Parameters affect Isodose Curves

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Nov 15, 2013 (3 years and 11 months ago)

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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
.