Zeiss INTRABEAM, Instructions For Use Manual

Page: 42 / 82

Operation INTRABEAM Water Phantom
Version 6.2
Page 42 G-30-1767-en
Distance of the probe tip from the IC ionization chamber
r
1
[mm] is the minimum distance of the probe tip from the IC ionization
chamber at which measurement is possible.
Z
1
[mm] is the Z position value you noted down which is displayed on the po-
sitioning unit when the probe tip is in the optimum Z position.
Z
1
[mm] is not the zero distance from the reference point of the ionization
chamber.
Z
1
[mm] is located at a distance of r
1
[mm] from the reference point of the ion-
ization chamber.
r
0
is the reference point of the ionization chamber.
In the depth dose curve, r
0
= 0 mm.
d
IC
is the distance of the reference point of the ionization chamber from the
upper surface of the ionization chamber housing. You can take this value from
the PTW calibration certificate where it is designated as "Distance between
entrance foil and chamber top".
d
H
is the thickness of the measuring chamber wall. The d
H
value is imprinted
on the measuring chamber.
d
A
is the air gap between the upper surface of the ionization chamber housing
and the inside of the measuring chamber wall. d
A
is constantly 0.5 [mm].
r
1
can be calculated using the following fomula:
r
1
= d
H
+ d
A
+ d
IC
For recording the depth dose curve, you have to move the probe tip upward
in steps, away from the optimum Z position.
You can define the step width to be used.
When you have changed the Z position by the selected step width on the po-
sitioning unit, note down the new Z value indicated by the counter and use it
to calculate the new distance according to the following formula:
r
i
= r
1
+ (Z
1
- Z
i
)