ESTRO 36 Abstract Book

S402 ESTRO 36 _______________________________________________________________________________________________

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).

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).

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.

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.