ESTRO 36 Abstract Book

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S397 ESTRO 36 2017
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concentration (4 wt%) and RI concentrations ranging from
6-16%. These formulations were cast in
spectrophotometer cuvettes and stored at <3°C prior to
irradiation. A passively scattered 225 MeV proton beam
with a 10 cm SOBP was selected and each formulation of
dosimeters was irradiated in a solid water phantom at four
depths along the beam profile: one in the dose plateau
and three along the SOBP. The photo-absorption spectra
were measured for each formulation. The optical
attenuation coefficients of the PRESAGE® samples were
compared to ion chamber measurements to determine the
quenching magnitudes.
Results
The photo-absorption spectra demonstrated consistent
absorption peaks, and all formulations responded linearly
with dose. The dose sensitivity of the dosimeters changed
by as much as 42% across all formulations. All formulations
with RI concentrations between 10-21% showed quenching
less than 3% at the proximal SOBP dose point but increased
quenching at other measurement points along the
SOBP. Formulations outside this RI concentration range
had greater quenching across all measurements. The
distal-most points of all formulations showed the greatest
quenching. When comparing these points, high LMG
formulations had lower quenching than those with low
LMG while RI concentrations were 12% or lower, but
quenching was greater when RI concentration was above
this range. The least quenching in the low LMG
formulations was 14.6% which occurred at 12% RI, while in
the high LMG formulations this occurred at 10% RI with a
maximum under-response of 8.4%. The highest quenching
observed was 73.8% in the low LMG, 30% RI formulation.
Conclusion
Previous studies have the only investigated the effects of
changing RI concentrations on the quenching magnitude of
PRESAGE® in a proton beam, but this study has
demonstrated that the quenching process is additionally
limited by LMG concentration. While a quenching
reduction limit for low LMG formulations was before it
could be fully eliminated, further reduction of quenching
by increasing LMG demonstrates that additional study into
PRESAGE® optimization of both of these components may
continue to improve accuracy in proton dosimetry.
PO-0761 Dosimetry with Farmer ionization chambers in
magnetic fields: Influence of the sensitive volume
C.K. Spindeldreier1,2, I. Kawrakow3, O. Schrenk1,2, S.
Greilich 1,2, C.P. Karger1,2, A. Pfaffenberger1,2
1German Cancer Research Center, Medical Physics in
Radiation Oncology, Heidelberg, Germany
2National Center for Radiation Research in Oncology,
Heidelberg Institute for Radiation Oncology, Heidelberg,
Germany
3ViewRay, Inc, Oakwood Village, USA
Purpose or Objective
Ionization chambers exhibit an altered dose response in a
magnetic field of an MR-linac due to the deflection of
secondary electrons by the Lorentz force. The actual dose
response depends on the magnetic field strength as well
as on the orientation between chamber axis, beam and
magnetic field [Meijsing PMB 54 2009, Reynolds Med Phys
40 2013, Spindeldreier DGMP 47 2016]. The purpose of this
study is to investigate the influence of dead volumes,
known to exist at the chamber base, on the response of a
thimble ionization chamber in the presence of a magnetic
field.
Material and Methods
The response of a Farmer chamber (PTW 30013) subject to
a 6 MV beam was measured in a small water tank
[Bakenecker Uni Heidelberg 2015] embedded in an
experimental magnet for magnetic field strengths
between 0.0 and ±1.1 T in the two magnetic field
orientations perpendicular to the beam and to the
chamber axis. The experimental setup was simulated using
the EGSnrc [Kawrakow Med Phys 27 2000, NRC PIRS 898
2009] user code egs_chamber [Wulff Med Phys 35 2008]. In
addition to computing the total dose deposited in the
chamber cavity for different sensitive volumes, a high
resolution dose map inside the cavity was obtained.
Results
A maximum of 8.1% and 7.0% increase in chamber response
was measured for the two orientations at a field strength
of ±0.9 T. In contrast, the calculated response was only
marginally different, when the entire air volume was
considered as a sensitive volume in the simulations. It was
possible to reproduce the experimentally observed
differences in dose response using a small dead volume
close to the chamber stem. The simulated dose
distribution within the chamber cavity was found to be
highly non-uniform with hot and cold spots at the chamber
stem and chamber tip, depending on the field orientation
(see Fig. 1).
Conclusion
In the presence of a magnetic field perpendicular to the
axis of thimble ionization chambers, the amount of
electrons entering the cavity from the tip and stem is
increased or decreased, depending on the field
orientation. The chamber response is therefore influenced
in a significant way by the presence of a dead region
known to exist at the chamber base near the stem.
Measurements with the chamber axis parallel to the
magnetic field are thus advantageous, as in this case the
dead volume has less impact due to the Lorentz force
acting radially. An optimized chamber design that