radiotherapy beam modifiers

1. Beam modifiers
DR/Saeed Albehairy
Lecturer of clinical oncology, BNS University, Egypt
MD of clinical oncology, Cairo University

2. Beam modifiers
• Definition: desirable modification in the spatial distribution of
radiation - within the patient - by insertion of any material in the
beam path aiming to increase tumor conformity and homogeneity or
protect normal tissue.
• Examples:
- as jaws, wedges, blocks, compensator, multi-leaf collimator (MLC),
electron cut-out and bolus

3. Beam modifiers

4. Jaws
• Conventional collimator as in cobalt 60
• Make rectangle or square shape
• May be symmetrical or asymmetrical
• Modern machines have 1-4 independent jaws
• Present in machine head and can rotate 3600
• Move in transverse or longitudinal axis
• collimator system enables field sizes from 5x5 cm 2 to 40x40cm 2
X
Y

5. collimator
Gantry

6. Independent jaw
• 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”.
• Here beam is blocked off at the central axis to remove the divergence.
• Use of independent jaws and other beam blocking devices results in
the shift of the isodose curves. This is due to the elimination of
photon and electrons scatter from the blocked part of the field.

9. Multileaf collimators(MLC)
1. Present in head of linear accelerators
2. Multileaf collimators are a bank of large number of collimating
blocks or leaves
3. Can be moved automatically independent of each other to generate
a field of any shape.
4. 40 pairs of leaves or more having a width of 1 cm on less
5. Thickness = 6 – 7.5 cm
6. Made of a tungsten alloy.
Primary x-ray transmission: Through the leaves < 2%. Interleaf
transmission < 3%. For jaws 1% Cerrobend blocks 3.5% .

10. Blocks
• Aim:
1. Protect critical structure
2. Avoid unnecessary radiation to normal surrounding structures
3. Field matching
• No complete block can be achieved due to gradual beam attenuation
and scattering
• The choice of the shielding material depends on the type and energy
of beam being used
• The most commonly used shielding material for photons is lead.

11. Blocks
• 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.
• The term half value-layer is a convenient expression for the
attenuation produced by any material.
• HVL is defined as the thickness of material, which will reduce the
intensity of the primary beam by 50%
• 4.5-5 HVLs of lead are used to achieve acceptable block

13. Lead thicknessBeam energy(photon)
5.5 cmCobalt-60
6 cm4 MV
6.5 cm6 MV
7 cm> 10 MV

14. Block placement
• 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 also as low energy has high scattering
and shieling with be lost if there is distance.
• 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

15. Custom blocks
• Custom made to achieve maximum dose conformity and minimize dose to
critical & normal structure.
• Most commonly used is Cerrobend (Bismuth 50%, lead 27 %, Tin 13 % and
cadmium 10%)
Advantages over Lead:
1. low melting point 700
2. Harder than Lead
Disadvantages over Lead:
Low density (83% of Lead) so thicker thickness is needed
Usually 7.5 cm block used( lead thickness x 1.2)
Buccal mucosa Cerrobend block

16. Custom blocks

17. Custom Blocks

18. Advantages of MLC over other shieldings
1. Less time consuming
2. More conformal
3. Remote controlled so less radiation hazards
4. Easy treat multiple fields

19. Disadvantages of MLC
1. Radiation leakage between leaves
2. Island block is impossible
3. Jagged boundaries make field matching difficult
4. Wider penumbra due to beam scattering

20. Primary x-ray transmission:
1. Through the leaves < 2%.
2. Interleaf transmission < 3%.
3. For jaws 1%
4. Cerrobend blocks 3.5% .

21. Bolus
• Tissue equivalent material placed directly on skin
• Aim: in megavoltage : reduce depth of Dmax ( build up bolus)
• 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

22. Bolus
• Commonly used materials are:
Cotton soaked with water.
Paraffin wax.
Superflab: synthetic oil gel
Superflab bolus