Beam ModificationDR :-Omer Hashim

Beam modification

• Define as desirable modification on spatial distribution radiation pathway in the patient by insertion of the any material in the beam pathway .

Beam modification

shielding

bolus

Filters

wedges

Shielding • Shielding of the vital organs is major concept

in radiotherapy also shielding of the normal tissue out side the tumor

Types of the shielding:-

Shielding block Custom block

Asymmetrical jaw MLC

Tray

Shielding block

Shielding blocks are most commonly made of lead. The sickness depend on the beam quality . The allowed transmission through the block .The primary transmission of the 5% is acceptable .(depend on the HVL of the lead)

• 4.5—5 HVL of the lead recommended

Half Value Layer

• = thickness of material that lowers beam intensity to ½ the original value

• If point T is completely shielded by the block still these is scatter radiation from the surrounding area

• So dose to point T will not be {0}due to :-1- Exponential attenuation of 1ry beam . 2- Scattered radiation of irradiated area.

Tray Factor: (Allowance for beam attenuation)

4 mm thick Lucite blocking tray will result in 3% absorption with 60Co.9 mm thick Lucite blocking tray will result in 5% absorption with 60Co.So the dose rate is increased by same factor (i.e. tray factor).

Site of the tray in the beam :It should be at least15 cm from the patient.

Custom block

This material consists of 50.0% bismuth, 26.7% lead, 13.3% tin, and 10.0% cadmium.

The main advantage of Cerrobend over lead is that it melts at 70 C . So easily cast into any shape. The minimum thickness require to shield is calculated relatively to the lead commonly use 7cms equivalent to 6cms pure lead

Independent jaw

Asymmetric fields are sometimes used to block off a part of the field without changing the position of the isocenter .used in rectangular blocking as in the matching fields or beam splitting.

Most modern machines are equipped with independently movable jaws. Some machines have one independent jaw, others have two independent pairs, and some have all four jaws as independent

Multileaf collimators

• It is bank of large number of collimator block or leave .that can move automatically and independently of each others .40 pairs or more of leaf with width of 1 cm and thickness of 6—7.5 cms .

Compensators &bolus• the surface irregularity gives rise to unacceptable

nonuniformity of dose within the target volume .• Bolus :-It is materials used to compensate for

missing tissues in order to use the isodose charts without modification.

• Bolus is a tissue-equivalent material placed directly on the skin surface to even out the irregular contours of a patient to present a flat surface normal to beam

Uses of Bolus:• To compensate for missing tissues treated

with orthovoltage radiations.• If we deliberately need to increase skin dose

during treatment with megavoltage radiations e.g. tumor ulcerating through the skin.

• Materials of compensators: 1. Materials with similar Z to muscles e.g. wax. – These materials are generally bulky and fragile

and may be fragmented before the end of radiotherapy course.

2. Materials with higher Z than muscles e.g. Aluminum, brass, lead.

The thickness ratio depend on the :-!) compensator to surface distance2) Thickness of the missing tissue 3) Fields size4) Beam quality2 Dimension compensator3 Dimension compensator

• Bolus layer :-which is thick enough to provide adequate dose buildup over the skin surface. The latter should be termed the buildup bolus.

• Placing of the bolus :-To preserve the skin-sparing properties of the megavoltage photon beams, the compensator is placed a suitable distance (≥20 cm) away from the patient's skin. (directly for orthovoltage machine)

Types of boluses:• Lincolnshire Bolus:

– It is formed of tiny spheres of about 1/4 mm diameter. These spheres consist of:

• 87% sugar and 13% Magnesium carbonate.

• Spiers Mixture:– It is formed of powder which is mixture of 60% Rice flour

and 40% sodium bicarbonate.• Paraffin wax mixed with equal amount of bee wax:

Wedge compensator :-

• For oblique beam incidence or curved surfaces for which the contour can be approximated with a straight line, standard compensating wedges are very convenient (73,74). Compensating wedges (C-wedges) are fabricated from a metal such as copper, brass, or lead. They are designed to compensate for a “missing” wedge of tissue, using the same design principles as discussed in bolus

wedge• What is the different between wedge filter

and wedge compensator ?• Wedge is abeam modifying device which

cause progressive decrease in the intensity causing the tilting in the isodose curve from there position . Degree of the tilting is depend on the slope of the wedge

• Maternal :- tungsten – lead – steel

The presence of a wedge filter decreases the output of the machine.Wedge factor

The ratio of doses with and without the wedge, at a point in phantom along the central axis of the beamMeasured at a suitable depth beyond dmax (5 to 10 cm)Sometimes incorporated into the isodose curves

Wedge Transmission Factor:_

Types of the wedges :-• Individualized wedge system

– A separate wedge for each beam width– to minimize the loss of beam output– To align the thin end of the wedge with the

border of the light field– Used in 60Co

• Universal wedge system– A single wedge for all beam widths– Fixed centrally in the beam– Used in Linac

Enhanced dynamic wedges substitute the physical wedges. This technique achieves

wedge-shaped dose distributions by computer-controlled movement of one of the collimator jaws (Fig. 3) under simultaneous adjustment of dose rate and speed of the

moving jaw during the irradiation (Salk and Röttinger, 2003).

• Wedge angle– the angle through which an isodose curve is titled

at the central ray of a beam at a specified depth. (10 cm)

– The angle between the isodose curve and the normal to the central axis

• The angle of isodose tilt to decrease with increasing depth in the phantom

Clinical application

• The dose gradient in the overlap region is minimized.

• The dose falls off rapidly beyond the region of overlap or the “plateau” region.

• Use of the wedge :-Relatively superficial tumors lying at a depth of a few cm below skin, and lying on one side of the body e.g. Maxilla or Parotid.

• with open fields:– The dose contribution from open anterior

field (100% weight) decreases with depth, • The lateral wedged fields provide a boost

to offset this decrease.– As a tissue compensator for missing wedge

of tissues. – Common wedge angle 15 -30-45 -60-

filters• Filtening filter is reduce the central exposure

rate relatively to the edge of the beam so the• The beam density become uniform .• In diagnostic and superficial x-ray energy

range• primarily aluminum filters are used to harden

the beam .usually made of (AL –CU )sickness 1• &called inherent filtration

Effect on Beam Quality :_Attenuating the lower-energy photons (beam hardening)By Compton scattering which results in energy

degradation (beam softening).In the Megavoltage x-ray beam :_ Inherent filtration of the transmission targetFlattening filter(Primary purpose: make beam intensity uniform in

cross-section