocenter and for partial arcs it is displaced towards the irradiated sector.

ELECTRONS• Delivers a reasonably uniform dose from the surface to a

specific depth, after which dose falls of rapidly , eventually to

near zero value.DEPTH DOSE CURVE

ELECTRONS PHOTONS

Most useful treatment depth , therapeutic range of electrons is given by the depth of 90% of the isodose curves……….

The PDD increases as the energy increases.However unlike photon beams , the percent of surface dose for electron beam increases with energy

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Electron Beams

For low energy electron beams, isodose curves bulging out for all dose levels

Isodose curves

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Electron beam cont…

Isodose curves

But bulge out for low dose levels

For high energy electron beams, isodose curves constrict for high dose levels

TAKE HOME POINTS

1.The dose at any depth is greatest on the central axis of the beam and gradually decreases toward the edges of the beams. ( horns in some LINACs overcompensate ).

2.The dose rate decreases rapidly as a function of lateral distance from the beam axis in the penumbra region. ( geometric penumbra with side scatter ↓)

3.It could be defined a physical penumbra as the lateral distance between two specified isodose curves at a specified depth. ( lateral distance between 90%10% or 8020%)

4.Therapeutic housing/source housing: lateral scatter from the medium and leakage from the head of the machine

HOME POINTS CONT…..5. The parameters that affect the single beamisodose distribution are:1.Beam quality 2.Source size, SSD, andSDD the penumbra Effect 3.Collimation and flattening filter 4.Field size.

6. Wedge Angle is The angle through which an isodose curve is titled at the central ray of a beam at a specified depth 10cm/50% isodose curves.

7. Isodose curves are different for Co60 , photon , & Electrons

8. Therapeutic range of electrons is given by the depth of 90% of the isodose curves.

THANK YOU

PAUL GEORGE