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the EGSnrc package, and 0.5, 1.0 and 1.5 T magnetic fields
were applied.
Results
Fig. 1 shows the derived kernels K(x-ξ) without and with
magnetic field for the three detector densities and two
beam qualities. The shape of K(x-ξ) without magnetic field
has been discussed in Looe et al 2015 in terms of the
electron density of the detector material. The effect of
the magnetic field on the secondary electrons’
trajectories in a non-water equivalent medium is
manifested as a distortion of K(x-ξ). It is worth mentioning
that function K(x-ξ) for water with normal density (middle
panels) does not vary in the presence of a magnetic field,
and the shape of this function merely represents the
geometrical volume-averaging effect.
Fig. 1. Area normalized K(x-ξ) for the cylindrical detector
voxels of 'low”, 'normal”, and 'enhanced” density without
and with, 0.5, 1.0 and 1.5 T magnetic field.
Conclusion
It has been shown for the first time that the lateral dose
response functions K(x-ξ) of non-water equivalent
detectors will be distorted by a magnetic field, showing
asymmetrical detector response, even if the detector’s
construction is symmetrical. The distortions are attributed
to the differences in charged particle trajectories within
the detectors having electron density other than of normal
water. The effect of a magnetic field on a detector’s
response can be characterized by the area-normalized
convolution kernel K(x-ξ, y-η). As previously proposed
(Looe et al 2015), corrections based on the convolution
model can be applied to account for the detector’s volume
effect in the presence of magnetic field:
PO-0771 The dose response functions of an air-filled
ionization chamber in the presence of a magnetic field
B. Delfs
1
, D. Harder
2
, B. Poppe
1
, H.K. Looe
1
1
University Clinic for Medical Radiation Physics, Medical
Campus Pius Hospital Carl von Ossietzky University,
Oldenburg, Germany
2
Prof em. Medical Physics and Biophysics, Georg August
University, Göttingen, Germany
Purpose or Objective
The development of therapeutic devices combining
clinical linear accelerators and MRI scanners for MR guided
radiotherapy leads to new challenges in the clinical
dosimetry since the trajectories of the secondary
electrons are influenced by the Lorentz force. In this
study, the lateral dose response functions of a clinical air-
filled ionization chamber in the presence of a magnetic
field were examined depending on beam quality and
magnetic field following the approach of a convolution
model (Looe
et al
2015, Harder
et al
2014).
Material and Methods
In the convolution model, the 1D lateral dose response
function
K
(
x-ξ
) is defined as the convolution kernel
transforming the true dose profile
D
(
ξ
) into the disturbed
signal profile
M
(
x
) measured with a detector. For an air-
filled ionization chamber, type T31021 (PTW Freiburg,
Germany), the lateral dose response functions were
determined by Monte-Carlo simulation using 0.25 mm wide
60
Co and 6 MV slit beams. The chamber was modelled
according to manufacturer’s detailed specification and
placed at 5 cm water depth in three different
orientations, i.e. axial, lateral and longitudinal. For each
chamber orientation, a magnetic field oriented
perpendicular to the beam axis was applied. Simulations
were performed for magnetic fields of 0, 0.5, 1 and 1.5 T
using the EGSnrc package and the
egs_chamber
code.
To verify the simulation results, the lateral dose response
functions without magnetic field were compared against
measurements with a 5 mm wide collimated 6 MV photon
slit beam using tertiary lead blocks following the approach
of Poppinga
et al
2015.
Results
Fig. 1 shows good agreement between the simulated and
measured dose response functions
K
(
x-ξ
) of the
investigated ionization chamber in the three investigated
orientations. The structures of the measured functions are
not as evident as those of the simulated functions possibly
due different scanning step widths used in the experiment
and the calculation.
Fig. 2 shows the lateral dose response function
K
(
x-ξ
) with
and without magnetic field obtained exemplary for the
detector in lateral orientation. The distortion of the dose
response function
K
(
x-ξ
) corresponds to the change in the
trajectory lengths of the secondary electrons in the air of
the ionization chamber due to the Lorentz force, as
compared to the trajectories in a small sample of water.
Fig. 1. Area-normalized simulated and measured dose
response functions
K
(
x
-
ξ
)
Fig. 2. Area-normalized dose response functions
K
(
x
-
ξ
) for
the T31021 in lateral orientation for magnetic fields of 0,
0.5,1 and 1.5 T
Conclusion
The distortions of the lateral dose response function
K
(
x-
ξ
) will alter the measured signal profile
M
(
x
) of a detector
in magnetic field, as demonstrated in this study. The
variety of the possible combinations of detector
orientation and magnetic field direction and the strong
dependence of the distortion on the magnetic field
strength require careful consideration whenever a non-
water equivalent detector is used in magnetic field.