PROCEEDINGS OF THE 7 INTERNATIONAL ... - Fizika

Measurement
points
S. Plaude et al. / Medical Physics in the Baltic States 7 (2009) 52 - 55
Fig.1. Test field configuration.
2.2. Test field configuration
Intensity modulated test field is made so that 10 cm
width symmetrical to the CAX the dose is high and
uniform, but 5-7 cm from the CAX there is low dose
region (Fig.1.).
A desired fluency was obtained using dynamic MLC. In
the middle part of the test field a speed of leaves is 0,4
cm/s, but at the edge of the field MLC leaves move at
2,0 cm/s. A gap between opposite leaves is 3 mm.
Measured points were chosen so that they cover the
high dose region as well as low dose region. Dose was
measured in 10 points with 1 mm interval between
them. There is a high dose gradient between two dose
levels.
2.3. Ionization chambers
Three radiation detectors were used. PTW 30006
“Farmer” type chamber with its active volume 0.6 cm 3 ;
PTW 31002 “Semiflex” type IC 0.125 cm 3 ;
Scanditronix Wellöfer semiconductor p-type diode
model PFD 3G (Fig.2.). Diameter of active area of the
detectors was 6.1 mm, 5.5 mm and 2.0 mm respectively
[7].
Detectors were positioned with their longitudinal axis
perpendicular to the beam axis. Measurements were
made in a depth of 5 cm with SSD (Source – Skin
Distance) =95 cm and SAD (Source – Axis Distance)
=100cm.
Dose profiles for dynamic field were acquired using
film dosimetry. Films were irradiated in water
equivalent phantom at a depth of 5 cm at the isocenter
plane. Kodak ® X-Omat films were used for the
measurements.
2.4. Dose modeling
CAX
A simple model using Microsoft Excel software was
developed to calculate the integrated dose in arbitrary
units inside volume of IC. Employed model was using
chamber volume as only parameter to model chamber
response to the photon fluency delivered by means of
moving leaf gap. This model allows to estimate the
response of the IC depending of the velocity of MLC
leaves and the gap between opposite leaves. It was also
possible to take into account a diameter of IC cross-
53
section, which is the main parameter of our interest.
a)
b)
c)
Fig.2. Radiation detectors: a) PTW 30006 “Farmer”; b) PTW
31002 “Semiflex”; Scanditronix Wellöfer semiconductor ptype
diode
3. Results and Discussion
Figure 3 shows calculated profiles in high gradient
region of test field with dynamic IM for Farmer and
Semiflex ICs. Figure 4 shows measured profiles in the
same high dose gradient region of test field. Dose
profile acquired by semiconductor detector and film are
shown as reference to illustrate actual dose gradient in
region of interest.
Figure 5 shows ratio of Farmer and Semiflex chamber
responses in region of interest – modelled using only
volume effect and derived from measured data.
Additional points show difference of the variation of
modelled and measured responses of the chambers. One
can see that simple use of volume effect of the chamber
gives discrepancy up to 19% with measurement results.
Results of measurements and modelling have shown
that volume effect of the chamber is important factor to
take into account during measurements in dynamic
fields. However, it is not possible to introduce only
volume based correction factor to arrive to agreement
with measured data. Variations of central electrode
effect, recombination, and polarisation in dynamically
modulated photon fields are important factors to
consider.
Additional modelling and measurements are needed to
take into account all important factors affecting
ionization chamber response in radiotherapy fields with
dynamically modulated intensity.
4. Conclusions
Using simple volume based modeling of chamber
response in dynamically IM radiotherapy fields is not
possible to predict with sufficient accuracy response of
the large volume ionization chamber in high dose
gradient regions of the field.
Results of such a model have differences up to 20%
with experimental results. It is not possible to evaluate
chamber response in the high dose gradient of the IM
field using chamber volume DVH based correction
factor. Additional modeling is needed to evaluate
chamber response in dynamically IM fields.