ESTRO 36 Abstract Book

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S399 ESTRO 36 2017
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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.
Material and Methods
Five different TL materials TLD100, TLD100H, TLD200,
TLD400 and TLD500, were investigated. Each type of TL
dosimeter was irradiated to the eight different qualities
of x-radiation. Mean of the response of the 5 dosimeters
for a certain x-radiation with effective energy Eeff was
taken as the energy dependent TL response of that type
of TL dosimeter. For each type of TL detector, energy
dependence curves were determined by fitting the
experimental results with a polynomial function. Tandem
curve pairs for six different combinations were generated;
;1.TLD100H, TLD200, TLD400, 2.TLD100H, TLD200,
TLD500, 3.TLD100H, TLD400, TLD500, 4.TLD100H,
TLD200, TLD400,5.TLD100, TLD200, TLD500 and
6.TLD100, TLD400, response ratios at different
energies was calculated and compared with two TL
material tandem systems.
Results
All Tandem curves exhibited maximum TL response ratio,
Emax, at approximately 45 keV, with reduction in TL
response ratios at energies above and below this energy
level. All tandem combinations, except the combinations
(1) and (4) showed that at energies in the 30 to 80 keV
range, where the TL response ratio of tandem pair (i) is
same, TL response ratio tandem pair (ii) differs by 20-30%,