# Parameters affect Isodose Curves

Πολεοδομικά Έργα

15 Νοε 2013 (πριν από 5 χρόνια και 1 μήνα)

177 εμφανίσεις

Clinical Physics

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

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

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

of a dense material,

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

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

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 simplicity and
reproducibility of setup

Homogeneous dose to the
tumor

Less chances of geometrical
miss

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
-
, 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
-
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
=
-
d

Rotation Therapy (
1
)

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

Ex:

MU

345
min

1.73
(MU/min)

200
set

be
to
MU
Total
min
73
.
1
144.8
250
time
Treatment
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.

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

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
.