To enable normal ditrbution data to be applied to treated zone when beam enters obliquely
This effect of blurring is more in kilo voltage than in megavoltage.
In this image we can see the effect of shielding on mv rad compared to kv radiation.. In megavoltage : the attenuation produced by the shielding material is more due to less amount of scatterd radiation Whereas in kv : less attenuation
And we can also see tat due to more scatter in kv there is low dose of radiation goin adjacent to shielded area THUS SHIELDING IS ACHIEVED BETTER IN MV THAN KV.
The choice of the shielding material is also dictated by the type of beam being used!! Most commonly used -- lead.
If n is the no of half value layers required to achieve this transmission of 5 % then the above is derived..
Thus a thickness of lead greater than 4.3 hvls would give less than 5% primary beam transmission nd is recommended hence practical thickness of lead used is bn 4.5 tp 5 HVLs . As no diff in attenuation at higher energies we don’t get higher HVLS
Because the absorber material is a source of electron scatter there will be an increase in skin dose ---to avoid this so that the main advantage of skin sparin by megavoltage is not lost..
Shadow tray – transparent plastic tray
not done as last minute alteration in field blockin
Divergent blocks are most suited for linac beams – as small source size hence small geometric penumbra
Any material can be used including lead
1.21 times thicker blocks necessary to produce the same attenuation.
Lead meltin point – 327 c Low melting point so enables to cast in any shape At room temp harder than lead
Care to be taken preparation : Cerrobend should be poured slowly to prevent air bubble formation Styrofoam block should be pressed tightly against the rubber pad at bottom to avoid leakage of material Inside wall of cavity to be sprayed with silicone to allow easy release of styrofoam pieces from block.
Hence this has given rise to the advent of multi leaf collimators and independent jawss.
2) In cobalt machines and older linear accelerators, where fixed jaws were present, half beam blocking was used to for beam splitting.
3)due to the elimination of photon and electrons scatter from the blocked part of the field
Generated electronically by creating wedged beam profiles through the dynamic motion of an independent jaw within the treatment field. In cobalt machines and older linear accelerators, where fixed jaws were present, half beam blocking was used to for beam splitting.
In addition, the use of asymmetric jaws as beam splitters, for field reductions, and with MLCs is helpful for most sites. asymmetric jaws to match supraclavicular and tangential fields for breast irradiation. reducing overall patient setup time the increased attenuation by the jaws reduces the dose to the contralateral breast and lung. A technique for matching lateral head and neck fields and the supraclavicular field using asymmetric jaws was done
The material of choice for leaf construction is tungsten alloy because it has one of the highest densities of any metal. Tungsten alloys are also hard, simple to fashion, reasonably inexpensive, and have a low coefficient of thermal expansion.
Non-focused (rounded) Leaf Ends: Leaf motion restricted to a single plane In principle, penumbra is somewhat greater than for focused collimators of divergent custom blocks Potential for greater leaf end transmission when leaves are abutted VARIAN ND ELEKTA USE THESE
Focused leaves – maintain geometric divergence
leaf and leaf size matches with beam divergence
Leaf transmission :Reduction of dose through the full height of the leaf Interleaf Transmission : Reduction of dose between adjacent leaves Leaf End Transmission :Reduction of dose between the ends of opposed leaves
Rounded edges is to provide constant beam transmision through leaf edge
In linac sharp dose fall ioff not achieved because dose fall off at the edge determined by scattered photons and electrons IN PGI WE HAVE SINGLE FOCUS LEAVES MLC
Mini – 2-5 mm leaf size Micro <2mm
Secondary : replace one of the jaws—upper or lower – elekta & siemens Tertiary : add on device in the head of the LINAC below the jaw collimators.--varian
The lower jaws can be split into a set of leaves as well. This design is used by SiemensTM and is double-focused. Both leaf ends and leaf sides match the beam divergence. That means that the collimator leaves move along the circumference of a circle centred at the x-ray tar- get of the linear accelerator, such that the end of the collimator is always tangential to the radius of the circle.1 The leaves of Siemens MLC can extend 10 cm across the field centreline, which allows a maximum leaf travel of 30 cm.
Advantage : MLC can be positioned just below the level of the standard upper and lower adjustable jaws . This design is used by VarianTM and was chosen to avoid lengthy downtime in the event of a MLC system malfunction. USING THIS APPROACH, IT IS POSSIBLE TO MOVE LEAVES MANUALLY OUT OF THE FIELD SHOULD A FAILURE OCCUR. THE MAJOR DISADVANTAGE OF PLACING THE MLC BELOW THE STANDARD JAW SYSTEM IS THE ADDED BULK AND CLEARANCE TO THE MECHANICAL ISOCENTRE. .
Electronic compensationn in head and neck Not useful in electron treatment and
because Time for shaping and inserting of custom blocks is not required.
2) because the procedure can be automated such that complex field shaped fields can be generated in sequence, reducing the set-up time.
3)As seen with physical compensators
1) Because the physical penumbra is larger than that produced by Cerrobend blocks treatment of smaller fields is difficult, as in shielding of critical structures, near the field. 3 done by cerrobend blocks
1) In total body irradiation
scatter near the field edges (example mantle fields), and horns in the beam profile.
Lincolnsire bolus nt used in mv bcoz difficult to adjust as per thickness
To compensate for these factors a tissue compensator always has attenuation less than that is required for primary radiation.
Compensator surface distance is the most important as the dist bn skin and compensator increases –thickness decreases.as
BCOZ A COMPENSATOR OF SAME THICKNESS AS THAT OF MISSING TISSUE WILL OVERCOMPENSATE DUUE TO DECREASE IN SCATTER ..
The term τ/ρc can be directly measured by using phantoms.
Used when proper mould room facilities are not available In this thin sheets of lead are glued together in a stepwise fashion.. Series of sticks placed Replaced by material – aluminum-shorter length Glued to the base plate
Various devices are used to drive a pantographic cutting unit. Cavity produced in the Styrofoam block is used to cast compensator filters.
Various systems in use for design of these compensators are: Moiré Camera – SPECIALLY DESIGNED CAMERA MOUNTED ON A SIMULATOR, ALLOWS TOPOGRAPHICAL MAPPING OF THE PATIENT AND THE DATA IS GENERATED Magnetic Digitizers – HANDHELD STYLUS WITH MAGNETIC SESOR WHICH SCANS THE PATIENT BODY CT based compensator designing systems – 3D TPS HAVE SUFFICIENT MULTILEVEL CT DATA TO AUTOMATE COMPENSATOR DESIGN
Need :when ones side of the pt is to treated bt more than one beam has to be used.
Less peripheral dose compared to physical wedge filter for example leess dose to the contralateral breast when using tangential breast irradiation technique
More in case of LINAC As cobaalt beam is monoenergetic .
The ratio of PDD at various points for wedged and non wedged fields are calculated And hence the thickness is determined. It also depends on the HVL of the beam for the filter material.
because of excessive reduction of beam output with smaller fields.
Using a combination of open and wedged field beams
It is mounted above the asymmetrical collimator and can be moved in and out of the radiation fields to generate desired wedge profiless.
STARTING POSITION RECEIVE MAX DOSE .
SPEED OF MOVEMENT OF COLLIMATOR LEAF DETERMINES THE ANGLE OF SLOPING ISODOSE CURVE. SIMPLE DYNAMIC , ENHANCED DYNAMIC
POOR MAN WEDGE Used in olden days when asymmetrical jaws opening was not possible and no planning TPS were available.
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.
To get uniform distribution.
When open field anteriorly and wedge field laterally are usedd…
technique is currently available on two Elekta VersaHD linear accelerator
less leakage outside of beam collimation and less variation of off-axis beam hardening FFF has increased dose rate; 1400 MU/min for 6 MV and 2400 MU/ min for 10 MV; less ;me for motion during treatment. FFF is softer than FF beam but surface dose is higher for FFF. Yet field size dependence of surface dose is smaller for FFF. 4)FFF has less photon head scatter and thus less field size dependence
a FFF linac for SBRT of liver and lung malignancies is associated with substantial improvement in treatment delivery time
In mv cant be used as a compensator as results in loss of skin sparing effect.
Thickness of bolus required to achieve 90-95% build up dose is substantially less than depth of Dmax
Water acts as bolus Cotton holds it
Lincolnshire bolus made up of tiny spheres of ¼ mm diameter Spiers mixture -powder
Designed to shape the dose distribution To conform closely to target volume
he potential benefit of a beam spoiler relative to bolus is preservation of skin-sparing characteristics for cases in which the skin surface does not require full dose.. In electron therapy to spread the beam or as bolus.
Leadin to xray production.. Compared to high atomic number target of xray tube –soft tissue has low atomic number hence less xray production
Produced when eectrons are slowed down by any material –more in high atomi no like lead.
For a low-energy electrons (<10 MeV), sheets of lead, less than 6 mm thickness are used. Design is easier, because the size is same as that of the field on the patients skin.
electron field shaping is done using lead cutouts placed on skin surface. Shields can also be supported at the end of the treatment cone if too heavy at the cost of greater inaccuracies. An electron cone is fitted to the treatment head, and is available in several preset sizes. This applicator has several collimators which attenuate any lateral scatter. It usually extends to just several centimetres from the surface. The most distal aperture may have an electron cutout fitted. This allows the field to be shaped appropriately, and is located in this position to prevent lateral scatter causing blurring of the beam edge.
IN PLACE OF FLATTENING FILTER. Nowadays dual foil systems are used, which compare well with scanning beam systems.
Against back scatter of electrons
Parts : Steel base plate Horizontal bar Lead block Transparent perplex plate Screws for distance adjustment
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
G .Lakshmi Deepthi
Desirable modification of the spatial distribution of
radiation within the patient by insertion of material in
the beam path.
Protect normal tissues
Uniform dose distribution
Better suitable for Radiotherapy
For special spatial distribution.
Radiation reaching any point is made up of primary and
Introduction of a modification device alters the dose
primary radiation is attenuated
secondary radiation blurs the effect
of beam modification.
Definition : Process in which a beam modifying device
is used for altering shape of the beam to reduce as far as
possible, eliminate the radiation dose at some specific
part or zone where beam is directed.
AIMS : protect critical structures
avoid unnecessary radiation to normal tissues
matching adjacent fields
Devices used for shielding :
Multi leaf collimators
A. Blocks :
An ideal shielding material should have
high atomic number
Block thickness :
Depends on the
Half value layer :
Thickness of a material required to attenuate the
intensity of primary beam to half of its original value.
Material which reduces beam transmission to 5% of
original value is acceptable .
1/2n = 5% or 0.05
Thus, 2n = 1/0.05 = 20
OR, n log 2 = log 20.
n = log20/log2 = 4.32
Cobalt -60 5.5cms
4 MV 6cms
6 MV 6.5cms
10 MV 7 cms
25 MV 7cms
Placement of blocks :
Kilo voltage :
sheets of lead are used
placed on the patients surface because :
Scattered radiation is more and all shielding
will be lost if distance between shield and the patient is
Low penetrating power of the beam.
placed high up in shadow tray at a distance of 15-
20cms from the skin surface to keep the dose to <50% of
to avoid increase in skin dose due to
Block divergence :
The blocks should be shaped or tapered so that their
sides follow geometric divergence of the beam ---------
to minimize block transmission penumbra.
Not much useful in cobalt due to large geometric
B .Custom blocks
These are custom made to achieve the greatest level of
dose conformity in order to minimize the dose to critical
structures and normal tissues.
Material :most common material used is
lipowitz metal ( Cerrobend )
other names : woods metal
Density -9.4g/cm3at 20°C (83% of lead).
melting point – 70°C.
Advantage over lead – low melting point
at room temperature, harder than lead
Disadvantage –thicker blocks necessary to produce
same attenuation. (Most commonly thickness of 7.5
Positive blocks – where central area is blocked .
Ex : lung , spinal cord
Negative blocks– where peripheral area is
Ex : tongue
Provide smooth boundaries by having continuous shape
around field size
No field size limitation.
Time consuming and expensive
When the materials are heated they emit toxic air due to
lead and cadmium
Sometimes due to excessive weight of the blocks can
cause injury to therapists.
C .Independent jaws :
Used when we want to block of the part of the field
without changing the position of the isocenter .
Allows us to shield a part of the field and can be used
for beam splitting
in this beam is blocked off at the central axis to
results in the shift of the isodose curves
Modern machines have 1-4 independent jaws
Clinical sites where asymmetric jaws are
typically used include breast , head and
neck, cranio-spinal , and prostate.
D . Multi leaf collimators (MLC) :
Are a bank of large number of collimating
blocks or leaves that can be driven automatically
independent of each other to generate field of
Typical MLC : 80 pairs of leaves or more
width of 1 cm on less.
Thickness : 6 -7.5cms
Density - 17 -18.5 g/cm3
Primary X-ray transmission :
Through leaves < 2%
Inter leaf <3%
Cerrobend blocks 3.5%
The primary beam transmission can be
further reduced by combining jaws with
MLC in shielding areas outside field
• In order to allow fast interleaf movement,
while reducing radiation transmission a
tongue and groove design is often used.
• leads to some under dosing in the region of
the tongue (10 – 25%).
• It leads to leak of upto 10%
MLC type A :
Y jaws are replaced by bank of MLC’s.
MLC type B :
Lower jaws X jaws are replaced
Siemens – double focused leaves
MLC type C :
MLC placed just below standard
upper and lower jaws
Applications of MLC :
To replace conventional blocking
Continuously adjusting the field shape in
To achieve beam-intensity modulation
Dynamic wedges and electronic compensation.
Less time consuming
Easy to treat multiple fields
No problem of hardening of the beam , scattered
radiation and increase in skin dose
Remote controlled– hence less radiation hazard
Constant control and continuous adjustment of field
shape during irradiation is possible.
Radiation leakage between leaves
Island blocking not possible
Jagged boundaries makes matching difficult
It is a beam modifying device which evens out the skin
surface contours while retaining the skin sparing
It compensates for the absent
tissues so that normal depth dose
distribution data and isodose
curves can be used for irregular
Kilo voltage : density wax or Lincolnshire bolus
Megavoltage : brass
Dimension and shape :
• 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 rather than in contact.
Thickness ratio : why ??
Thickness of a tissue equivalent component
Missing tissue thickness along same ray
Depends on :
Compensator surface distance
Thickness of missing tissue
The formula used for calculation of compensator thickness
is given by:
TD x (τ/ρc )
TD is the tissue deficit
ρc is the density of the compensator.
τ is the thickness ratio.
when d is ≤ 20 cm, a fixed value of thickness ratio (τ) is
used for most compensator work, (~ 0.7).
The term compensator ratio is the inverse of the thickness
ratio. (ρc /τ ).
2D : varies along a single dimension only
ex : lead, aluminum, Lucite taking into account their
effective attenuation coefficient, electron densities and the
coefficient. This results in
production of a laminated
3D : measures tissue deficits in both transverse
and longitudinal cross sections.
designed by Moire camera
tissue heterogeneity ( Ellis )
improve dose uniformity where dose irregularities arises
due to reduced scatter near beam edges and high dose
regions or horns in beam profile.
For Total Body Irradiation – as lung compensator
A wedge shaped beam modifying device, which causes a
progressive decrease in intensity across the beam,
resulting in tilting of the isodose curves from their
2 classes –physical
Physical wedge filter :
Wedge shaped absorber that causes a progressive
decrease in intensity across the beam resulting in a tilt of
the isodose curves from their normal positions
Tilt occurs towards the thin end
Tilt depends on slope of the wedge filter.
sigmoid – straighter isodose curves
Material: tungsten, brass. Lead or steel.
Nonphysical wedge filter :
Electronic filter that generates a tilted dose distribution
profile similar to a physical wedge by moving one of the
collimating jaws from one end of the field to the other.
Ex : Varian's enhanced dynamic wedge
Siemens virtual wedge
Advantage : automation of treatment delivery
less peripheral dose
Internal : internal motor slides the wedge into
placed above secondary collimating jaws.
External : manually inserted into beam
placed on a transparent plastic tray that can
be inserted into the head of the machine.
minimum distance of 15cm is required
between any absorber in the beam and surface in
order to keep skin dose below 50% of Dmax
Wedge isodose angle :
It is the compliment of the angle through which
isodose curve is tilted with respect to central ray of
the beam at a specified depth .
Angle decreases with increasing depth due to
The choice of the reference depth varies:
• 10 centimeters.
• 1/2 - 1/3rd of the beam width.
• At the 50% isodose curve (kV)
Depends on :
Hinge angle : Angle between the central rays
of two intersecting beams
Θ = ϕ/2
Wedge separation – distance between thick
ends of wedge filters
Wedge transmission factor :
The ratio of doses with wedge and without
the wedge at a point in phantom along the
central axis of the beam .
Presence of wedge decreases output of the
Wedge factor increases with increasing
depth and field size due to beam
hardening and extra scatter from wedge .
“X” is width and
“Y” is 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
cant use the wedge
without risking a hot
Parts of a wedge :
Individual wedge :
Separate wedge is used for each beam width
Used in cobalt beams
Width is fixed
Angles--15°, 30°, 45°, 60°.
Using bigger wedge will
reduce output of machine
6W x (15)
8W x (15)
10W x (15)
Universal wedge :
Single wedge serves for all beam widths
fixed centrally in the beam irrespective of
Not useful for cobalt
Angles - 15°, 30°,
Come in one size of
20 x 30 cms.
Motorized wedge :
Physical wedge integrated into head of the
unit and operated remotely.
Single 60 physical wedge which generates
desired angle from 0 to 60
Dynamic wedge :
Wedge effect produced by movement of one
of the independently movable collimator
leaf across the field.
2 types :
Simple Dynamic Enhanced Dynamic
Can provide wedge
angles – 15,30,45,60
Wedge angles of
Only for symmetric
fields of size upto 20cm
Both symmetric and
asymmetric field size
upto 30cm width
created by opening of small field in large field
small field gives 2/3rd dose while large field
gives 1/3rd dose
wedged dosimetry produced by movement of
collimators planned at advanced TPS of the
Advantage : wedge factor not needed.
Omni wedge :
here the wedged fields are combined along the X
axis and Y axis with proper weight
provides diagonal wedge profile
Compensating wedges :
used where the contour can be approximated
with a straight line for a oblique beam (for missing
wedge of a tissue)
Differences from wedge filters :
standard isodose curves can be used
no wedge transmission factor are required
partial field compensation can be done
Wedge application : wedge pair field
For treatment using perpendicular beam
arrangement the superficial region of
tumor receives high dose or hot spot
To avoid this wedges are placed thick
edges adjacent to
Open and wedge field combination :
from anterior field
decreases with depth
attenuation at thicker
Boost to deeper area
by thin end
Flattening filters :
Exposure rate at the center is greater that that
towards the beam edge .
inverse square law
natural spatial distribution
more scattered radiation in the center.
Thus flattening filters are the devices which
reduce the central exposure rate relative to that
near the edge
Used for megavoltage
Role in kilovoltage : initially used toavoid
excess dose to normal tissues– but not
preferred as it is complicated due
togreater chance of scattered radiation.
Material – copper or aluminum
Design – thickest at the center
It doesn't’t alter central axis percentage depth
Accurate positioning is important
prescribed at 10cms
extent of flatness should be +_3% along
should cover >80% of the field
USES : in treatment of pituitary ,prostate
Flattening filter free linacs :
It is a tissue equivalent material placed
directly on the skin surface
kilovoltage – to even out irregular
contours of a patient (as a compensator)
Megavoltage – to reduce the depth of
(build up bolus)
Ideal bolus :
Same atomic no as that of tissue
Same Electron density
Same absorption and scattering properties
Coblat -60 2-3mm
Commercially available :
Superflab : made of synthetic oil gel
thick and doesn’t deform
Superstuff : water + powder
pliable gelatin like material
Elastogel :gel enclosed in plastic sheets
good adhesive property
thereby minimizing air gap
Bolus in electron therapy :
Electron bolus is defined as water or near water
equivalent material that is normally placed either in
direct contact with skin surface or inside body cavity .
Uses– to flatten out irregular surface
increase surface dose
shape coverage of treatment volume
Placement : such that 90% dose reaches skin surface
1) flexible sheet Ex: superflab
placed directly on skin surface
thickness increments of 0.3-0.4cms
use- chest wall irradiation
2) Rigid bolus sheet
Also known as scatter plate– as it scatters electron beam
in addition to decreasing energy .
Used in treating highly irregular or sensitive skin
Standard thickness of PMMA (0.125 to 0.25 cm) are
placed perpendicular to beam.
Should be in contact or close to patient to avoid large
penumbra which occurs due to air gap.
Beam spoilers :
The beam spoiler is composed of a sheet of material
which has a low atomic number,typically lucite.
used to increase the build-up dose near the surface for
treatment of superficial treatment areas
First used by Doppke to increase dose to superficial
neck nodes in head and neck cancers using 10 MV
Electron beam modifiers :
Electron traveling in a medium loses energy due to :
Ionization and excitation – soft tissue
Electrons are generated in a similar way to photons in a
linear accelerator, with the major difference being:
Lack of a tungsten target
A scattering foil instead of a flattening filter
An electron applicator or electron cone which provides
added collimation of the beam
Electron Cone :
To avoid variation in output and electron scatter, jaws
cannot be used to collimate electron beams.
An electron beam cone is therefore used to provide the
collimation– fitted to treatment head.
A primary collimator is provided close to source –
defines the maximum field size.
A secondary collimator, near the patient defines the
Scattering foil :
A device to widen the thin pencil beam (3 mm) of
Metallic plates of tin, lead or aluminum are used.
◦ Beam attenuation.
◦ Generation of bremsstrahlung radiation.
◦ Less prone to mechanical errors.
◦ Less expensive.
◦ Requires less instrumentation.
Internal shield :
Stops electrons that enter the body surface before they
reach any critical structures and deposit any significant
Tungsten is used for testis
A tissue equivalent material
is coated over the lead shield
like wax/ dental acrylic/
Thickness of lead required –0.5mm/Mev
When lead is used for shielding ,it needs to be
supplemented by other shielding material because less
efficient—hence bolus placed over skin surface.
Ex : salivary glands
Breast cone :
It is a beam modifying and directing
device used for tangential fields therapy
Directs beam to the central axis of the area of
interest where a tangential beam is applied to a
Helps position the patient with an accurate SSD
Endplate provides compensation ,enhances
surface dose and presses down the tissue
Effective shielding of lungs
Penumbra trimmers :
Penumbra refers to the region at the edge of
beam where dose rate changes rapidly as
function of distance from beam axis.
Types : geometric – source size
transmission – edge of collimator block
physical – lateral distance between 2
isodose curves at specified depth
Width depends on :
depth below skin
SDD (inversely related)
Trimmers consist of extensible ,
heavy metal bars to attenuate
the beam in the
They increase the SDD
distance ,thereby reducing
Beam modification is essentially a fundamentally
important aspect of modern radiotherapy treatment
planning and delivery.
Beam modification increases conformity allowing a
higher dose delivery to the target while sparing more
normal tissues simultaneously
Megavoltage radiation is more favorable for beam
modification due to its favorable scatter profile
However necessitates close scrutiny of every phase of
planning and treatment process—daily
“Price of safety is