ESTRO 36 Abstract Book

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S405 ESTRO 36 2017
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Regarding MLC errors affecting the field size, large
deviations from reference were observed in the IQM
signal, while unidirectional shifts introduced deviations
below detection limit. A similar behavior was observed for
2D γ and DVH parameters. Figure 1 illustrates the
correlation of Dmean(PTV) and IQM signal deviation,
indicating that clinically relevant errors can be identified.
Conclusion
The deviation between calculated and measured signal is
relatively high, therefore a measurement should be
defined as reference. With this limitation, the system is
not yet capable of treatment plan verification but is a
powerful tool for constancy testing.
The detector provides excellent signal stability and is very
sensitive regarding error detection. The signal deviation
correlates with 2D γ and DVH metric deviations; this
information can be used for identifying action limits for
the IQM.
PO-0773 Three-dimensional radiation dosimetry based
on optically-stimulated luminescence
M. Sadel1, E.M. Høye2, P. Skyt2, L.P. Muren2, J.B.B.
Petersen2, P. Balling 1
1 Aarhus University, Department of Physics and
Astronomy, Aarhus, Denmark
2 Aarhus University Hospital, Department of Medical
Physic, Aarhus, Denmark
Purpose or Objective
Modern radiotherapy employs complex 3D radiation fields
to deliver therapeutic doses during treatment, and
detailed quality assurance is a prerequisite. Methods
based on luminescent passive detectors, such as optically
stimulated luminescence (OSL), are widely applied,
especially for personal dosimetry and phantom
measurements. Reusability is one advantage of using OSL
for dosimetry; the OSL particles can be reset by
temperature or light-bleaching. Furthermore, the OSL
material used in this study has a wide dynamic range and
linear dose response, and the dosimeter matrix consists of
a flexible material that can be cast into anthropomorphic
shapes and simulate organ deformations during RT. In this
abstract we propose a new, reusable 3D dosimetry system
based on OSL material embedded homogenously inside a
transparent matrix.
Material and Methods
Cuvette-sized prototypes of the dosimeter were produced,
consisting of a matrix; 4 g of a transparent silicone
elastomer (SE) (Sylgard 184, Dow Corning), and a
homogeneously embedded OSL material; 0.3 g of lithium
fluoride (LiF) doped with magnesium, copper and
phosphorus (LiF:Mg,Cu,P - MCP).
Three samples were prepared in standard OSL-reader
aluminum trays; a reference sample with silicone
elastomer, and two samples with OSL powder embedded
in the SE matrix, containing 0.06 mg and 0.2 mg OSL
powder (sample 1 and 2 respectively). They were read-out
using a Risø TL/OSL DA-20 reader. Samples were irradiated
with 1 Gy beta radiation and stimulated for 100 s with blue
light emitting diodes (LEDs), with emission centered at 470
nm and an intensity of ~80 mW/cm2.
Results
The transparency of the dosimeter (see Fig. 1) depended
on the concentration of MCP powder, which must be
optimized as a compromise between signal level per
volume and overall transparency. The refractive-index
match between LiF and the SE is quite good for visible
wavelengths, which minimizes light scattering from the
particles.
Approximately 10,000 and 40,000 counts were detected in
1 second per 1mm3 voxel from samples 1 and 2,
respectively, corresponding to the anticipated signal
levels. Also, the silicone matrix in itself did not add to the
OSL signal (see Fig. 2). 3D distributions can be obtained
without the need for inversion algorithms, for example, by
stimulating the OSL dosimeter with a light sheet (from a
laser source), and imaging the luminescence intensity
across that sheet (by a combination of optical filters and
a camera), and shifting this plane across the dosimeter.
Conclusion
A new 3D dosimeter system based on OSL material has
been presented. It has the potential to verify complex 3D
RT doses with high spatial resolution, while maintaining
the advantages known from personal-dosimetry use of
OSL.
PO-0774 Investigation of dose-rate dependence at an
extensive range for PRESAGE radiochromic dosimeter
E.P. Pappas1, E. Zoros1, K. Zourari 2, C.I. Hourdakis2, P.
Papagiannis1, P. Karaiskos 1, E. Pantelis1
1National and Kapodistrian University of Athens, Medical
Physics Laboratory - Medical School, Athens, Greece
2Greek Atomic Energy Commission, Division of Licensing
and Inspections, Athens, Greece
Purpose or Objective
The purpose is to investigate dose-rate dependence
effects for a recent formulation of the commercially
available PRESAGE radiochromic dosimeter (Heuris Inc,
NJ) in a wide range of dose delivery rates extending to
three orders of magnitude (0.018 – 19 Gy/min).
Material and Methods
In order to achieve an extensive dose rate range, this work
was divided into two separate studies. Lower dose rates
were delivered by 60Co beams while higher dose rates were
achieved by a flattening-filter-free (FFF) linear
accelerator. For the low dose rate part of this study, 10
PMMA cuvettes (1×1×4 cm3), filled with PRESAGE samples,
were irradiated to the same dose with 5 different dose