BEAM
DATA ACQUISITION
Generally
for Beam Data Acquisition a water phanthom is used:
- Ionization
chamber is placed on central axis
- Central
axis depth dose data is taken and then plotted
- This
data is then converted to PDD and then the maximum dose is found
and all the other doses are compared to it (this can also be called
normalization to Dmax set equal to 100%)
 - next
beam profile data is aquired and then plotted; eventually we will
have a reading for every point in the phanthom on
the plane we are looking at (all these reading are
called the DOSE MATRIX)
- now
lines are drawn to connect point with the same value within the
Dose Matrix. These lines are called isodose curves.
Information
that can be aquired from an Isodose Curve:
- fields
size
- SSD
- Depth
of Dmax
- Beam
flatness
- Penetration
- Penumbra
Normalization
– is the comparison of the dose distribution to
a particular point inside the patient (generally the point of intrest
is the center of the tumor or at the isocenter.
- We
can normalize to any value we want; generally the number choosen
is 100%
Example
of normalization of Dose Matrix:
Original
Dose Matrix:
Normalizing
Value to Point choosen (for this example this will be 56):
Dose
Matrix Normalized to 56:
This
can also be done for the Isodose Curves:
Original
Isodose Curves:
New
Isodose Curves Normalized to 56:
WEDGES:
Wedges
:
is an absorber that is placed in the path of the beam that is thick
at one end and thin at the other. (Usually made of steel)
The
general purpose of a wedge is to bend the radiation field to match
the patient contour and/or tumor so as to achieve a uniform dose
distribution.
- There
are various wedges that are employed
- The
distance that a wedge is placed is always at least 15 cm from
the skin surface, so as to avoid electron contamination and thus
retaining skin sparing.
This
is the dose matrix with a wedge in place:
- This
data has been normalized to depth dmax on the central axis to
100%. (this is one of the most common places to normalize wedge
data, or any other data for that case)
- One
other place that is also chosen for normalization is the maximum
dose in the field of radiation (in the case shown below that would
be 132%, we would normalize that point to 100%)
How
we name Wedges:
The
name of a wedge is determined by:
- The
angle that the 50% isodse makes with a line perpendicular with
the central axis (in low energy machines)
- The
angle that the isodose curve make at a depth of 10 cm with a line
perpendicular with the central axis (in high energy machines)
(the 50% isodose curve in a high energy machine is too deep for
most treatments)
Naming
Wedge for Low Energy Machines:
Naming
Wedge for High Energy Machines:
|
