Beam Modification devices in

RadiotherapyDr Susmita Sadhukhan

Dr Anirban HalderIPGME&R

Contents :

What is beam modification?Why is it needed?Type of beam modifiaction devicesWhat are block,wedge and MLC and their application in modern radiotherapy

BEAM MODIFICATION

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

Why Beam modification required:

Tight conformation Shielding of vital organs Shaping of field Tissue compensation Modification of skin sparing Matching of adjacent fields Achieve flattening of beam

Types of beam modification There are four main types of beam modification:

– Shielding: To eliminate radiation dose to some special parts of the zone at which the beam is directed.

– Compensation: To allow normal dose distribution data to be applied to the treated zone, when the beam enters obliquely through the body or where there is irregular or sloping surface .

– Wedge filtration: Where a special tilt in isodose curves is obtained.

– Flattening: Where the spatial distribution of the natural beam is altered by reducing the central exposure rate relative to the peripheral

Devices to be discussed

o Field blocking and shaping devices: Shielding blocks. Custom blocks. Asymmetrical jaws. Multileaf collimators.

o Wedge filterso Beam spoilers

o Compensators o Beam flattening filters.o Boluso Breast cone.o Penumbra trimmers.o Electron beam

modification

Shielding Blocks

• Since radiation attenuation is exponential and because of scattering, complete shielding can never be achieved.

• The aims of shielding are:– To protect critical organs.– Avoid unnecessary irradiation to surrounding normal

tissue.– Matching adjacent fields.

• An idea shielding material should have the following characteristics:– High atomic number.– High-density.– Easily available.– Inexpensive.

• The choice of the shielding material is also dictated by the type of beam being used!!

• The most commonly used shielding material for photons is lead.

• The thickness used depends upon the energy of the radiation.• For practical purposes, the shielding material which reduces

beam transmission to 5% of its original is considered acceptable.

• If n is needed HVL to achieve this transmission,then-

½ⁿ = 0.05

• Ideally the blocks should be shaped(divergent) to minimize transmission penumbra.

• BUT divergent blocks give little advantage for beams with LARGE geometric penumbra

Shielding

100%

50%

250 KV

4 MV

Lesser amount of scattered radiation with megavoltage

radiation means that the attenuation produced by

shielding is also more.

The higher scatter contribution to the overall dose results in

lower dosage adjacent to the shielded area in kilovoltage

radiation.

Beam energy Required lead thickness

4 MV 6.0 cm

6 MV 6.5 cm

10 MV 7.0 cm

Co60(1.25 MeV) 5.0 cm

• 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)--1.Avoids increase in skin dose

due to electron scatter.2.Also impossible to place

the heavy block on the body !!

Custom blocks• 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).• 1.21 times thicker blocks

necessary to produce the same attenuation.

• Most commonly thickness of 7.5 cms used.

Lead, 26.70%

Bismuth, 50.00%

Cadmium, 10.00%Tin,

13.30%

Bismuth Lead

Tin Cadmium

Electrically heated wire pivoting around a point (simulating the source)

cutting the styrofoam block

Cavities in the styrofoam block being used to cast the

Cerrobend blocks.

CERROBEND BLOCKShielding blocks can be of two

types:– Positive blocks, where

the central area is blocked.

– Negative blocks, where the peripheral area is blocked.

CAUTION TO BE TAKEN :

Cerrobend should be poured slowly to prevent bubble formation

Styrofoam block should be pressed tightly against a rubber pad at the bottom to prevent leakage.

Inside walls of the cavity may be sprayed with silicone for easy release.

Advantage of cerrobendblock:

Can be easily cast into any shape.

Edges focused towards the target-less penumbra

Disadvantages of cerrobend block:

The blocks are cast from alloys which need careful handling as they contain toxic materials.

The density of these alloys is usually lower than lead and therefore they need to be thicker.

Manufacturing is time consuming.

Special Shielding

Wedge Filters

• Most commonly used beam modifying device

• causes a progressive decrease in intensity across the beam, resulting in tilting the isodose curves from their normal positions.

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

• Material: tungsten, brass. Lead or steel.

WEDGE ISODOSE ANGLE

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

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

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

Type of wedges:– Manual wedges:1.Individualized wedge .2.Universal wedge– Internal or Motorized wedge– Dynamic wedges– Virtual wedges – Pseudo wedgesA motorized or internal or automatic wedge is a similar

device, a physical wedge integrated into the head of the unit and controlled remotely.

A dynamic or virtual wedge produces the same wedged intensity gradient by having one jaw close gradually while the beam is on. Dynamic wedges are used to achieve any arbitrary wedge angle in the range 0°–60º.

Automatic wedge filter

• Fixed jaws can be used to produce pseudo wedges where part of the treatment field requiring greater dose would be irradiated using smaller field sizes.

• Virtual wedge (Siemens GmbH) – the speed of collimator is constant and the dose rate is changeable. The smallest opening distance of collimator is 1 cm. The dose rate varies for every 2 mm movement of collimator jaws. The monitor units appropriate for prescribed dose are calculated on the base

• Enhanced dynamic wedge (Varian ) – the dose rate is constant and the speed of collimator is changeable. The Multiple Asyncronous Parallel Processing controlled the dynamic wedge system. The treatment monitor unit is determined on the base of Segmented Treatment Tables. The following figure shows the comparison of Varian dynamic and Siemens virtual wedge of algorithm.

UNIVERSAL WEDGE INDIVIDUALIZED WEDGE

Same wedge can be used for all field size

Center of wedge is fixed at beam center axis

Only a small portion is effective ,unwedged portion unnecessarily reduce the beam intensity.

Simpler to use and more useful for LINAC

• The width (W) of the wedge is fixed and important.

• Require separate wedge for each beam width

• Optimally designed to minimize the loss of beam output

• Thin edge is alligned with the border of the light field

• More useful for cobalt machines.

• Come in one size of 20 x 30 cms (except 60°).

Individualized wedge Universal wedge

• The presence of the wedge decreases output of the machine.

• That need to be corrected using WTF• In some isodose charts used in cobalt machines the

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

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

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

RULES OF WEDGE USE

• Wedged fields are generally used for relatively superficial tumors

• Beams are usually directed from the same side of the patient.• The broad edges of the wedges should be aligned together.• The wedge angle choosen depends on the angle between the

central rays of the two beams also called the “hinge angle”• Wedges:

– Reduce the hot spots at the surface – Rapid dose falloff beyond the region of overlap.

• Thus the 2 factors on which the wedge angle is choosen are:– The hinge angle.– The wedge separation

Multileaf Collimators• Multileaf collimators are a bank of

large number of collimating blocks or leaves

• Can be moved automatically independent of each other to generate a field of any shape.

• 40 pairs of leaves or more having a width of 1 cm on less (projected at the isocenter).

• 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%

• MLC systems may have double focus or single focus leaves.• Edges may be rounded to ensure constant beam transmission

through a leaf edge regardless of its possition in the field,though this can lead to significant beam transmission (20%) when the leaves abut each other.

• In order to allow fast interleaf movement, while reducing radiation transmission a tongue and groove design is often used.

• This design in turn leads to some under dosing in the region of the tongue (17 – 25%).

• MINI MLC:width 2-5mm• Micro MLC: width below 2mm

Penetration through curved leaves is independent of leaf position

• Technical parameters that characterize the nce of MLC are-

1.The maximum field size2.Width3.Max overtravel4.Interdigitation5.Configuration of MLC with respect to the collimator

jaws 6.Min &max leaf speed7.Precision of leaf positioning

MULTILEAF COLLIMATOR

• Accessory MLC(add-on):

high resolution MLCUsed in conjunction with stereotactic CRTAttached to the accessory holder of the t/t head Leaf resolution in the range of 1.5-4mm

MLC CONFIGURATION IN THE TREATMENT HEAD:

1.Total replacement of upper jaws2.Total replacement of lower jaws3.Tertiary collimator configuration

Multileaf Collimators• The degree of conformity

between the planned field boundary and the boundary created by the MLC depends upon:– Projected leaf width.– Shape of target volume.– Angle of collimator rotation.

• RCI = Treated Volume (inside 95% isodose curve) / PTV

• The direction of motion of the leaves should be parallel with the direction in which the target volume has the smallest cross-section.

The advantages are: Time for shaping and inserting of custom blocks is not required. The hardening of beam,

scattered radiation, and increase in skin doses and doses outside the field, as seen with physical compensators is avoided.

Automation of reshaping and modulation of beam intensity in IMRT.

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

The disadvantages are:• The physical penumbra is a

problem.t/t of small fields is difficult

• Island blocking is not possible.

• The jagged boundary of the field makes matching difficult.