Zeiss INTRABEAM, Instructions For Use Manual

Page: 52 / 82

Operation INTRABEAM Water Phantom
Version 6.2
Page 52 G-30-1767-en
The correlation between the two measurement methods is described by the
following formula:
Where f'(r) represents the conversion function. This formula has a standard
deviation of max. 5.1%. The values for the conversion function itself can be
obtained from the calibration documents of the XRS 4.
In summary, the conversion function comprises 3 different parts:
1 Use of two different ionization chamber models (PTW TN34013 vs.
TN23342) in the water phantom. The diameter of the chamber entrance
film of the active volume of the TN23342 is approx. 5.2 mm in contrast to
only 2.9 mm for the TN34013. This leads to different angular resolutions
and to different "true" distances r (typical geometric and absorption prob-
lem for point sources). Therefore, the smaller distance r is, the greater the
deviations between the two depth dose curves will be.
2 Use of two different designs for the ionization chamber holders. This leads
to different measured dose rates, which becomes especially noticeable for
small distances r.
3 Use of two different algorithms to calculate the water energy dose rate
(standard ion dose rate with a roentgen to gray conversion factor of 8.81
mGy/R in contrast to air kerma with the conversion factor
from
PTW), which leads to a constant offset in the entire measuring range.
Ḋ
w
(r){TARGIT} = f'(r) . Ḋ
w
(r){Calibration v4.0}
K
Ka D
W