OPTICAL FIBER DETECTORS AS IN-VIVO DOSIMETRY METHOD OF QUALITY ASSURANCE IN RADIATION THERAPY

A new in-vivo dosimetry system has been under development for some time using radioluminescent phosphors [1,2]. These phosphors are activated, metal ion doped glasses (Ex: Cu 1+ -doped quartz fiber), have excellent optical transparency and oer several potential advantages for radiation dosimetry, including: small size, high sensitivity, linearity of dose-response insensitivity to electromagnetic interference. The utility of these phosphors as a detection modality has been limited in real-time dosimetry applications due to the production of Cerenkov radiation in the carrier fiber, which produces a contaminant signal proportional to dose rate as well as the size of the radiation field. One possible method for eliminating this signal is using an electronic gating signal from the accelerator to delay data acquisition during the actual beam pulse, when Cerenkov radiation is produced. The dosimeter was tested using an external beam radiotherapy machine that provided pulses of 6 MeV x-rays. Gated detection was used to discriminate the signal collected during the radiation pulses, which included contributions from Cerenkov radiation, from the signal collected between the radiation pulses. Gated detection of the Phosphorescence provided accurate, real-time dose measurements that were linear with absorbed dose, independent of dose rate and that were accurate for all field sizes studied.