Yb-doped silica optical fibers for real-time dosimetry in radiotherapy

Over the years, many efforts have been made to develop radiation detectors to handle the complex issues of small field dosimetry and achieve the increasing accuracy, precision, and in vivo dose monitoring required by the new advanced treatment modalities. In this context, interest has surged in the development of sensors based on radioluminescent (RL) optical fibers. However, the employment of such systems for monitoring extended radiation fields with energies above the Cerenkov threshold is still challenging, since a spurious luminescence, namely, the “stem effect,” is also generated in the passive fiber portion exposed to radiation. Previous investigations demonstrated that the RL of Yb3+, displaying a sharp emission line at about 975 nm, is free from any spectral superposition with the spurious luminescence. This paved the way to their use in applications where an efficient stem effect removal is required [1, 2]. In this work, the near infrared (NIR) RL and dosimetric properties of Yb-doped silica optical fibers, coupled with an optical detector prototype based on an avalanche photo-diode, were studied by irradiating the fibers with clinical beams [3]. Yb-doped fibers have been prepared by sol-gel. Irradiations were carried out with photons and electron beams generated by a Varian Trilogy accelerator. The performances of the system in standard and small field sizes have been investigated comparing the output factor, percent depth dose and off axis ratio measurements of the prototypal dosimetric system with other commercial sensors. The NIR scintillation proves to be unaffected by the stem effect, even in unfavourable large field irradiations. The system shows a satisfactory reproducibility, good sensitivity, linear dose rate response, independence of the signal of dose rate and impinging beam orientation. The results are in good agreement with reference dosimeters in terms of relative dose profiles and output factors. The results of this study demonstrate that the drawback due to the stem effect in Yb-doped silica optical fibres can be managed in a simple but effective way by optical filtering. The robustness of the system in complex dosimetric scenarios and the accuracy achieved by Ybdoped fibres in relative dose assessments suggest an effective use of the system or real-time in vivo dosimetry.