Beam modification devices

1. 13/21/2015

2. Outline
Modification of beam
Need of beam modification
Types of beam modification
Devices modifying the beam geometry
Working of beam modifiers

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

4. Four 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 where different types of tissues
are present
• 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

5. Beam modification devices
4 main types of beam modifying devices
Wedge filter
Compensator
Bolus
Shielding blocks

6. other modification devices
Beam spoilers
Beam flattening filters
Breast cone
Penumbra trimmers
Electron beam modification

7. Hurdles
• Radiation reaching any point, is made
up of primary and scattered photons.
• introduction of the modification devices
results in alteration of dose distribution
• The phenomena scattering results in an
“blurring” of the effect of the beam
modification

8.  The higher scatter
contribution to the
overall dose results in
lower dosage adjacent
to the shielded area in
kilovoltage radiation.
 Lesser amount of
scattered radiation with
megavoltage radiation
means that the
attenuation produced by
shielding is also more.

9. Shielding
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
• It should have following characteristics
high atomic number
high density
easily available
inexpensive

10. Shielding
Shielding Blocks
Independent Jaws
Multi Leaf Collimators
Custom Blocks

11. Lead as a shielding material
• Choice of shielding is also dictated by the type
of beam being used.

12.  It’s thickness depends on
• Attenuation of shielding 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

13. Placing of Shielding
 In Kilovoltage radiation shielding is readily achieved by
placing sheets of lead on the surface directly
It is necessary because of the lower penetrating power of 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

14. Customised blocks
• Material used for custom locking is known as
the Wood's metal (Cerrobend)
• Commercial names
 Lipowitz's alloy
 Cerrobend,
 Bendalloy,
 Pewtalloy
 MCP 158
• Melting point 70°C.
• Density 9.4 g /cm3 at 20°C (83% of lead)

15. Composition Of Cerrobend
Lead, 26.70%
Bismuth, 50.00%Cadmium,
10.00%
Tin, 13.30%
Bismuth Lead Tin Cadmium

16. • Major advantage over lead is its low melting
point which enables it to cast in any shape.
• At room temperature it is harder than lead.
• 1.21 times thicker blocks necessary to
produce the same attenuation.
• Most commonly thickness of 7.5 cms used.

17. Construction of blocks
Outline of the treatment
field being traced on
radiograph using a
Styrofoam cutting
device.
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.

18. Custom blocks
 Shielding blocks can be of two types:
• Positive blocks,
where central area is blocked
• Negative blocks
where peripheral area is blocked
 Diverging Block
– Ideally blocks should be shaped or tapered so
that their sides follow the geometric
divergence which minimizes the block
transmission penumbra

19. 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 & can be used for “beam
splitting”
 Beam is blocked off at the
central axis to remove the
divergence
 Use of independent jaws
results in the shift of the
isodose curves & penumbra
 This is due to the elimination
of photon and electrons scatter
from the blocked part

20. 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.
• MLCs are available from all
the major medical LINAC
manufacturers : Elekta,
Siemens & Varian
• Independent
manufacturers:BrainLab,
Radionics, Direx,
NOMOS,Novalis

21. MLC
Conventional
Secondary Tertiary
Singly Focused
Doubly Focused
OR
Mini Micro
Single Plane
Double Plane
OR
Miniature

22. Intraleaf transmission:
radiation transmitted through the full height of the leaf
Interleaf transmission:
transmitted radiation measured along a line passing between leaf sides
Leaf end transmission:
transmitted radiation measured along a ray passing between the ends of
opposed leaves in their most closed position
Leaf Terminology

23.  Secondary MLC: replaces one of the
secondary jaws (upper or lower) - Elekta &
Siemens
 Tertiary MLC: add-on device in the head of
linac below the jaw collimators - Varian
 Leaves
– Elekta (80, microSRS )
– Siemens (58 and 80 leaves)
– Varian (52, 80 and 120 leaves)

24. ELEKTA MLC: Y-Jaw replacement
No. of leaves 80
Material Tungsten
Thickness (height) 7.5 cm
Arrangement Tongue and Groove
Width at isocentre 1.0 cm
Movement path Linear
End design Curved/rounded
Focusing Single
Interdigitization of opposite leaves Possible
Position Sensor Optical/CCD camera
Back-up jaw thickness 3.0 cm
Field size at isocentre 40 x 40 cm2

25. SIEMENS MLC:X-Jaw Replacement
No. of leaves 58
Field size at isocentre 40 x 40 cm2
Material Tungsten
Thickness (height) 7.6 cm
Arrangement Tongue and Groove
Width at isocentre 1.0 cm – inner 27 pairs
6.5 cm – outer 2 pairs
Movement path Circular arc
End design Straight
Focusing Double
Interdigitization Not Possible
Position Sensor Linear encoders

26. VARIAN MLC: Tertiary
Material Tungsten
Thickness (height) 5.0 cm
Arrangement Tongue and Groove
Movement path Linear
End design Curved/rounded
Focusing Single
Interdigitization Possible
Position Sensor Linear encoders

27. Comparison of leaf assembly
(End-on view)
Elekta Varian Siemens

28.  Merits
• beam shaping is simple and less time consuming
• these can be used without needing to enter treatment
room
• correction and changing of field shape is simple
• overall treatment time is shortened
• constant control and continuous adjustment of the
field shape during irradiation is possible
 Demerits
• step edge effect
• radiation leakage between the leaves
• wider penumbra

29. Tissue compensators
• 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.
To compensate for dose irregularities arising due to
reduced scatter near the field edges.

30. • 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

31. Thickness ratio or density ratio
Required thickness of a
tissue equivalent
compensator
Missing tissue thickness
along same ray

32. • The thickness ratio depends on:
– Compensator to surface distance,d
– Thickness of the missing tissue
– Field size
– Depth
– Beam quality
• Distance is the most important factor when d
is ≤ 20 cm
• A fixed value of thickness ratio (τ) is used for
most compensator work (~ 0.7)

33. Two-dimensional compensators
 Thickness varies, along a single dimension only.
 Can be constructed using thin sheets of lead,
lucite or aluminum

35.  3-D compensators are designed to measure
tissue deficits in both transverse and
longitudinal cross sections
 Cavity produced in the Styrofoam block is
used to cast compensator filters
 Various devices are used to drive a
pantographic cutting unit
Three-dimensional compensators

36. Wedge filters
 Wedge shaped absorber causes a progressive
decrease in intensity across the beam
 Results in tilt of the isodose curve & degree of
the tilt depends upon the slope of the wedge
filter
 Material: tungsten, brass. Lead or steel

37. Isodose lines in a wedge system

38. Wedge filters
 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

39. Types of wedges
Physical wedges
– Individualized
– Universal
– Motorized wedge
Dynamic wedges
Enhanced Dynamic wedges

40. Wedge parameters
• The two dimensions of
wedges are important – “X”
or width and “Y” or length.
• All wedges are aligned so that
the central axis of the beam is
at the central axis of the
wedge.
• If the X dimension of field is
longer then we can’t use the
wedge without risking a hot
spot!!
X
This area will have a
hot spot.

41. Wedge isodose angle (θ) is the complement of the
angle through which the isodose curve is tilted with respect to
the central ray of the beam at any specified depth
This depth is important because the
angle will decrease with increasing
depth.
The choice of the reference depth
varies:
10 centimeters
1/2 - 2/3rd of the beam width
At the 50% isodose curve

42. Hinge angle,φ
It is the angle between central axes of two
beams passing through the wedge
Relationship b/w φ & θ
Wedge angle,θ= 90 – φ/2

43. • Degree of separation between wedges
– Distance between the thick ends of wedge filters
as projected on the surface
• 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
– Resultant reduction in output results in an
increase in the treatment time

44. Physical wedge
• 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
• Two types
– Individualized
– Universal

45. 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
• The width (W) of the wedge is fixed
• All systems have the following four angles 15°, 30°, 45°,
60°.
• The wedge systems available are:
– 6W ( x 15)
– 8W ( x 15)
– 10W ( x 15)

46. Universal wedge
• Single wedge serves for all beam widths. It is fixed
centrally in the beam irrespective of field size.
• 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

47. It is a physical wedge integrated into the head of the
unit and controlled remotely
( 60° wedge)
Motorized wedge

48. Dynamic Wedge
• It produces the same wedged intensity
gradient by having one jaw close
gradually while the beam is on

49. Working principle
Wedge Pair Fields
• For treatment using perpendicular beam
arrangement 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

50. Wedge Pair Fields

51. Working principle
Open and Wedged field combinations
• For treatment of some tumors when open field
anteriorly and wedged field laterally is used
• Dose contribution from anterior field decreases
with depth
• Bilateral wedges produce compensation and
attenuation at thicker end
• Boost to the deeper area by thinner end

53. Beam fattening filter
• Intensity is more at central axis and
decreases as we move away
• Non-uniform dose at any given depth
• FF is used to uniform it
• Usually made up of Al or Brass

54. Bolus
• A tissue equivalent material
used to reduce the depth of
the maximum dose (Dmax)
• 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.

55.  Properties of an ideal bolus:
– Same electron density and atomic number
– Pliable to conform to surface
– thickness of the bolus used varies according to the
energy of the radiation
 In megavoltage radiation:
• Co-60 : 2 - 3 mm
• 6 MV : 7- 8 mm
• 10 MV : 12 - 14 mm
• 25 MV: 18 - 20 mm

56. • 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)
– Lincolnshire bolus (sugar and mag carbonate in form of
spheres)
– Spiers Bolus (rice flour and soda bicarb)
• Commercial materials:
– Superflab: Thick and doesn't undergo elastic deformation.
Made of synthetic oil gel
– Superstuff: Add water to powder to get a pliable gelatin
like material
– Bolx Sheets: Gel enclosed in plastic sheet

57. 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

58. Penumbra trimmers
• Consists of extensible,
heavy metal bars to
attenuate the beam in the
penumbra region.
• Increase the source to
diaphragm distance,
reducing the geometric
penumbra.
• Another method is to use
secondary blocks placed
close to the patient ( 15 –
20 cms).

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

63. Beam
Modification
Of Electrons

64. 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

65. 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.

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

67. Conclusion
 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

68. Drugs are not always necessary.
Belief in recovery always is.
Norman Cousins
American political activist

69. Thank u…