ESTRO 36 Abstract Book

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S403 ESTRO 36
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Conclusion
Rare-earth doped scintillating silica, thanks to their high
light yield and favorable spectral properties, offer a true
alternative to perform optical fiber dosimetry, in different
clinical and preclinical conditions, eliminating in a reliable
and robust way the influence of the stem effect, without
the need of complex and time-consuming calibrations.
PO-0763 Characterizing the response of Gafchromic
EBT3 film in a 1.5 T magnetic field
Y. Roed1,2, H. Lee2, L. Pinsky 1, G. Ibbott2
1 University of Houston, Physics, Houston, USA
2 The University of Texas MD Anderson Cancer Center,
Radiation Physics, Houston, USA
Purpose or Objective
To assess the influence of a magnetic field (B-field) on the
response of radiochromic film. Irradiation at different
orientations of the film with respect to the B-field was
assessed as well as different durations of exposure of the
films to the B-field.
Material and Methods
EBT3 films were placed at 5 cm depth in an acrylic
phantom and irradiated to 2, 4, and 8 Gy using a cobalt
source while exposed to the B-field from an
electromagnet. The film surfaces were perpendicular to
the incident beam while a reference film edge was
oriented either parallel (RE0) or perpendicular (RE90) to
the B-field. Two B-field strengths of B = 0 T and B = 1.5 T
were used. All films were exposed to the B-field for 7 min.
A subsequent set of films was irradiated to 4 Gy using the
same setup and B = 1.5 T field strength. The films were
exposed to the B-field for 6, 10, or 30 minutes.
All films were scanned with an Epson 10000 XL flatbed
scanner prior to and 24 hours after irradiation, first with
the reference edge parallel (PA) to the scan direction and
then perpendicular (PA90).
Red channel pre- and post-scans were analyzed with
ImageJ software. Percent differences (%diff) with respect
to B = 0 T were calculated for PA and PA90 films. %diff
between PA and PA90 were also determined.
Results
All films exhibited an under-response at each dose level
when compared to irradiation at B = 0 T. Less than -2.0 %
difference was determined in the PA scan direction at all
dose levels for both orientations in the B-field (solid and
dashed black in Figure 1). The under-response increased
with increasing dose for RE90 films scanned in PA90
direction (dashed grey in Figure 1). For RE0 the maximum
%diff was -1.1 % (solid grey in Figure 1).
The %diff in scan direction increased with increasing dose
from 11.0 % to 12.4 % in RE0 orientation and from 9.6 % to
11.1 % in RE90 orientation.
Table 1 shows that increasing the time the films remained
in the B-field resulted in less than 1.0 % over-response for
6 and 30 min and -0.9 % under-response for 7 and 10 min.
The %diff in scan direction was about 12 % for all times.
Conclusion
Radiochromic films can measure doses delivered by a
magnetic resonance-image guided radiotherapy treatment
unit and can be considered for quality assurance of MR-
guided treatment units. The duration of exposure to the
B-field did not affect the response of the film and neither
did the orientation of the reference edge as all
determined %diff were less than the uncertainty of film
measurements. However, the orientation of the reference
edge with respect to scan orientation did have a
significant effect on the response of the film. Maintaining
consistent orientation of films both during the irradiation
in a B-field and also on the flatbed scanner still remains
essential to acquire results with the lowest %diff.
PO-0764 A study of Tandem systems incorporating
three thermoluminescent dosimetry materials.
V. Nelson1
1Macarthur Cancer Therapy Centre, Medical Physics,
Campbelltown, Australia
Purpose or Objective
Tandem systems, incorporating a pair of TL materials,
have been shown to be very useful for the determination
of effective energy in radiation beams with unknown
radiation energy spectrum. Tandem curves in all these
tandem systems exhibits maximum TL response ratio
between 40-50 keV and similar TL response ratios on
either side of this energy (Emax), making it difficult to
determine if the effective energy is less than or greater
than the Emax (Fig.1(a)). However, if a third TL material,
with energy dependent TL response different to the other
two materials, is included in the tandem, two tandem
curves can be obtained. If these two tandem curves
significantly differ from each other, the effective energy
of the radiation beam can be identified as either less than
or greater than Emax, Fig. 1(b), hence improving the
dosimetry in this energy range. The aim of this work was
to test a number of different TL materials and find a TL
material which fulfils this criteria.