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Results
A maximum of 8.1% and 7.0% increase in chamber response
was measured for the two orientations at a field strength
of ±0.9 T. In contrast, the calculated response was only
marginally different, when the entire air volume was
considered as a sensitive volume in the simulations. It was
possible to reproduce the experimentally observed
differences in dose response using a small dead volume
close to the chamber stem. The simulated dose
distribution within the chamber cavity was found to be
highly non-uniform with hot and cold spots at the chamber
stem and chamber tip, depending on the field orientation
(see
Fig.
1).
Conclusion
In the presence of a magnetic field perpendicular to the
axis of thimble ionization chambers, the amount of
electrons entering the cavity from the tip and stem is
increased or decreased, depending on the field
orientation. The chamber response is therefore influenced
in a significant way by the presence of a dead region
known to exist at the chamber base near the stem.
Measurements with the chamber axis parallel to the
magnetic field are thus advantageous, as in this case the
dead volume has less impact due to the Lorentz force
acting radially. An optimized chamber design that
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.