Improving QA: Increasing resolution of detector arrays in dosimetry systems using a simple add-on
Michiel van den Bosch,
The Netherlands
OC-0121
Abstract
Improving QA: Increasing resolution of detector arrays in dosimetry systems using a simple add-on
Authors: Michiel van den Bosch1, Sander van het Schip1, Hans van Velzen1, Willy de Kruijf1
1Institute Verbeeten, Department of Medical Physics & Instrumentation, Tilburg, The Netherlands
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Purpose or Objective
In
radiotherapy, dosimetry systems with diodes or ionization chambers are commonly
used to verify whether the linear accelerator can deliver the treatment plan
correctly. By adding more detectors, the measurement resolution can be
increased.
Another way
to increase the measurement resolution is to shift the dosimetry system
slightly after the first measurement, repeat the measurement and combine both
results. For example, this method is used for the Delta4 (Scandidos;
cylindrical solid PPMA phantom with two orthogonal planes with diodes) in the
craniocaudal direction. Due to its cylindrical shape the outer contour does not
change when shifting the dosimetry system in that direction. Unfortunately, a shift in
the other directions will result in a change of the outer contour. This results
in a change of the attenuation of the incident beam and consequently a
dosimetric error.
In this
study, we introduce shells that prevent geometry changes when shifting the dosimetry
system between measurements (patent pending). By combining the measurement values the resolution
can be improved.
Material and Methods
Two
shells were printed with a 3D printer. They were made of Acrylonitril Butadieen
Styreen (1.01g/cc) and printed in 3 parts to ensure an easy (un)mount of the
shell onto the Delta4.
In the
first measurement a shell is used to increase the body diameter with 10mm. The
shell is also simulated in the treatment planning software and incorporated in
the dose calculation. In the second measurement the first shell is removed and the
second shell is added. The outer diameter of this shell is identical to the
first shell, yet the position of the inner surface is shifted 2.5mm (see Figure
A). The wall is 7.5mm thick at one side and 12.5mm in the opposite direction.
The couch (PerfectPitch, Varian) of the linear accelerator (TrueBeam, Varian)
is shifted 2.5mm to compensate for the asymmetry of this shell. At start of the
second measurement the outer geometry is identical to the first measurement,
but the positions of the Delta4 diodes are shifted with 2.5mm. The results of
the first and second measurement are combined afterwards to increase the
resolution with a factor 2.
Results
It was
possible to add and remove the shells without changing the orientation and
position of the Delta4 dosimetry system. Figure B shows the measurement profile
of a patient VMAT plan. The points of the first measurements are shown in blue,
the second in orange. In the central region (width 6cm) the extra measurements
points are situated centrally, since the distance between the diodes is 5mm in
this central region of the Delta4. In the outer region, the Delta4 resolution
is 10mm.
Conclusion
It is
possible to increase the measurement resolution using 3D printed shells. This
solution is not limited to the Delta4, but can also be applied to other dosimetry
systems. Furthermore, different designs (e.g. wall thickness) can be used to shift
diodes differently with respect to the outer contour.