ESTRO 36 Abstract Book

416 / 1096

S401 ESTRO 36
_______________________________________________________________________________________________
For depth dose measurement, both substrates show a
great agreement within ±2% when compared to the IC
response.
Conclusion
The difference detector Si substrates show the difference
in degradation of the detector sensitivity. The M512-Epi
demonstrate 3.5 times better radiation hardness in
comparison with M512-Bulk while show more the dose per
pulse dependence. However, for typical treatment when
SSD <150 cm for all beam angles the sensitivity of the
detector decreases within 2% for both substrates making
M512 -Epi more preferable choice as QA detector for
dosimetry in SRS and SBRT.
PO-0760 Investigation of PRESAGE formulation on signal
quenching in a proton beam
M. Carroll1,2, M. Alqathami2, G. Ibbott2
1 University of Texas at Houston, Graduate School of
Biomedical Sciences, Houston, USA
2 The University of Texas MD Anderson Cancer Center,
Radiation Physics, Houston, USA
Purpose or Objective
PRESAGE®, a radiochromic polyurethane dosimeter, has
shown potential as a 3D dosimetry system for conventional
radiotherapy systems. When irradiated by protons,
however, signal quenching is observed in high-LET
regions. This quenching may result from either (or both)
the local saturation of the Leucomalachite green (LMG) or
recombination of the radical initiator (RI) along proton
tracks. This work studied the magnitude of these
quenching mechanisms and the effects of changes to
formulaic concentrations of these components to further
minimize or eliminate the quenching effect.
Material and Methods
Ten formulations of PRESAGE® were manufactured under
standardized conditions but with RI concentrations ranging
from 3-30 (wt%) and low LMG concentration (2 wt%). Six
more formulations were then manufactured with high LMG
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.
Greilich1,2, 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.