2. KILOVOLTAGE RADIATION
OUTPUT VALUES
- Surface doses were limiting factor in treatment with
kilovoltage radiations
- The standard reference point for dosage statements is
at the centre of the field on the surface
- Surface output: The exposure rate (R) at any point in an
irradiated material is made up of two components –
- 1) Primary radiation( directly from the tube)
- 2) Scattered radiation ( from the irradiated material)
3. PRIMARY RADIATION
The exposure rate of primary radiation depends
on
- The amount of radiation generated at the target of
the Xray tube
- The volume and type of material it has to pass
through to reach the surface
- The distance of the surface from the target
I - Tube current in milliamperes
E – Applied kilovoltage
Z – Target atomic number
4. SCATTERED RADIATION
The exposure rate of the scattered radiation will
always be directly proportional to exposure rate of
the primary radiation
Percentage scatter: The exposure rate of the
scattered radiation expressed as a percentage of
the primary exposure rate which is producing the
scatter
For a point on the surface its called as
percentage back scatter
Depends on the size and shape of the Xray field
and radiation quality
5.
6. BEAM DIMENSIONS:
- Percentage back scatter does not vary in direct
proportion with beam size ( The scattered radiation
generated in outer parts of an irradiated zone
suffers more attenuation in reaching the reference
point)
- Saturation value: Point where a further increase in
beam size produces practically no increase in the
scattered radiation reaching the centre.
- Beam shape – Percentage backscatter is same for
circle and square of the same area but lesser for
rectangle of the same area
10. The likelihood of radiation being scattered (
Scatter attenuation coefficient) decreases as
the energy of radiation increases
The steady fall in the percentage backscatter is
expected at the higher quality end (Megavoltage
radiation) due to relative reduction in the amount
of scattered radiation
At lower energies what is unexpected is the
marked falling off of scattered radiation
12. It is not only the scatter caused by the primary
radiation but by any radiation that has to be
taken into account (for large and rectangular
beams)
The greater amount of scattering of the lower
energy radiations is due to its much lower
penetrating power
The magnitude of the total scattered radiation
depends on contributing volume and how much
radiation is scattered per volume at low energies
At higher energies, the percentage backscatter
depends entirely on the amount of scatter
produced only and so it decreases steadily with
increasing energy
15. Beam defining devices like diaphragms and
applicators may add scattered radiation to the
beam which will be a contributory factor in the
variation of tube output
How the magnitude of scattered radiation varies
with beam size and shape can be determined
only by measurements
With a well designed system the contribution of
scattered radiation will be small but will vary from
one equipment to another
Output calibration measurements of one
equipment can seldom be applied to another
The surface is not the ideal place for making
output calibration measurements
17. The reason for the falling off of scattered radiation
towards the beam edge is that this edge is further
away from much of the beam than is the centre ,
therefore scattered radiation reaching the edge has
generally suffered more attenuation
The primary exposure rate is smaller at the edge
than at the centre of the field because of the inverse
square law. The magnitude of the effect depends on
beam size being much greater for larger beams
18. There is considerable amount of scattered
radiation beyond the geometric edges of the
beam ( in the region receiving no primary
radiation)
Clinical implication: Organs and tissues outside
the
geometric beam may well be exposed to
amounts of
radiation that are not negligible
While in the plane at right angles to the electron
stream the Xray emission is symmetrical on
either side of the central axis, but in the plane of
the stream more radiation emerges from the
target side of the central axis of the beam than
19. SUMMARY OF OUTPUT
Primary exposure rate depends on
1) Tube current
2) Tube kilovoltage
3) S.S.D
4) Any added filter
5) Tube wall thickness and material
6) Target material
7) Voltage wave form
20. Percentage backscatter depends on
1) Size of the beam
2) Shape of the beam
3) Quality of radiation
4) Design and detail of collimation
21. Depth dose data
Percentage depth dose = Absorbed dose at the point x 100
Absorbed dose at the surface
Absorbed dose rate at any point = Exposure rate at that point X
Exposure to absorbed dose conversion factor ( roentgen to rad)
Percentage depth dose = Exposure rate at the point X 100
Output
22. Factors influencing percentage depth
dose values
1) Depth of that point below the surface
- The greater the depth the smaller the percentage
depth dose(D)
- Explained by inverse square law and increasing
attenuation suffered with increasing thickness
23. 2) Beam dimensions
- Steady increase in D though not linear as area increases
- Smaller values of D for rectangular than square or circular
beams
24. 3) Radiation quality
- Penetrating power of the radiation
- D increases with increasing half value layer
- Magnitude of increase affected by scattered radiation.
Therefore more pronounced for small beams
- Not very efficient to increase PDD values by increasing
filtration
So for greater PDD values, higher radiation energies are
needed
To achieve this radiotherapy turned from kilovoltage range
to
megavoltage range
25. 4) S.S.D
- D increases as SSD increases
- Effect of SSD on output : Surface output is inversely
proportional to square of SSD
- Device working in 200- 300 kV range : SSD is 50 cm
26.
27.
28. 5) Position in the beam
- The exposure rate falls to either side of the central ray
- Explained by inverse square law and attenuation
- The PDD also varies: greatest at the central axis and falling
off towards the beam edge
29. ISODOSE CURVES
Definition :
Lines joining the points of equal Percentage
Depth Dose (PDD).
The curves are usually drawn at regular intervals
of absorbed dose
Expressed as a percentage of the dose at a
reference point.
ISODOSE CHARTS :
Contour maps of dosage distribution in and
around the beam
which consists of a family of isodose curves
33. Any falling off in output should prompt a
distribution check since it may result from ‘pitting’
which may upset the distribution resulting in
asymmetrical distribution
Routine checks of radiation distribution should be
done periodically ‘ in air’ to ensure that
satisfactory conditions are maintained or to
detect any changes
34. MEGAVOLTAGE RADIATIONS
> 1 MV
Greater penetrating power
Smaller scattering
Scatter occurs in ‘forward’ direction
Difference in ionization by the electrons
Difference in spatial distribution between the
kilovoltage (primary electrons in all directions)
and megavoltage radiations( primary
electrons in forward direction)
35. IONIZATION:
- The effect of X-radiation are produced by ionizations
and excitations produced in turn by electrons
liberated when photons interact with matter
- Kilovoltage radiations liberate electrons which
travel only a fraction of a millimetre in tissue ,
water, air . So the exposure at a point is a
direct measure of the absorbed dose at that point
- In Megavoltage radiations , electrons liberated
travel 1 mm to 8 cm before being brought to rest.
Because of the BRAGG
EFFECT the electrons produce most of their
ionization towards
the final end of their track. Here the absorbed dose
at any point
36. Central axis depth dose values
- Most striking difference between kilovoltage and
megavoltage radiation is the pattern of their
respective absorbed dose variation with depth
- ‘The Build up’
37.
38. The total ionization at any place will be the
sum of all the effects shown and numbers
representing this are given at the foot of the
above diagram
The build up is the same for all depths greater
than 4 mm
In practice the ionization decreases beyond
the peak of the build-up because of the effects
of the inverse –square law and photon
attenuation
39. The electrons ejected by megavoltage photons
travel predominantly ‘forward’ and there is an
increase of ionization along their tracks towards
the end of their range
‘Build up ‘ effect is counterbalanced by any
scattered radiation travelling in the opposite
direction( as its electrons are ejected towards the
surface), so this build up effect is not seen in kV
radiation
In megavoltage radiation ‘build up’ occurs
because there is minimal scattered radiation
41. • Exposure which is a function of the beam is
maximum at the surface and falls steadily with
depth because of the joint influence of the inverse
square law and beam attenuation
• Beyond the peak of the build up, Exposure and
absorbed dose vary in the same way
• At these depths exposure (roentgen) can be
directly converted into absorbed dose(Rad) as for
lower voltage radiations
DOSAGE REFERENCE POINT
- Not on the phantom surface
- At a depth of the maximum of the depth dose
curve
42. OUTPUT
• Filtration effect is negligible in megavoltage
radiations unlike kilovoltage radiations
• Transmission type target is always used in
high energy tubes because Xrays are
mainly produced in the forward direction.
Therefore it undergoes inherent filtration
before emerging out of the tube
• Output is independent of field shape and
size because of minimal scattered radiation
43. PERCENTAGE DEPTH DOSE
VALUES
Generating voltage
S.S.D
Beam size and shape
Depth
Position in the beam
Isodose charts
45. S.S.D
Directly proportional like in Kv radiations but change
is greater
The smaller the scatter , the greater the influence of
S.S.D on percentage depth doses
Much greater S.S.D values are used for
megavoltage radiations
For eg, with 4 M.V apparatus 100 cm S.S.D is used
BEAM SIZE AND SHAPE
MV beams PDD is almost independent of beam size and
shape due to minimal scatter
DEPTH
-Slower rate of decrease with depth
47. Beam flattening filter or compensator is
necessary in MV radiations to eliminate the
rapid variation of primary exposure rate
across the beam
Compensators help in minimising the
exposure variation across megavoltage fields
48. ISODOSE CHARTS
Striking difference with kV radiation – Much
smaller limits of scattered radiation outside
the geometric limits of the beam AND
smaller changes in PDD across the beam at
any depth
Flatter isodose curves are the result of the
beam flattening compensators whose
efficiency at considerable depths is due to the
reduced scatter for high energy radiation
