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dose and dose-rate gradients, high dose rates and
softening of photon energy spectrum with depth. A single
crystal synthetic diamond detector PTW 60019 (marketed
as microDiamond) (PTW, Freiburg, Germany) has a small
active volume and was designed for such measurements in
high energy photon, electron and proton beams. It can be
read out directly with standard electrometers used at
radiotherapy departments, unlike thermoluminescent
detectors, which are currently the most used dosimeters
in BT but have to be pre- and post-processed with
dedicated equipment. Hence the purpose of this study was
to evaluate the suitability of a microDiamond for the
determination of absorbed dose to water in an HDR
192
Ir
beam quality. The use of three microDiamond samples also
allowed for assessment of their individual reproducibility.
Material and Methods
In-phantom measurements were performed using the
microSelectron HDR
192
Ir BT treatment unit. Oncentra
treatment planning system (TPS) was used to create
irradiation plans for a cubical PMMA phantom with a
microDiamond positioned at one of the three source-to-
detector distances (SDDs) (1.5, 2.5 and 5.5 cm). The
source was stepped by 0.5 cm over the total length of 6
cm to yield absorbed dose of 2 Gy at the reference point
of the detector. A phantom correction factor was applied
to account for the difference between the experimental
phantom and the spherical water phantom used for
absorbed dose calculations made with the TPS. The same
measurements were repeated for all three detectors
(mD1, mD2, mD3).
Results
Experimentally determined absorbed dose to water
deviated from that calculated with the TPS from -1 to +2
% and agreed to within experimental uncertainties for all
the detectors and SDDs (Figure 1). The mD2 overestimated
absorbed dose to water by up to 2% compared with the
estimates by the other two detectors. A decrease in the
difference with increasing SDD suggests that it might be
related to differences in the position of the active volume
inside the detector which is of higher importance closer to
the source where dose gradients are steeper. The
combined relative uncertainty in experimentally
determined absorbed dose to water did not exceed 2% ()
for all the detectors and SDDs. A variation in raw readings
was within 2% over the investigated range.
Conclusion
Preliminary results indicate that the dosimetric properties
of a microDiamond obviate the need for multiple
correction factors and facilitate dosimetry of HDR
192
Ir BT
sources. This, together with the convenience of use, shows
high potential of a microDiamond for quality assurance of
HDR BT treatment units at clinical sites. It must be noted,
nevertheless, that individual characterization of a
microDiamond is required to achieve high accuracy.
PO-0770 The distortions of the dose response
functions of dosimeters in the presence of a magnetic
field
H.K. Looe
1
, B. Delfs
1
, D. Harder
2
, B. Poppe
1
1
Carl von Ossietky University, University Clinic for
Medical Radiation Physics, Oldenburg, Germany
2
Georg August University, Prof em.- Medical Physics and
Biophysics, Göttingen, Germany
Purpose or Objective
The new developments of MRgRT have opened new
possibilities for high precision image-guided radiotherapy.
However, the secondary electrons liberated within the
medium by the primary photon beam are subjected to the
Lorentz force. Therefore, the trajectories of the
secondary electrons in non-water media, such as an air-
filled cavity or a high-density semiconductor, will differ
significantly from that in water. In this work we
demonstrate, using simple geometries, that the shape of
the lateral dose response functions of clinical detectors
will depend on the electron density of the detector
material, the beam quality and the magnetic field. The
dosimetric implications are discussed and correction
strategies are proposed.
Material and Methods
Based on the convolution model (Looe et al 2015), the one-
dimensional lateral dose response function, K(x-ξ), acting
as the convolution kernel transforming the true dose
profile D(ξ) into the measured signal profile M(x), was
derived by Monte-Carlo simulation for a simple cylindrical
detector placed at 5 cm depth in water using
60
Co and 6
MV slit beams. The cylinder with 1.13 mm radius and 2 mm
height was filled with water of normal density (1 g/cm
3
),
low density (0.0012 g/cm
3
) and enhanced density (3
g/cm
3
), where the latter two represent the density of an
air-filled ionization chamber and a semiconductor
detector respectively. Simulations were performed using
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