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.