Beam modification techniques include blocks, wedges, and compensators to modify the spatial distribution of radiation. Shielding blocks are commonly made of lead and are used to shield organs at risk. Wedge filters cause a progressive decrease in beam intensity to tilt isodose lines and provide a dose boost. Compensators are customized materials that account for tissue irregularities and ensure uniform dose distribution within the target. Flattening filters reduce the central exposure rate relative to the beam edges to produce a uniform beam density.

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

5. Shielding
• Shielding of the vital organs is major concept
in radiotherapy also shielding of the normal
tissue out side the tumor

6. Types of the shielding:-

7. 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

8. Half Value Layer
• = thickness of material that lowers beam
intensity to ½ the original value

11. • 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.

12. 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.

14. 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

21. 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

23. 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 .

33. 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

34. 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.

35. • 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 distance
2) Thickness of the missing tissue
3) Fields size
4) Beam quality
2 Dimension compensator
3 Dimension compensator

36. • 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)

37. 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:

47. 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

50. 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

53. 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
beam
Measured at a suitable depth beyond dmax (5 to 10
cm)
Sometimes incorporated into the isodose curves
Wedge Transmission Factor:_

54. 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

56. 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).

57. • 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

58. 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.

59. • 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-

65. 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

66. 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 target
Flattening filter
(Primary purpose: make beam intensity uniform
in cross-section