Ce-doped SiO2 optical fibres for real-time dosimetry of external radiotherapy beams

The introduction of new technologies in ra diation therapy aimed at improving the dose distribution to the tumour volume, calls for the development of innovative devices able to assure the quality of radiation beams a nd in-vivo real time dosimetry measurements. This work describes the use of a novel glass composite fibre dosimeter for the characterization of different t ypes of radiotherapy beams. The dosimeter is based on the radioluminescence emission of a small portion of SiO 2 fibre doped with cerium prepared by means of the sol gel and powde r in tube techniques. The doped portion is connected by fusion splicing to a commercial optical fibre which is optically coupled to a photomultiplier tube operating in photon counting mode. The signal is processed by a circuitry and software properl y designed. In order to subt ract the Cerenko v contribution to the signal response a double optic al fibre geometry is adopted. The system was tested by using conven tional radiotherapy photon and electron beams, as well as stereotactic fields gene rated by a 6 MV linear accelerator equipped with cone stereotactic collim ators. Characterisation measur ements were also performed using 138 MeV proton pencil beams at the Ce nter for Proton Therapy of the Paul Scherrer Institute (PSI), using the discrete spot scanning technique. The system showed interesting dosimetr ic properties in terms of sensitivity, reproducibility, linearity, energy and angul ar dependence. Moreover, the small dimension of the fibre and the possibility to perform real time measurements of the dose rate, proved to be particularly useful for a reliable evaluation of the output factors of small stereotactic fields and for resolving different scanning seque nces of proton pencil beams.