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minimizes the dead volume will also reduce the influence
of the magnetic field.
Acknowledgements:
We thank Dr. E. Schuele as well as R.
Kranzer (PTW) for giving detailed information on the
chamber geometry.
PO-0762 Real-time dosimetry with rare earth doped
silica
G. Loi
1
, E. D'Agostino
2
, I. Veronese
3
, N. Chiodini
4
, A.
Vedda
4
1
Azienda Ospedaliera Maggiore della Carità, Medical
Physics Department, Novara, Italy
2
DoseVue NV, R&D, Mol, Belgium
3
Università degli Studi di Milano and INFN, Dipartimento
di Fisica, Milano, Italy
4
Università di Milano Bicocca, Dipartimento di Scienza
dei Materiali, Milano, Italy
Purpose or Objective
Modern radiotherapy techniques as Cyberknife or VMAT
are characterized by high daily doses regimes and steep
dose gradient, often associated to small irradiation fields.
Optical fiber based dosimetry represents a very attractive
alternative to perform measurements under these
conditions, thanks to its compactness, real-time response
and high sensitivity. The use of such technology has
however been hampered by the complex calibration
procedures needed to handle the so-called stem signal.
Rear earth doped silica, coupled to optical fibers,
represent an efficient and robust way to solve this
problem.
Material and Methods
Different types of rare earth doped silica were produced
by sol-gel technique. They were coupled to diff erent type
of fibers and tested under several conditions.
The radioluminescence and dosimetric propert ies of Yb-
doped silica optical fibers, were studied by irradiating the
fibers with photons and electron beams generated by a
Varian Trilogy accelerator and comparing its performances
with other existing state of the art dosimeters. The
scintillation was detected with a laboratory prototype
based on an avalanche photodiode (APD).
Beside the clinical measurements, a second set of
measurements exploiting a cerium-doped silica fiber, was
also performed on a preclinical irradiator (Xrad Smart
from PXI inc). Measurements were performed during high
resolution CT imaging as well as during irradiation.
Results
The Yb-doped silica system, tested under clinical
conditions, showed a satisfactory sensitivity,
reproducibility, and a linear dose-rate response. A reliable
dose evaluation was obtained independently of the dose
rate and of the orientation of the impinging beam, clearly
demonstrating that stem signal (and, more specifically, its
Cherenkov component) was very efficiently suppressed,
even in very unfavorable large field irradiation conditions.
The results showed a good agreement with reference
dosimeters in terms of relative dose profiles and output
factors. Figure 1 shows the outcome of output factor
measurements, performed on the linear accelerator, for
different field sizes, comparing the Yb-doped fiber to a
micro ionization chamber from Standard Imaging (A16).
As for the preclinical irradiations, the very high
scintillation yield from the doped silica allows its use
without further handling of the stem signal. Figure 2 shows
an example of signal obtained for a 20x20 mm² and 40x40
mm² fields. The difference between the curves is related
to the output factor. This was previously determined to be
equal to 0.94 for the 20x20 mm² field, versus the 40x40
mm² field.
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. Roed
1,2
, H. Lee
2
, L. Pinsky
1
, G. Ibbott
2
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