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Beam directionBeam direction Practice & principles moderator - Mr. P. Goswami department of radiotherapy
PGIMER, chandigarh
DefinitionDefinition
Whole plan of the treatment is worked out in advance of the actual treatment and certain devices are used to direct the beam towards the tumor
Why Beam direction ?Why Beam direction ?
Homogenous tumor low normal tissue
dose dose
better therapeutic ratio
Prerequisites for beam Prerequisites for beam directiondirectionPatient factors:
◦Early stage of disease◦Good general condition◦Good nutritional status◦Radical treatment intent (curative)
Machine factors:◦Isocentric◦Non isocentric
StepsSteps
LocalisationPositioningImmobilisationField selectionDose distributionCalculationsExecution & verification
LOCALISATIONLOCALISATION
The target volume and critical normal tissues are delineated with with respect torespect to patient’s external surface contour.
What to localize?◦Tumor◦Organ
Methods?◦Clinical examination◦Imaging
Why Localize?Why Localize?
Irradiate the tumor and spare the normal tissue.
Allow calculations and beam balancing.
Define radiation portals.Use the beam directing devices.
Clinical localizationClinical localization
Advantages:◦ Available everywhere. ◦ Cheapest and quickest(?).◦ Needs little additional equipment.
Disadvantages:◦ Error prone in the wrong hands.◦ Accessible areas required.◦ Volumetric data not easily obtained.
Clinical localization is mandatory despite advanced imaging – need to know what to image!
Imaging LocalizationImaging Localization
Imaging:◦ X-rays:
Plain Contrast Studies
◦ CT scans◦ MRI scans ◦ USG scans◦ PET scan◦ Fusion imaging
Type of study selected depends on:◦ Precision desired.◦ Cost considerations◦ Time considerations◦ Labour considerations
X raysX rays
The most common and cheapest modality available.
However 2-D data acquired only.
Orthogonal films can be used with appropriate contrast enhancement for localization in 3 dimensions
X raysX raysPlain films:
◦ Head and neck region◦ Cervix (radio-opaque
markers)Contrast
◦ Esophagus◦ Rectum◦ Bladder◦ Stomach
Estimation of depthEstimation of depth
From data gained by localization studies:◦CT / MRI – Accurate data◦Lateral height method◦Tube shift method
Depth estimation necessary for:◦Calculations◦Selection of beam energy
Lateral height methodLateral height method
d
d
H1 + H22
d =
H1H2
Tube shift methodTube shift method
Image shift and tube shift are interrelated WHEN the tube to target distance remains constant.
Goal: To obtain a graph of different object heights against the tube shift.
Serial measurements of image shift measured (for same tube to film distance) while varying the height of the markers above the table.
Tube shift principlesTube shift principles
Marker
d2
y
f
S
Tumor
x1
x2
d1
CalculationCalculation
d1
f
yyd2
x1x2
x1
S=
d1
f – d1
S
x2
S=
d2
f – d2
yy = d2 – d1
= fx2 + S
x2 -x1 + S
x1
Tumor
Marker
CT scansCT scans
Provides electron density data which can be directly used by the TPS.
Volumetric reconstruction possible.
Good image resolution - better where bony anatomy is to be evaluated.
The image is a gray scale representation of the CT numbers – related to the attenuation coefficients.
Hounsfield units =
(μtissue – μwater) x 1000/ (μwater)
253 265 235
125 125 112
56 450 156
135 158 247
269 300 65
36 123 598
CT scan perquisitesCT scan perquisites
Flat table top Large diameter scan aperture
(≥ 70 cm). Positioning, leveling and
immobilization done in the treatment position.
Adequate internal contrast – external landmarks to be delineated too.
Preferably images to be transferred electronically to preserve electron density data.
MRI scansMRI scans
Advantages:◦ Imaging in multiple planes without
formatting.◦ Greater tissue contrast – essential for proper
target delineation in brain and head and neck◦ No ionizing radiation involved.
Disadvantages:◦ Lower spatial resolution◦ Longer scan times◦ Inability to image calcification or bones.
Fusion ImagingFusion Imaging
Includes PET – CT imaging and Fusion MRI.
Allows “biological modulation” of radiation therapy.
Expensive : requires additional software
Final clinical utility – still remains to be realized
POSITIONINGPOSITIONING
Patient positioning is the most vital and often the most NEGLECTEDNEGLECTED part of beam direction:
Good patient position is ALWAYS:◦Stable.◦Comfortable.◦Minimizes movements.◦Reproducible.
Standard PositionsStandard Positions
Supine:◦ MC used body position.◦ Also most comfortable.◦ Best and quickest for
setup.◦ Minimizes errors due to
miscommunication.
Prone:◦ Best for treating
posterior structures like spine
◦ In some obese patients setup improved as the back is flat and less mobile.
Positioning aidsPositioning aidsHelp to maintain patients in non
standard positions.These positions necessary to
maximize therapeutic ratio.Accessories allow manipulation of
the non rigid human body to allow a comfortable, reproducible and stable position.
Positioning aids…Positioning aids…
Pelvic Board
Prone Support
Breast Board
Breast BoardsBreast Boards
Disadvantages:◦ Possibility of skin
reactions in the infra mammary folds
◦ Access to CT scanners hampered
Solutions:◦ Thermoplastic
brassieres.◦ Breast rings.◦ Prone treatment
support.
Allow comfortable arm up support ► brings arms out of the way of lateral beams.
Positions patient so that the breast / sternum is horizontal ► avoiding angulation of the collimator.
Pulls breast down into a better position by the pull of gravity.
Breast boards…Breast boards…
Modern Breast Board
Indexed Arm supports
Indexed wrist support
Head rest
Carbon fiber tilt board
Wedge to prevent sliding
Belly boards Belly boards
Mould makingMould making
Mould making : Contd..Mould making : Contd..
Mould making : Contd..Mould making : Contd..
ThermoplasticsThermoplasticsThermoplastics are
long polymers with few cross links.
They also possess a “plastic memory” - tendency to revert to normal flat shape when reheated
Foam systemsFoam systemsMade of polyurethaneAdvantages:
◦ Ability to cut treatment portals into foam.
◦ Mark treatment fields on the foam.
◦ Rigid and holds shape.Disadvantages:
◦ Chance of spillage◦ Environmental hazard during
disposal
Vacuum bagsVacuum bags
Consist of polystyrene beads that are locked in position with vacuum.
Can be reused.However like former immobilization not perfect.
Bite BlocksBite BlocksA simple yet
elegant design to immobilize the head.
A dental impression mouthpiece used.
The impression is attached to the base plate and is indexed.
SRS devicesSRS devices
Sterotactic frames.
Gill Thomas Cosman System.
TALON® Systems – NOMOS corp.
Patient ContouringPatient ContouringContour is the representation of
external body outline.Methods:
◦Plaster of Paris◦Lead wire◦Thermoplastic contouring material◦Flurographic method◦CT/MRI
RADIATIONRADIATION FIELD
Types:◦ Geometrical: Area DEFINED by the light beam at any
given depth as projected from the point of origin of the beam.
◦ Physical: Area encompassed by the 50% isodose curve at the isocenter. In LINACs often defined at the 80% isodose.
Single FieldSingle Field
Criteria for acceptability:1. Dose distribution
to be uniform (±5%)
2. Maximum dose to tissues in beam ≤ 110%.
3. Critical structures don’t receive dose exceeding their normal tolerance.
Situations used:◦ Skin tumors◦ CSI◦ Supraclavicular
region◦ Palliative
treatments
2 Field techniques2 Field techniquesCan be :
◦ Parallel opposed◦ Angled
Perpendicular Oblique
◦ Wedged pair
Advantages:◦ Simplicity◦ Reproducibility◦ Less chance of
geometrical miss◦ Homogenous dose
• Dose homogeneity depends on:• Patient thickness• Beam energy• Beam “flatness”
Disadvantage of 2 field techniques -large amount of normal tissue
gets radiation -if separation is more there is an
arc like distribution so, in ca cx if separation is >16 cm four fields are
used.
3 field techniques3 field techniquesUsed in -deep seated tumors -to save vital structuresExample -ca esophaghus -ca lung -ca UB -ca nasopharynx with ant extention -ca maxilla with ethmoidal
extention
4 field techniques4 field techniquesUsed in -ca cx -ca rectum
Multiple fieldsMultiple fieldsUsed in 3DCRT & IMRTUsed to obtain a “conformal” dose
distribution in the modern radiotherapy techniques.
Disadvantages:◦Integral dose increases◦Certain beam angles are prohibited
due to proximity of critical structures.◦Setup accuracy better with parallel
opposed arrangement.
Magna fieldMagna field
Radical treatment of lymphoma(HD)
Whole body irradiationHemi body irradiation
DOSE DISTRIBUTION ANALYSIS
Done manually or in the TPS.Manual distribution gives a hands
on idea of what to expect with dose distributions.
Inefficient and time consuming.Pros:
◦ Cheap◦ Universally available◦ Adequate for most clinical situations.
CalculationsCalculations
Techniques:◦SSD technique (PDD method)◦SAD technique◦Clarkson’s technique◦Computerized
PRESCRIPTIONMandatory
statements:◦Dose to be
delivered.◦Number of
fractions◦Number of
fractions per week
SSD techniqueSSD technique
PDD is the ratio of the absorbed dose at any point at depth d to that at a reference depth d0.
D0 is the position of the peak absorbed dose.
Dmax is the peak absorbed dose at the central axis.
Total Tumor dose
Number of fieldsx
Number of #s
=T
Incident dose =
T x 100
PDD
Time =ID
Output
SAD TechniqueSAD Technique
Uses doses normalized at isocenter for calculation.
In this technique the impact of setup variations is minimized.
Setup is easier but manual planning difficult.
Time taken for treatment reduced.
SAD calculationsSAD calculationsTotal Tumor dose
Number of fieldsx
Number of #s
=T
Incident dose =
T x 100
TMR/TAR
Time =ID
Output
SSD vs SAD techniqueSSD vs SAD technique
SSD treatments:◦ Relatively less
homogenous dose distribution
◦ Setup possible without requiring expensive aids e.g. Laser
◦ PDD charts can be used for simple dose calculations
◦ More skin reactions
SAD treatments:◦ Less number of
MUs required◦ Time taken is less◦ Impact of setup
inaccuracies is minimized in 2 field techniques
◦ Ease of setup reproducibility in multi field treatments.
TAR vs. SSDTAR vs. SSDTAR = Tissue Air
RatioTAR introduced by
Jones for rotation therapy.
Allows calculation of dose at isocenter WITHOUT correcting for varying SSDs.
TAR is the ratio of dose at a point in the phantom to the dose in free space at the same point (Dq /D0)
DqD0
TARTARTAR removes the influence of SSD
as it is a ratio of two doses at the SAME point.
However like PDD the TAR also varies with:◦Energy◦Depth◦Field Size◦Field Shape
EXECUTION & VERIFICATIONCan be done using:
◦Portal Films◦Electronic Portal images◦Cone Beam CT mounted on treatment
machines (IGRT).◦Seen during treatment
-CCTV camera -lead glass -mirror -infrared camera (in imrt)
Port filmsPort films
◦Cheapest.◦Legal necessity(?)◦But have several disadvantages.
Port film disadvantagesPort film disadvantages
Factors leading to poor image contrast:◦High beam energy (> 10 MV)◦Large source size ( Cobalt)◦Large patient thickness (> 20 cm)
Slow acquisition times.Image enhancement not possible.Storage problems.
Electronic Portal ImagingElectronic Portal ImagingVideo based
EPIDSFiber optic
systemsMatrix liquid
ion chambersSolid state
detectorsAmorphous Si
technology*
BEAM DIRECTION DEVICESBEAM DIRECTION DEVICES
The main beam direction devices are:◦Collimators◦Front pointer / SSD indicator◦Back Pointer◦Pin and arc◦Isocentric mounting◦Lasers
CollimatorsCollimatorsCollimators provide beams of desired
shape and size.Types:
◦ Fixed / Master collimator.◦ Movable / Treatment collimator.
Fixed CollimatorsFixed Collimators
Protects the patient from bulk of the radiation.
Dictates the maximum field size for the machine.
Maximum beam size is when exposure at periphery is 50% of that of the center.
In megavoltage radiotherapy beam angle used is 20°.
Master Collimator : DesignMaster Collimator : Design In megavoltage x ray
machines beam energy is maximum in forward direction.
• Beam energy is equal in telecurie sources so primary collimators are spherical.
Movable CollimatorsMovable Collimators
Define the required field size and shape.
Placed below the master collimators results in trimming of the penumbra.
Types:◦Applicators◦Jaws / Movable diaphragms
Applicators: DesignApplicators: Design
Metal Plate with hole
Lead Sheet
Box
Plastic Cap
Applicators Applicators
Advantages:◦ Indicate size and
shape of beam.◦ Distance maintained.◦ Direction shown.◦ Plastic ends allow
compression.◦ Compression allows
immobilization.◦ Penumbra
minimized.
Disadvantages:◦ Useful for low
energy only.◦ Separate sizes
and shapes required.
◦ Costly.◦ Shapes may
change due frequent handling.
JawsJawsHandling of heavy
weight not required.Skin sparing effect
retained.Jaws moved
mechanically – accurately.
Jaw border lies along the line radiating from
focal spot
Jaws: DisadvantagesJaws: Disadvantages
Disadvantages Remedy
Size and shape of field remain unknown
Light beam shining through the jaws
Patient to source distance unknown
SSD indicator used.
Compression not possible
A Perspex box may be applied to the head
Front & back pointersFront & back pointers
This method requires the identification & marking on the patient’s surface, of two points lying on a line passing through the tumor centre.
Entry point- ATumor centre- TExit point- B
Front Pointer/ SSD indicatorFront Pointer/ SSD indicator
Detachable device to measure the SSD and align the beam axis.
Designed so that it may be swung out of the beam path during treatment.
Back PointerBack PointerThe pointer can be moved in the sleeve.A nipple is used to allow compression.The arrow lies along the central ray.
Sites usedSites used
Front pointer and back pointer used in the following situations:◦Head and Neck◦Breast◦Brain tumors
LimitationsLimitations
Requires skin marks – inherently unreliable.
Back pointer is unreliable when compression is desired.
Both front and back points must be accessible.
Accurate localization of tumor center is mandatory.
Pin & ArcPin & Arc
Pin
Arc
Bubble
Principle Principle
Based on the principle of parallelogram
How does it work?How does it work?
arrangement of pin & arc is such that when pin is at it’s lowest position, it’s lower end is on the central axis of beam & on centre of curvature of arc.
Depth of the tumor is already known.
Pin is withdrawn the reqd. distance & it’s lower end is brought in contact with the surface mark.
So long as the pin is vertical the rest of equipment & applicator will rotate about the centre of tumor and central ray will always pass through it
Thus keeping the pin vertical & in contact with surface mark any particular angle can be selected
The oblique distance can be read off the scale or bar by applying principle of parallelogram.
Advantages of Pin & ArcAdvantages of Pin & Arc
Allows Isocentric treatment of ◦Deep tumors.◦Eccentric tumors.
Can be used with compression e.g. in treating deep seated tumors.
Can be used for manual verification of Isocentric placement of machines
Sites where usedSites where used
Mostly in midline tumors situated at a depth◦Esophagus◦Cervix◦Bladder ◦Rectum◦Vagina◦Lung sometimes
Isocentric MountingIsocentric MountingFirst used by Flanders
and Newberg of Hammersmith Hospital for early linear accelerators.
The axis of rotation of the three structures:◦ Gantry◦ Collimator◦ Couch
coincide at a point known as the Isocenter.
Why Isocentric Mounting?Why Isocentric Mounting?
Enhances accuracy.Allows faster setup and is more
accurate than older non isocentrically mounted machines.
Makes setup transfer easy from the simulator to the treatment machine.
LasersLasersLASER = Light Amplification Of
stimulated Emission Of RadiationTypically 3 pairs are provided with
the machine and intersect at the isocenter.
Also define:◦Beam Entry◦Beam Exit
LasersLasers
Other uses:◦Checking the isocenter◦Reproducing the setup on the
simulator at the treatment couch.Fallacies:
◦Accurate setup depends on proper alignment of the lasers themselves
◦Lasers known to move frequent adjustments needed.
ConclusionConclusionBeam direction devices &
methods are important part of radiotherapy which aids in accurate treatment.
To neglect the extra accuracy that can be gained by beam direction is to throw away much of the value of the powerful and expensive apparatus now in use in radiotherapy.
Thank you.