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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/cm
2
.
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 1mm
3
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. Pappas
1
, E. Zoros
1
, K. Zourari
2
, C.I. Hourdakis
2
, P.
Papagiannis
1
, P. Karaiskos
1
, E. Pantelis
1
1
National and Kapodistrian University of Athens, Medical
Physics Laboratory - Medical School, Athens, Greece
2
Greek 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
60
Co 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 cm
3
), filled with PRESAGE samples,
were irradiated to the same dose with 5 different dose
rates. Irradiations were performed with a
60
Co PICKER unit
in a secondary standard calibration laboratory. The
samples were divided into groups of two and each group
was placed at a different distance (56.65 - 427 cm) from
the
60
Co source at a 5cm depth within a water phantom.
Irradiation times varied in order to deliver the same dose
of 1 Gy at the center of all cuvettes with dose rates in the
range of 0.018 – 1.0 Gy/min. For the high dose rate study,
a similar methodology was employed. Four couples of
PRESAGE cuvettes were placed within a slab in a solid
water phantom and irradiated at different dose-rates by
varying the dose delivery rate of an ELEKTA Versa HD FFF
linac from 2.5 up to 19 Gy/min. Dose delivery of 1 Gy for
all dose rates was verified by ion chamber measurements.
Irradiation induced optical density (OD) change was
measured from pre- and post-irradiation scans with a
digital spectrophotometer operated at 633 nm. Mean OD
change for each group was normalized to the value for the
highest dose rate in each study.
Results
Results presented in figure 1 show a trend of increasing
PRESAGE dose sensitivity with decreasing dose rate with
the over-response reaching up to 16% at 0.018 Gy/min.
Although in a first approach such low dose rates could be
considered extremely low in external radiotherapy, recent
studies have shown that in advanced radiotherapy
techniques (e.g. VMAT) dose rate varies drastically across
dose distributions delivered and a considerable
contribution of the delivered dose could come from very
low dose rates (0.01 - 0.1Gy/min). Regarding the high dose
rate study, all responses agree within experimental
uncertainties, indicating that PRESAGE sensitivity is not
significantly affected.
Figure 1: Dose rate dependence of PRESAGE response for
both studies included in this work. Error bars correspond
to 1 standard deviation of all experimental uncertainties
involved.
Conclusion
Results of this study indicate a significant over-response
of this PRESAGE formulation in very low dose rates that
should be considered when they are used in applications
involving wide range of dose delivery rates.
Acknowledgement: This work was financially supported by
the State Scholarships Foundation of Greece through the
program ‘Research Projects for Excellence IKY/SIEMENS’.
PO-0775 Contributions to detector response in
arbitrary photon fields
S. Wegener
1
, O.A. Sauer
1
1
University Hospital, Radiation Oncology, Würzburg,
Germany
Purpose or Objective
Due to their small active volumes, diodes are often the
detectors of choice for many commissioning tasks
including the measurement of output factors, especially in
small fields. However, high-atomic number material in the
chip, detector shielding or other components and a finite
active volume size have been found to alter the signal
compared to the dose ratios measured in water in the
absence of such a detector. As a consequence, correction
factors need to be applied to correct the obtained signals.
Using three experimental setups (fig. 1), the different
contributions to the detector signals were separated and
analyzed: the response to scatter, the primary beam and
the combination of both.
Material and Methods
Signal ratios were obtained for three different
experimental setups (fig. 1): First, the standard open field
geometry. Secondly, fields in which the central part of the
beam was blocked out by a 4 mm aluminum pole and the