Clinical Physics
Dr.Aida Radwan
Assistant Professor of Medical Physics
National Cancer Institute
Cairo University
Chapter (
2
)
Treatment Planning
ISODOSE
CURVES
Photons
•
The central axis depth dose distribution is not sufficient
to characterize a radiation beam which produces a
dose distribution in
a
3
D
volume
•
in order to represent volumetric or planar variation in
absorbed dose, distributions are depicted
روصت
by
means of Isodose curves.
•
Isodose curve
–
Lines passing through points of equal dose
–
Drawn at regular intervals of absorbed dose
–
Expressed as a percentage of the dose at a reference point
Isodose Distribution
Isodose
Chart (1)
SSD=
80
cm
SAD=
100
cm
Properties of x

γ

ray dose distribution
1.
The dose at any depth is greatest on the central axis of the beam
and gradually decreases toward the edges of the beam
,
with the exception of some
linac
x

ray beams which exhibit
areas of high
dose or ‘
horns
’ near the surface in the
periphery
of the
field which is created by
the
flattening
filter
to over
compensate
near the surface in order to obtain flat
isodose
curves at greater
depths.
2
. Near the edges of the beam “the
penumbra
region” the
dose
rate decreases rapidly as a function of lateral distance from the
beam
axis.
The
width of geometric penumbra depends on
source size
,
distance from the source
, and
source

to

diaphragm distance
.
The dose variation across the field at a specified depth. Such a
representation of the beam is known as the
beam profile.
3.
Near the beam edge, the “
Falloff
of the
beam”
is caused by:
–
By the geometric penumbra
–
By the reduced side
scatter
4.
Outside the geometric limits of the beam and
the
penumbra the
dose variation is the result of
side scatter from the field and both leakage and
scatter from the collimator system.
Beam Profiles
•
flatness
–
usually specified at
10
cm
–
within
3
%. over
80
% of
the field
•
symmetry
–
usually specified at
10
cm
–
within
2
%. over
80
% of
the field
80
% field size
Field size
80
%
20
%
penumbra
50
%
50
%
Measurement of Isodose Curves
•
Ion chambers
–
Relatively flat energy response and precision
–
Waterproof and small
•
Solid state detectors
•
Radiographic films
The computer

driven devices for measuring
isodose
curves
Automatic Isodose Plotter System
•
Two ion chambers
–
Detector A
–
To move in the tank of water to sample
the dose rate
–
Monitor B
–
fixed at some point in the field to monitor
the beam intensity with time
–
The final response A/B is independent of fluctuations
in output
.
Sources of Isodose Charts
Isodose distributions can be obtained from
manufacturers of radiation generators or from other
institutions having the same unit.
However, the user is cautioned against accepting
isodose
charts from any source and using them as basis
for patient treatment without adequate verification.
The first and most important check to be performed is to
verify that the central axis depth

dose data correspond
with percent depth

dose data measured independently
in a water phantom.
Sources of Isodose Charts
•
To verify
the central axis depth dose
data
correspond with
PDD
data measured
independently
in a water phantom
•
A deviation of
2
% or less
in local dose
is
acceptable
up to depth of
20
cm.
•
For selected
Field Size
and depths, an agreement
within
2
mm in the penumbra
region
is
acceptable.
Parameters
affect
Isodose
Curves
•
The parameters affect the single

beam
Isodose curves are
:
–
Beam
quality
–
Source size, SSD, and SDD

the penumbra effect
–
Collimation and flattening filter
–
Field size
Beam Quality
•
The depth of a given
isodose
curve increases
with increase of beam quality.
•
Greater lateral scatter
associated with
lower

energy beams
•
For
megavoltage beams
, the scatter outside
the field is minimized as a result of forward
scattering and becomes
more a
function of
collimation than energy
.
200
kVp
,
SSD=
50
cm
60
Co, SSD=
80
cm
4
MV, SSD=
100
cm
10
MV, SSD=
100
cm
Source Size, SSD, and SDD
•
Source size, SSD, and SDD affect the
isodose
curves by
virtue of
the
geometric penumbra.
•
The SSD affects the PDD and the depth of the
isodose
curves.
•
The dose variation across the field border is a
complex function of
geometric
penumbra,
lateral scatter
, and
collimation
.
Collimation and Flattening Filter
Collimation
–
Blocks
The flattening filter
•
The cross

sectional variation of the filter thickness
causes variation in the photon spectrum or beam quality
across the field.
•
Other absorbers or scatter between the target and the
patient
Field Size
•
One of the most important parameters in treatment
planning
•
adequate dosimetric coverage of the tumor requires a
determination of appropriate field size
•
this determination must always be made
dosimetrically rather
than geometrically.
•
so a certain
isodose
curve (e.g.,
90
%
) enclosing the treatment
volume
should be the guide in choosing a field size
rather than
the geometric dimensions of the field.
•
Great caution should also be exercised in using field sizes smaller
than
6
cm in which
–
A Relative
large
part of the field is in the penumbra
region
–
The
isodose
curve for small field sizes tend to be Bell
shape
–
TPS should be mandatory for small field size.
Wedge Filters
•
Special filters or absorbing
blocks are placed in the path
of a beam to modify its
isodose
distribution.
•
The
most commonly used
beam

modifying device
made
of a dense material,
such as lead or steel
•
Mounted on a (transparent
plastic) tray
•
Arranged at a distance of at
least
15
cm from the skin
surface
Wedge Isodose Angle
•
Wedge angle:
The solid angle of the metallic wedge used,
15
o
,
30
o
,
45
o
,
60
o
.
•
Wedge
isodose
angle
–
the angle through which an
isodose
curve is
tilted 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
10
cm
wedge
angle
Wedge Transmission Factor
•
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
d
max
(
5
to
10
cm
)
Wedge Systems
•
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
60
Co
•
Universal wedge system
–
A single wedge for all beam widths
–
Fixed centrally in the beam
–
Used in
Linac
Wedges types
In older units a number of wedges, typically
15
°
,
30
°
,
45
°
and
60
°
, could physically be placed in the
treatment head by the radiographer.
The wedge, made from a dense material such as lead
or steel attached to a backing plate that would fit into a
wedge holder, was usually situated between the
ionization chamber and the mirror.
The use of
manual wedges
has largely been replaced,
first by
motorized wedges
, which consist of a
physical
wedge of a large wedge angle, usually around
60
°
.
The wedge is permanently situated in the treatment
head and can be moved into the beam automatically for
part of the treatment to give the desired wedge angle.
Another method of obtaining a modified beam profile is
through
dynamic
or
virtual wedges
. As the name
suggests
there is no physical wedge but the effect is
created by moving one of the collimators across the
beam at a pre

calculated speed during the treatment to
give the same effect on the beam profile as a wedge
Any of the four collimator jaws could create this effect,
but in practice it is limited to one set of jaws.
One
advantage
of this system of wedging the beam is
that
the average beam energy remains constant across
the full width of the beam.
With a
physical wedge
there will be
a degree of beam
hardening
that will depend on the thickness of filter
traversed.
Wedges are needed to improve dose uniformity
within the
planned target volume (PTV), compensate
for missing tissue and beams coming in with different
hinge angles.
The angle does not
relate to the edge itself but rather
the angle through which the
isodose
is turned, the
wedge angle
being defined as
“
the angle between the
central axis and a line tangent to the Isodose curve at
the depth of
10
cm”.
Effect of wedges
on Beam Quality
•
Attenuating the lower

energy photons (
beam
hardening
)
•
For
x

rays, there can be
some beam hardening
,
especially the PDD
at large depths
.
•
TARs
and TMRs may be assumed unchanged
for small depths (
less than
10
cm
)
Design of Wedge Filters
A
B
C
E
G
I
K
M
O
Q
S
T
U
Nonwedge
isodose
40
55
62
65
67
68
68
68
67
65
62
55
40
Wedge
isodose
35
39
41
47
53
60
68
76
86
95
105
110
115
Wedge/
nonwedge
0.875
0.7
10
0.66
0
0.72
0
0.79
0
0.88
0
1.00
1.12
1.28
1.46
1.70
1.20
2.88
Transmission
ratio
0.38
7
0.42
5
0.46
2
0.51
5
0.59
0.66
0.75
0.86
1.0
mm Pb
15.2
13.6
12.2
10.5
8.3
6.5
4.5
2.3
0
Single Field Technique
•
Criteria
–
The dose distribution within the tumor volume is
reasonably uniform (
5
%).
–
The maximum dose to the tissue in the beam is
not excessive (not more than
110
% of the
prescribed dose).
–
Normal critical structures in the beam do not
receive doses near or beyond tolerance.
Parallel Opposed Fields
•
The advantages
–
The simplicity and
reproducibility of setup
–
Homogeneous dose to the
tumor
–
Less chances of geometrical
miss
•
A disadvantage
–
The excessive dose to normal
tissues and critical organs
above and below the tumor
A
, Each beam weighted
100
at
D
max
.
B
, Each beam weighted
100
at the isocenter.
Patient Thickness
v.s
. Dose Uniformity
•
Tissue lateral effect
–
If the
patient
thickness
is ( large ) or
the beam
energy
is ( low )
the central axis maximum dose near the surface
is (high ) relative
to the midpoint dose.
Edge Effect
•
The edge effect or the tissue lateral damage
–
For parallel opposed beam
, treating with one field
per day
produces
greater biologic damage to
normal subcutaneous tissue
than
treating with
two fields per day.
–
The problem becomes more severe when larger
thickness (
20
cm
) are treated with one field per
day using a lower

energy beam (
6
MV
).
Integral Dose
•
A measure of the total energy absorbed in the
treated
volume
•
For a uniform dose, the integral dose is the product of
mass
and dose.
•
For a single beam,
Mayneord
formulation
unit : gram

rad
, kg

Gy
or J
(
1
Gy
=
1
J/kg)
•
Where
:
•
= Integral Dose
•
D
0
= is the peak dose along central line
•
A = geometrical
area
of the field.
•
d = total thickness of the patient
•
= depth of
50
% depth dose.
=correction for geometric divergence of the beam
2
/
1
d
)
88
.
2
1
(
2
/
1
SSD
d
)
88
.
2
1
)(
1
(
44
.
1
2
/
1
/
693
.
0
2
/
1
0
2
/
1
SSD
d
e
d
A
D
d
d
Multiple Fields (
1
)
To deliver maximum dose to the
tumor
and minimum dose to the
surrounding
tissues
1

Using
fields of appropriate
size.
2

Increasing
the number of
fields.
3

Selecting
appropriate beam
directions.
4

Adjusting
beam
weights.
5

Using
appropriate beam
energy.
6

Using
beam
modifiers if needed.
Multiple Fields (
2
)
Certain beam angles are
prohibited.
Why
?
–
The presence of critical organs in those
directions.
The
setup accuracy of a treatment may be better with
parallel opposed beam arrangement
–
The acceptability of a treatment plan depends not on the
dose distribution but also on
•
The practical
feasibility
ايلمع بسانم
•
Setup accuracy
•
Reproducibility of the treatment technique
Isocentric Techniques
•
The
isocenter
is the point of intersection of
the collimator axis
and
the gantry axis of
rotation
.
•
Isocentric technique
Placing
the isocenter at a depth with the
patient
and
directing the beams from different directions
SSD
=
SAD

d
Rotation Therapy (
1
)
•
The beam moves continuously about
the patient, or the patient is rotated
while the beam is held fixed.
•
The Rotation Therapy
is For
small and
deep

seated
tumors
Not for
too large tumors
Or if The
external surface
differs
markedly
from a
cylinder.
Or if The
tumor is too far off center.
Rotation Therapy (
2
)
T
D
D
ref
iso
TMR
S
S
D
D
p
c
iso
0
=
the reference dose rate related to the quantity
which
may be average TAR or TMR
T
The dose rate at the isocenter
Using TMR system
=
the
D
max
dose rate for a
10
10
field at the SAD
Ex:
MU
345
min
1.73
(MU/min)
200
set
be
to
MU
Total
min
73
.
1
rad/min
144.8
rad
250
time
Treatment
rad/min
8
.
144
746
.
0
99
.
0
98
.
0
200
iso
D
Wedge Field Techniques
The dose gradient in the overlap region is minimized.
The dose falls off rapidly beyond the region of
overlap or the “plateau” region.
The Wedge Angle
=
90
º

/
2
= the wedge
angle
= the hinge angle
S
= the
separation (the
distance between
the
thick ends of
the
wedge filters as
projected
on the
surface)
The wedge should be such that
the
Isodose
curves from
each field are parallel to the bisector of the hinge angle
.
When the
isodoses
are combined, the resultant
distribution is uniform
.
Uniformity of Dose Distribution
Because
wedge pair
techniques are normally
used for treating small, superficial tumor
volumes, a high

dose region of up to
+
10
%
within the treatment volume is usually
acceptable. These hot area occur under the
thin ends of the wedges and their magnitude
increases with
field size and wedge angle.
This
effect is related to the differential attenuation
of the beam under the thick end relative to
the thin end.
Open and Wedged Field Combinations
•
The principle of this technique is that as the dose
contribution from the anterior field decreases with
depth, the lateral beam provides a boost to offset this
decrease
Terminology (
1
)
•
Gross tumor volume (
GTV)
The
demonstrated tumor
•
Clinical target volume (
CTV)
The
demonstrated tumor
and volumes
with suspected(subclinical
)
tumor
•
Internal Target Volume (ITV)
added to CTV to compensate for internal physiological
movements and variation in size, shape, and position of the CTV
during therapy in relation to an internal reference point and its
corresponding coordinate system.
•
Planning
target volume (
PTV)
The
CTV and a margin to account for variations in size, shape, and
position relative to the treatment
beams
Graphical representation of the volumes of interest, as defined in ICRU
Reports No.
50
and
62
.
Contours of GTV, CTV, PTV and organs at risk (bladder and rectum) have
been drawn on this CT slice for a prostate treatment plan.
•
Planning Organ at Risk Volume
The organ(s) at risk (OR) needs adequate protection just
as CTV needs adequate treatment. Once the OR is
identified, margins need to be added to compensate for
its movements, internal as well as set

up. Thus, in
analogy to the PTV, one needs to outline planning organ
at risk volume (PRV) to protect OR effectively.
•
Treated volume
The volume that receives a dose that is considered
important for local cure or palliation.
•
Irradiated volume
The volume that receives a dose that is considered
important for normal tissue tolerance.
Maximum Target Dose
The highest dose in the target area is called the maximum target dose,
provided this dose covers a minimum area of
2
cm
2
.
Higher dose areas of
less than
2
cm
2
may be ignored in designating the value of maximum target
dose
.
Minimum Target Dose
The minimum target dose is the lowest absorbed dose in the target area
.
Mean Target Dose
If the dose is calculated at a large number of discrete points uniformly
distributed in the target area, the mean target dose is the mean of the
absorbed dose values at these points. Mathematically
:
where N is the number of points in the matrix and D
i,j
is the dose at lattice
point i,j located inside the target area
(
Terminology (
2
)
Median Target Dose
The median target dose is simply the value between the
maximum and the minimum absorbed dose values within the
target
.
Modal Target Dose
The modal target dose is the absorbed dose that occurs most
frequently within the target area. If the dose distribution over a
grid of points covering the target area is plotted as a frequency
histograph, the dose value showing the highest frequency is
called the modal
dose.
hot spot
A hot spot
is an area outside the target that receives a higher
dose than the specified target dose.
Like the maximum target
dose, a hot spot is considered clinically meaningful only if it
covers an area of at least
2
cm
.
2
Target volume

dose frequency curve.
Specification of Target Dose
The absorbed dose distribution in the target volume is usually not
uniform.
Although a complete dosimetric specification is not possible
without the entire dose distribution, there is value in having one
figure as the main statement of target dose.
The use of the term tumor dose is not recommended .
The quantity maximum target dose alone cannot be used for
reporting, since it can conceal
يفخأ
serious under dosages in some
parts of the target volume.
Although local tumor control depends on the minimum target
dose, this quantity alone is not recommended by the ICRU (
23
),
because it is difficult to determine the extent of the tumor, and
therefore,
The selection of the minimum target dose becomes difficult if not
arbitrary. Moreover, if most of the target volume receives a dose
that is appreciably different from the minimum, this may also
reduce its clinical significance. A statement of both the maximum
and minimum values is useful, but it is not always representative
لثمت
of the dose distribution.
Furthermore, this would do away with the simplicity of having
one quantity for reporting target dose.
The mean, median, and modal doses are not generally
recommended, because they usually require elaborate
calculations for their accurate determination and may not be
feasible by institutions having limited computation facilities.
The ICRU Reference Point
The target dose should be specified and recorded at what is called
the ICRU reference point. This point should satisfy the following
general criteria:
1.
The point should be selected so that the dose at this point is
clinically relevant and representative
لثمت
of the dose throughout
the PTV;
2.
The point should be easy to define in a clear way;
3.
The point should be selected where the dose can be accurately
calculated;
4.
The point should not lie in the penumbra region or where there is
a steep dose gradient.
In most cases the ICRU reference point should lie well within the
PTV, provided it generally meets the above mentioned criteria.
Stationary Photon Beams
For a single beam, the target absorbed dose should be
specified
on the central axis
of the beam placed
within the
PTV.
For parallel opposed
,
equally weighted
beams, the point
of target dose specification should
be on the central axis
midway between the beam entrances
.
For parallel opposed,
unequally weighted beams
, the
target dose should be specified
on the central axis placed
within the PTV.
For any other arrangement of two or more intersecting
beams, the point of target dose specification should be
at
the intersection of the central axes of the beams placed
within the PTV
.
Rotation Therapy
For
full rotation
or arcs of at least
270
degrees, the target
dose should be specified
at the center of rotation
in the
principal plane.
For
smaller arcs
, the target dose should be stated in the
principal plane, first,
at the center of rotation
and,
second,
at the center of the target volume
. This dual

point specification is required because in a small arc
therapy, past

pointing techniques are used that give
maximum absorbed dose close to the center of the target
area.
Single field
Dose distribution of a single posterior

oblique
photon beam, designed to cover the target
volume(white outline.
parallel opposed
,
equally weighted
parallel opposed
,
unequally weighted beams
For any other arrangement of two or more intersecting beams, the point of target dose
specification should be
at the intersection of the central axes of the beams placed
within the PTV
.
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