Post on 14-Apr-2017
transcript
Isodose linesdefinition, influencing factors,
clinical application
LIBIN SCARIAResident Medical Physicist
Beams of ionising radiation have characteristic
process of energy deposition, in
order to represent volumetric &
planar variations in absorbed dose are
depicted by isodose curves
Dose distribution
PERCENT DEPTH DOSE
Quotient expressed as percentage, of the absorbed dose at any depth d to the absorbed dose at dmax along the central axis of the beam
Isodose curves– Lines joining the points of equal percentage depth
dose(PDD)– Curves are usually drawn at regular intervals of
absorbed dose & expressed as a percentage of dose
• Isodose charts– Family of isodose curves– PDD values are normalised at Dmax or reference
depth
Field size (dosimetrical):lateral distance between the 50% isodose lines at a reference depth
Field size (geometrical) : field defining light is made to
coincide with 50% isodose lines of the radiation beam projected on a
plane perpendicular to the beam axis and at standard SSD or SAD
Beam profile• Falloff of the beam– By the reduced side scatter– Physical penumbra width
• High dose or ‘horns’ near the surface in the periphery of the field
• Outside the geometric limits of the beam and the penumbra, the dose variation is the result of– side scatter from the field– both leakage and scatter from the collimator system
PENUMBRA• Dose transitions near the boarders of the
field• Region at the radiation beam over which
the dose rate changes rapidly as a function of distance from the central axis
Geometric penumbraTransmission penumbraPhysical penumbraScatter penumbra
For cobalt-60 teletherapy: P ~ 1cmFor linacs : P~ 3mm
Measurement of isodose curves• Ion chambers• Solid state detectors
– TLD,Silicon diodes,• Radiographic films• Computer driven devices
Ion chamber is the most reliable method due to its relatively flat energy response and precision
Beam Analysing System3D water phantom- IBA Wellhofer Blue Phantom
• Two ion chambers :– Detector A: to move in the tank
of water to sample the dose rate– Detector B: fixed at some point
in the field to monitor the beam intensity with time
• Final response A/B is independent of fluctuation in the output
Sources of isodose chartAtlases of premeasured isodose
chartsGenerated by calculations using
different algorithmsManufacturers of radiation generators
Parameters of isodose curvesBeam quality
Source size, SSD, SDD Penumbra
Collimation and flattening filterField size
Beam Quality• Depth of a given isodose curve increases with
beam quality• Greater lateral scatter associated with lower-
energy beams• For megavoltage beams, the scatter outside the
field is minimized as a result of forward scattering – becomes more a function of collimation than energy
4 MV 6MV 15MV
Field size One of the most important parameters in
treatment planning PDD increases as field size increases Field size dependence of PDD is less
pronounced for higher energy than for lower energies
Field size smaller than 6 cmRelative large penumbra regionBell shapeTPS should be mandatory for small field size
5*5 cm2 10*10 cm2 15*15cm2
SSD affects the PDD and the depth of the isodose curvesPDD icreases with SSD
Beam fattening filter• Intensity is more at central axis and
decreases as we move away• Non-uniform dose at any given depth• FF is used to uniform it• Usually made up of Al or Brass
Wedge filters Wedge shaped absorber causes a progressive
decrease in intensity across the beam Results in tilt of the isodose curve & degree of
the tilt depends upon the slope of the wedge filter
Material: tungsten, brass. Lead or steel
15o Wedge
30o Wedge
45o Wedge
60o Wedge
Wedge isodose angle (θ) is the complement of the angle through which the isodose curve is tilted with respect to the central ray of the beam at any specified depth
This depth is important because the angle will decrease with increasing depth.
Angle of isodose tilt to decrease with increasing depth in the phantom.
The choice of the reference depth varies:
10 centimeters1/2 - 2/3rd of the beam widthAt the 50% isodose curve
Hinge angle,φIt is the angle between central axes of two
beams passing through the wedge
Relationship b/w φ & θ
Wedge angle,θ= 90 – φ/2
Bolus• A tissue equivalent material
used to reduce the depth of the maximum dose (Dmax)
• In megavoltage radiation bolus is primarily used to bring up the buildup zone near the skin in treating superficial lesions.
• The thickness is usually 0.5 cm to 1.5 cm
Combined Open Field Technique
• Criteria:– The dose distribution within the tumor volume is
reasonably uniform (±5%).– The maximum dose to the tissue in the beam is not
excessive (not more than 110% of the prescribed dose)
– Normal critical structures in the beam do not receive doses near or beyond tolerance
Parallel Opposed Fields
• Advantages– The simplicity and reproducibility of setup– Homogeneous dose to the tumour– Less chances of geometrical miss
• Disadvantage– Excessive dose to normal tissues and critical
organs above and below the tumour
Multiple fields To deliver maximum dose to the
tumour and minimum dose to the surrounding tissues
• 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
Multiple fields Certain beam angles are prohibited
• Presence of critical organs in those directions• 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
• Practical feasibility• Setup accuracy• Reproducibility of the treatment technique
Isodose lines of Electrons
Isodose curves• Scattering of electrons
determines shapes of isodose curves– Expansion– Lateral constriction
• Larger field size required at surface
Proton isoeffective dose
“Charged Particle Therapy” to better preserve healthy tissues and organs at risk
Medicine is
a science of uncertainity
and an art of
probabilityDr. William Osler
A canadian Physician and one of the four founding professors of John Hopkins Hospital,
USA
Thank u…