BEAM MODIFICATIONS PRINCIPLES AND METHODS

PRESENTED BY MODERATED BY:

DR SANDIP NAVIN SINGH

Beam modification is defined as desirable modification to the spatial distribution of radiation within the patient by inserting of material into the beam

TYPES OF BEAM MODIFICATIONS1. Shielding2. Compensators3. Wedge filters4. Beam flattening

TYPES OF BEAM MODIFYING DEVICES Field blocking and

shaping devices:◦ Shielding blocks.

◦ Custom blocks.

◦ Asymmetrical jaws.

◦ Multileaf collimators.

Compensators.

Wedge filters.

Beam flattening filters.

Bolus

PROBLEMS IN BEAM MODIFICATIONS

The radiation reaching any point in a scattering medium is made up of a mixture of primary and scattered photons

The result of introducing any beam modifying device depends on the relative amounts of primary and scattered radiation

The aim of giving a point complete protection from radiation thus would not be achieved due to scattered radiation

Another phenomenon called blurring is produced in primary beam by attenuation in beam modifier

SHIELDING

DEFINITION Alteration to the shape of

the beam to reduce or,as far as possible eliminate the radiation dose at some special parts of zone at which beam is directed.

Shielding is achieved more easily with high energy radiation than low energy due to low scattering in high energies

The effect of a shielding block on a beam of kilovolt and megavolt

An ideal shielding material should have1. High atomic number

2. High density

3. Easy availability

4. Inexpensive

Shielding blocks are most commonly made of lead. Shielding can be of two types1. Positive: Where central area is blocked, eg. lung block2. Negative :Where peripheral area is blocked.eg head &

neck

SHIELDING

The thickness of lead required for adequate protection to shielded area depends on

1. Beam quality2. Allowed transmission through the block

A primary beam transmission of 5% through the block is considered acceptable

A thickness of lead between 4.5 and 5.0 half value layer is recommended for clinical shielding

Half value layer is defined as the thickness of material which will reduce the intensity of primary beam by 50 %

Beam Quality Lead thickness

COBALT 60 5cm

4MV 6 cm

6MV 6.5 cm

10 MV 7 cm

25MV 7cm

CUSTOM BLOCKING Uses a low melting point alloy

LIPOWITZ metal(cerroband)

Made up of bismuth,lead,tin,cadmium

It melts at 70 c and can be easily cast to any shape

At room temperature it is harder than lead

In Megavoltage range of photon beams the most commonly used thickness is 7.5 cm

BiPbSnCd

Used when we want to block the part of the field without changing the position of the isocenter.

Independently movable jaws, allows us to shield a part of the field, and this can be used for “beam splitting”.

Here beam is blocked off at the central axis to remove the divergence.

There is change in the physical penumbra .

This causes elimination of photons and electron scatter from the blocked portion of field,reducing dose near the edges.

INDEPENDENT JAWS

It consists of a large number of collimating block or leaves that can be driven automaticallly

independent of each other to generate a field of any shape Typically consists of 80 leaves

Indivisual leaf has a width of 1 cm or less projected at isocentre

Leaves are made of tungsten alloy

Have a thickness of 6 to 7.5 cm

Primary x ray transmisssion through the leaves is <2%

MULTILEAF COLLIMATORS

The degree of conformity between the planned field and the jagged field depends on

1. Projected leaf width2. Shape of target volume3. Angle of rotation of the collimator

The use of multi leaf collimators in blocking and field shaping is ideally suited for treatment req large number of multiple fields

The advantages are:◦ Time for shaping and inserting

of custom blocks is not required.

◦ Reduction in set up time of multiple fields

◦ The hardening of beam, scattered radiation, and increase in skin doses and doses outside the field, as seen with physical compensators is avoided.

◦ MLCs can also be used to as dynamic wedges and electronic compensators (2D).

◦ Modern treatment like 3DCRT,IMRT are dependent on it

The disadvantages are:◦ Because the physical

penumbra is larger than that produced by others treatment of smaller fields is a drawback

◦ Difficult when blocking is required close to critical structures

◦ The jagged boundary of the field makes matching difficult.

DEFINITION Alterations to enable normal distribution data to be applied

to all or part of the treated zone when the beam enters the body obliquely and/or it passes through different types of tissues or through curved irregular surface

First used by Ellis

Standard isodose charts are usually obtained from the measurement made in cubic phantoms at right angle to surface

In kilovolt range unit density wax or lincolnshire bolus is used In megavoltage aluminium or brass compensators are used

COMPENSATORS

The dimentions and shape of the compensators is adjusted because of

1. Beam divergence2. The relative linear attenuation coefficient of filter material

and soft tissue3. The reduction in scatter at various depths when the

compensator is placed at a distance from the skin

To compensate for these factors its attenuation is less than that required for primary radiation only

PRINCIPLE OF TISSUE COMPENSATORS

Compensators are made out of aluminium or brass blocks using a matrix of square columns corresponding to irregular surface

The compensator thickness should be such that the dose at a given depth is same whether the missing tissue is replaced with the bolus in contact or with the compensator at given distance from skin surface

The thickness ratio or density ratio defined as thickness of a tissue equivalent component along a

ray/missing tissue thickness along the same ray

It depends upon1. Compensator surface distance

2. Thickness of missing tissue

3. Field size

4. Depth

5. Beam quality

Average value =0.7 if d> or equals to 20 cm for Co 60,4Mev,10Mev

Thickness ratioTc= TD x (τ/ρc),

where TD is the tissue deficit and ρc is the density of the compensator.

Compensator ratio(CR) defined as ratio of missing tissue thickness to compensator thickness necessary to give the dose for a particular field size and depth (ρc /τ ).

Production of practical compensator

Two-dimensional compensators

Used when proper mould room facilities are not available.

Thickness varies, along a single dimension only.

Can be constructed using thin sheets of lead, lucite or aluminum. This results in production of a laminated filter.

Three-dimensional compensators 3-D compensators are designed to measure tissue deficits in

both transverse and longitudinal cross sections.

Cavity produced in the Styrofoam block is used to cast compensator filters

Various systems in use for design of these compensators are:◦ Moiré Camera.◦ Magnetic Digitizers.◦ CT based compensator designing systems.

DEFINITION: Alterations to produce special spatial distributions ,i.e to

modify its isodose distributions

First devised by Ellis & Miller

It is a wedge shaped absorber thick at one end ,tapers at the other

It causes progressive decrease in the intensity across the beam resulting in a tilt of isodose curve

WEDGE FILTERS

Degree of tilt depends on slope of wedge filters

Wedge filters makes the isodose curves for two intersecting fields parallel,resulting in an uniform irradiation.

Material used are tungsten,brass,lead,steel,Al,Cu

Wedge angle or wedge isodose angle is defined as the angle through which the 50% isodose curve has been turned from its position in a normal beam

Thus wedge angle=90-hinge angle/2

Hinge angle is angle between the central rays of two intersecting fields

Since the range of hinge angle is some what limited wedge angle of(35,45,55)will cope up with majority of cases

This solves the problem of different value of wedge angle required for different beam angle

TYPES OF WEDGE SYSTEMS1. Indivisualised wedge2. Universal wedge3. Dynamic wedge4. Virtual wedge5. Pseudo wedges

Indivisualised wedge systems requires a separate wedge for each beam.

Mainly used for cobalt systems

Universal wedge system is a single wedge system that serves for all beam width

Used for linear accelerator beams.

Dynamic wedges or motorised wedges is a 60 degree wedge mounted in the treatment head to create the wedge profile beam directed

Virtual wedge or dynamic enhanced are moving jaws that are moved by computer control to create wedge beam

COMPENSATING WEDGES Used for oblique beam incident on curved surface whose

contour can be approximated with a straight line.

Metals of copper,brasss,lead used In a compensating wedge standard isodose curve can be

used without modification. No transmitting factors are required for C-wedges. It can be used for partial field compensation i.e to

compensate only a part of contour ,irregular in shape.

BOLUS

A tissue equivalent material used to reduce the depth of the maximum dose (Dmax).

A bolus can be used in place of a compensator for kilovoltage radiation to even out the skin surface contours.

In megavoltage radiation bolus is primarily used to bring up the buildup zone near the skin in treating superficial lesions

The thickness of the bolus used varies according to the energy of the radiation.

In megavoltage radiation:◦Co60 : 2 - 3 mm◦6 MV : 7- 8 mm◦10 MV : 12 - 14 mm◦25 MV: 18 - 20 mm

Properties of an ideal bolus:◦ Same electron density and atomic number.◦ Pliable to conform to surface.◦ Usual specific gravity is 1.02 -1.03

Commonly used materials are:◦ Cotton soaked with water.◦ Paraffin wax.

Other materials that have been used:◦ Mix- D (wax, polyethylene, mag oxide)

◦ Lincolnshire bolus (sugar and mag carbonate in form of spheres)

◦ Spiers Bolus (rice flour and soda bicarb) Commercial materials:

◦ Superflab: Thick and doesn't undergo elastic deformation. Made of synthetic oil gel.

◦ Superstuff: Pliable gelatin like material.◦ Bolx Sheets: Gel enclosed in plastic sheet.

FLATTENING FILTERS A beam flattening filter reduces

the central exposure rate relative to that near the edge of the beam.

Used for Linear accelerators.

Due to the lower scatter the isodose curves are exhibit “forward peaking”.

The filter is designed so that the thickest part is in the centre.

Material: copper or brass.

Beam modification allows us with the liberty to treat a specific part ,while protecting sensitive and vital organs.

Although devices like wedges and compensators plays a pivotal role in treatment of patients ,they are likely to be superseded by newer technologies like multi leaf collimators,IMRT etc.

And lastly I would like to mention the line in our physics book

“THE PRICE OF SAFETY IS ETERNAL VIGILENCE “

CONCLUSION

THANK YOU