BEAM MODIFYING DEVICES

Dr Nanditha Kishore DNB 1st year

Introduction

Defined as desirable modification in the spatial distribution of radiation - within the patient - by insertion of any material in the beam path.

Problem in beam modification Radiation reaching any point, is made

up of primary and scattered photons.

Any introduction of the modification devices results in alteration of dose distribution, due to these two phenomena.

The phenomena scattering results in an “blurring” of the effect of the beam modification

Beam energy

Types of beam modification 1.Shielding

2.Compensation

3.Wedge filtration

4.Flattening

Types of beam modification devicesShielding And Shaping A. Shielding Blocks B. Custom Blocks C. Independent Jaws D. Multi Leaf Collimators Compensators Wedge FiltersBolus

Other devices

Beam flattening filters Beam spoilers Breast cone Penumbra trimmers Electron beam modification

Shielding

It is a method of modification of beam to protect the critical structures around the treated volume by using various devices.

The aims of shielding are: To protect critical organs. Avoid unnecessary irradiation to

surrounding normal tissue. Matching adjacent fields.

Ideal shielding material

It should have following characteristics 1.high atomic number 2.high density 3.easily available 4.inexpensive Most common shielding material

used for photons is LEAD.

Choice of shielding is also dictated by the type of beam being used.

Thickness of shielding material It depends on 1.Attenuation of shielding material The term half value-layer is a

convenient expression for the attenuation produced by any material.

Half-value layer is defined as the thickness of an absorber required to attenuate the intensity of beam to half its original value.

For practical purposes, the shielding material which reduces beam transmission to 5% of its original is considered acceptable.

The number of HVL (n)1/2n = 5% or 0.05 Thus, 2n = 1/0.05 = 20 . OR, n log 2 = log 20.n = 4.32

The relationship holds true, only for mono energetic x-ray beams.

Practically thickness of lead between 4.5 - 5 half-value layers results in 5% or less of primary beam transmission.

Placement of shielding In kilovoltage radiation shielding is readily

achieved by placing sheets of lead on the surface directly.

This is necessary, because of the lower penetrating power of the beam.

In Megavoltage radiation, Thicker blocks used. Placed higher up in shadow trays (15 -20

cm). Avoids increase in skin dose due to

electron scatter. Also impossible to place the heavy block

on the body !!

CUSTOM BLOCKS These are introduced by POWER’S et

al. Material used for custom locking is

known as the Lipowitz metal or Cerrobend.

Melting point 70°C. Density 9.4 g /cm3 at 20°C (83%

of lead). Major advantage of cerrobend

block over lead is its low melting point which enables it to cast in any shape.

At room temperature it is harder than lead.

Composition Of Cerrobend

Lead, 26.70%

Bismuth, 50.00%

Cadmium, 10.00%Tin,

13.30%

Bismuth Lead

Tin Cadmium

1.21 times thicker blocks necessary to produce the same attenuation.Most commonly thickness of 7.5 cms used.

BLOCK DIVERGENCE

Ideally the blocks should be shaped or tapered so that their sides follow the geometric divergence .

This minimizes the block transmission penumbra.

Divergent blocks are most suited for beams having small focal spots.

Constructing cerrobend blocks Steps 1.Drawing the outline of treatment

field including the areas to be shielded on a simulator radiograph .

2.constructing divergent cavities in a styrofoam block

3.filling the cavities with cerrobend material in liquid state.

Shielding blocks can be of two types: Positive blocks,

where the central area is blocked.

Negative blocks, where the peripheral area is blocked.

Independent jaws

Used when we want to block of 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”.

Use of independent jaws and other beam blocking devices results in the shift of the isodose curves.

These have replaced half beam blocks as beam splitters.

Asymetric collimation produced by these independent jaws has an effect on

1.Physical Penumbra 2.Tilt of isodose curves This is by eliminating photon and

eletron scatter from blocked portion of field thereby reducing dose near the edge.

A beam modifying device which evens out the skin surface contours, while retaining the skin-sparing advantage.

It allows normal depth dose data to be used for such irregular surfaces.

Compensators can also be used for To compensate for tissue heterogeneity.

This was first used by Ellis, and is primarily used in total body irradiation.

To compensate for dose irregularities arising due to reduced scatter near the field edges (example mantle fields), and horns in the beam profile.

Compensators

Compensator

The dimension and shape of a compensator must be adjusted to account for :

Beam divergence. Linear attenuation coefficients of the

filter material and soft tissue. Reduction in scatter at various depths

due to the compensating filters, when it is placed at the distance away from the skin.

To compensate for these factors a tissue compensator is always has an attenuation less than that required for primary radiation.

Thickness ratio or density ratio

Required thickness of a tissue equivqlent compensator

Missing tissue thickness along same ray

A tissue equivalent compensator designed with thee same thickness f missing tissue will over compensate.

To compensate for this decrease in scatter one should use appropriate thickness of compensator material.

The thickness ratio depends on: Compensator to surface distance. Thickness of the missing tissue. Field size. Depth. Beam quality

Of the above distance is the most important factor when d is ≤ 20 cm.

Therefore, a fixed value of thickness ratio (τ) is used for most compensator work (~ 0.7).

The formula used for calculation of compensator thickness is given by:

TD x (τ/ρc), where TD is the tissue deficit and ρc is the

density of the compensator.

The term τ/ρc can be directly measured by using phantoms.

The term compensator ratio is the inverse of the thickness ratio. (ρc /τ ).

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.

Various devices are used to drive a pantographic cutting unit.

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

Medium density materials are preferred to reduce errors.

Various systems in use for design of these compensators are: Moiré Camera.

Magnetic Digitizers.

CT based compensator designing systems.

Compensating wedges These are used in cases such as curved

surfaces and oblique beam incidences in which contour can be approximated with a straight line.

These are fabricated from a metal such as copper,brass or lead.

Designed to compensate for a missing wedge of tissue.

Compensating wedgesThree important differences between

compensating wedges and wedge filters are:

Standard isodose curves, can be used since the c-wedges are not designed to produce tilt in isodose curves unlike standard wedges.

No wedge transmission factors are required.

Partial field compensation can be done for only part of contour which is irregular in shape.

Compensator set up

At the filter-surface distance calculated ≥ 20 cm.

Nominal SSD measured from a plane perpendicular to beam axis touching the highest point in the contour.

In SAD technique the depth of the isocenter is measured from the same elevated point only.

Multi leaf collimators A Multi leaf collimator(MLC) for

photon beam consists of a large number of collimating blocks or leaves that can be driven automatically independent of each other to generate a field of any shape.

Typical MLC consists of 80 leaves or more and this number depends on sophistication of machine.

Basic geometry of MLCs Multi leaf collimators are a bank of

large number of collimating blocks or leaves

Thickness = 6 – 7.5 cm Made of a tungsten alloy. Density of 17 - 18.5 g/cm3. Primary x-ray transmission:

Through the leaves < 2%. Interleaf transmission < 3%. For jaws 1% Cerrobend blocks 3.5% .

Considerations in the use of MLC 1.Conformity between the planned

field boundary which is continuous and jagged step wise boundary created by MLC in a stationary field.

2.optimization of MLC rotation. 3.The physical penumbra which is

larger than that produced by the collimator jaws or cerrobend blocks.

Comparison of MLCsVARIAN SIEMENS

1.in terms of position it is positioned as a

tertiary system below standard adjustable jaws.

2.Shape

rounded edge

3.Non focussing leaves so increased chance of penumbra through rounded edges

MLC replace the lower x-jaws.

Straight edges

Double focussing leaves i.e both the leaf edges and leaf ends are according to beam divergence.

VARIAN SIEMENS4.Tongue and groove model

5.Inter leaf movements present.

6.Rounded ends produce better confirmity in treatment volume.

7.With of leaf varies from 0.5 to o.25cm so more accurate delineation of volume

Blunt ends

No inter leaf movement

Sharp ends produce step egde effect.

Width of leaf is 1cm

VARIAN SIEMENS

Leaf carriage system present so maximum horizontal field opening is upto 30cm from centre.

Since MLC is placed in tertiary position placement and repair is sufficient without disturbing machine function.

Treats by DYNAMIC ARC METHOD OR SLIDING WINDOW TECHNIQUE.

There is no leaf carriage system so maximum horizontal field size is 19cm.

Any repair of MLC entitles entire machine to be stopped for servicing.

TREATS BY STEP AND SHOOT METHOD

Advantages of Multi leaf collimators 1.beam shaping is simple and less time

consuming.

2.these can be used without needing to enter treatment room.

3.correction and changing of field shape is simple.

4.overall treatment time is shortened.

5.constant control and continuous adjustment of the field shape during irradiation in advanced conformal radiotherapy is possible

Disadvantages

1.step edge effect 2.radiation leakage between the

leaves 3.wider penumbra

The use of MLCs in blocking and field shaping is ideally suited for treatments requiring large numbers of multiple fields because of automation of procedure and decrease in set up time.

Thus the importance of MLC is not just the replacement of cerrobend blocking .

Greater impact of this technology is in automation of field shaping and modulation of beam intensity.

Wedge Filters It is the most commonly used beam

modifying device. It works by producing tilt in the isodose curves.

Degree of the tilt depends upon the slope of the wedge filter.

Material: tungsten, brass. Lead or steel.

Usually wedges are mounted at a distance of 15 centimeters from the skin surface.

The sloping surface is made either straight or sigmoid in shade.

A sigmoid shape produces a straighter isodose curve.

Mounted on trays which are mounted on to the head of the gantry.

Types of wedge filtersPhysical wedges A physical wedge is an angled piece of lead or

steel that is placed in the beam to produce a gradient in radiation intensity. Manual intervention is required to place physical wedges on the treatment unit’s collimator assembly.

Motorized wedge Is a physical wedge integrated into the head of

the unit and controlled remotely. Dynamic Wedge IT produces the same wedged intensity

gradient by having one jaw close gradually while the beam is on.

Wedges come in 4 angles 15,30,45 and 90 degrees.

As the angle increases Attenuation produced by the thicker

end (heel)increases . Dose transmission from thinner

end(toe) thus tilting of isodose curve increases.

Selection of wedge This depends on 1.Wedge isodose angle or wedge

angle The angle through which an isodose

curve is tilted at the central ray of beam at a specified depth(1/2 or 1/3 of beam width or at 50% isodose line).

2.Hinge angle It is the angle between central axes of

two beams passing through the wedge.

3.Degree of separation between wedges

Distance between the thick ends of wedge filters as projected on the surface.

4. Wedge transmission factor defined as ratio of dose with and

without wedge at a point in phantom along the central axis of beam.

Usually measured at a suitable depth below the Dmax usually 5 -10 cms.

The resultant reduction in output results in an increase in the treatment time

Wedge transmission factor In some isodose charts used in cobalt machines

the wedge transmission factor is already incorporated, and no further correction is necessary.

Use of wedge will result in a preferential hardening - more pronounced in case of linear accelerators.

This is because the Co 60 beam is monoenergetic .

For small depths (<10 cms) most of the calculation parameters however remain unchanged.

At larger depths however, the PDD can be altered specially in case of linear accelerator beams

Wedge angle

Hinge angle=90-wedge angle 2

Working principleWedge Pair FieldsFor treatment using perpendicular

beam arrangement (gantry angles o degree and 90 degree) the superficial region of tumor receives higher dose or hot spot occurs,

To avoid this wedges are placed with thick ends adjacent to each other to get uniform distribution.

Open And Wedged field combinations For treatment of some tumors when

open field anteriorly and wedged field laterally is used

a. Dose contribution from anterior field decreases with depth

b. Bilateral wedges produce compensation and attenuation at thicker end

c. Boost to the deeper area by thinner end

Wedge systems Wedge filters are of two main types 1.Individualized wedge system This requires a separate

wedge for each beam width optimally designed to minimize the loss of beam output.

These are used in cobolt teletherapy .

2.Universal wedge system Single wedge serves for all beam

widths. It is fixed centrally in the beam irrespective of field size.

This is useful as it saves time. However not suitable for cobalt

beams because of excessive reduction of beam output with smaller fields.

Come in one size of 20 x 30 cms

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. Penetrating power should not increase

as this will alter the PDD as well as reduce the flattening.

Beam flatteningfilter

The beam flatness is specified at 10 centimeters.

The extent of flatness should be ± 3% along the central axis of the beam at 10 centimeters.

Should cover 80% or more of the field, or reach closer than one centimeter from the edge.

There is usually over flattening of isodoses, near the surface. This results in production of “horns” or hot spots.

No point parallel to the surface should receive a dose > 107% of the central axis dose.

Bolus A tissue equivalent material used to reduce

the depth of the maximum dose (Dmax). Better called a “build-up bolus”.

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.

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) Temex rubber (rubber) Spiers Bolus (rice flour and soda

bicarb)

Commercial materials: Superstiff: Thick and doesn't

undergo elastic deformation. Made of synthetic oil gel.

Superflab: Add water to powder to get a pliable gelatin like material.

Bolx Sheets: Gel enclosed in plastic sheet

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

Breast cone A beam modifying and directing device

used for a tangential fields therapy. Advantages:

Directs beam to the central axis of the area of interest, where a tangential beam is applied to a curved surface.

Helps position, the patient with an accurate SSD.

Endplate provides compensation, enhances surface dose and presses down the tissue.

Effective shielding of lungs.

Penumbra Trimmers Refers to the region at the edge of the

beam where the dose-rate changes rapidly as a function of distance from the beam axis.

Types: Transmission penumbra:

Transmission through the edge of the collimator block.

Geometrical penumbra : Finite size of the source.

Physical penumbra: Lateral distance between to specified isodose curves at a specific depth (90% & 20% at Dmax).

Takes scattered radiation into account.

Penumbra width depends upon: Source diameter. SSD. Depth below skin. Source to diaphragm distance

(inversely)

Consists of extensible, heavy metal bars to attenuate the beam in the penumbra region.

Increase the source to diaphragm distance, reducing the geometric penumbra.

SSD

SDD

d

. P = AB ( SSD + d – SDD)/ SDD

Beam Spoilers

First used by Doppke to increase dose to superficial neck nodes in head and neck cancers using 10 MV photon beams.

Special beam modification device where shadow trays made from Lucite are kept at a certain distance from the skin.

Based on the principle that relative surface dose increases when the surface to tray distance is reduced.

Beam Modification Of Electrons the most clinically useful energy

range for electrons is 6 to 20 Mev.Electron beams can be used for

treating superficial tumors with characteristic sharp drop off in dose beyond the tumor.

Distinct advantages are dose uniformity in target volume and sharp dose fall off beyond the tumor.

Beam shaping

1.Electron beam cone 2.Internal shielding 3.Scattering foil 4.lead cutouts

Internal shielding A lead shield can be placed

where shielding of structures against backscatter electrons is required.

A tissue equivalent material is coated over the lead shield like wax/ dental acrylic/ aluminum.

Example of areas requiring these techniques are the buccal mucosa and eye lids.

Scattering foil A device to widen the thin pencil beam

(3 mm) of electrons. Metallic plates of tin, lead or aluminium

are used. Disadvantages:

Beam attenuation. Generation of bremsstrahlung

radiation. Advantages:

Less prone to mechanical errors. Less expensive. Requires less instrumentation.

Lead cut-outs

For a low-energy electrons (<10 MeV), sheets of lead, less than 6 mm thickness are used.

The lead sheet can be placed directly on the skin surface.

Shields can also be supported at the end of the treatment cone if too heavy at the cost of greater inaccuracies.

Design is easier, because the size is same as that of the field on the patients skin.

Beam modification increases conformity allowing a higher dose delivery to the target, while sparing more of normal tissue simultaneously.

Megavoltage radiotherapy is better suited for most forms of beam modification due to it’s favourable scatter profile.

However any beam modification necessitates a close scrutiny of every phase of the planning and treatment process.

Thank you