On the Importance of Fluorescence Within the Stem Effect of Scintillation Detectors
F Therriault-Proulx1,2*, L Beaulieu2,3, L Archambault2,3, S Beddar1, (1) University of Texas MD Anderson Cancer Center, Houston, Texas, (2) Departement de Physique, de Genie Physique et d'Optique, Universite Laval, Quebec, QC, (3) Centre Hospitalier Univ. de Quebec, Quebec, QCWE-G-BRB-5 Wednesday 4:30:00 PM - 6:00:00 PM Room: Ballroom B
Purpose: To quantify the nature and composition of the light produced in optical fibers under different irradiation conditions and evaluate its impact on dosimetry.
Methods: Irradiation of a bare PMMA optical fiber (Mitsubishi ESKA Premier) was performed using a superficial therapy unit, an Ir-192 HDR brachytherapy source, a Co-60 external-beam unit as well as photon and electron beams from a linear accelerator. Spectra of the radiation-induced visible light in the fiber were acquired and signals were compared as a function of depth and irradiation type. Irradiation of a 75 kVp beam from the superficial therapy unit was used to isolate the fluorescence spectrum. Isolation of the Cerenkov spectrum component was obtained from irradiation of a 15 MeV electron beam at a 45 degree angle. Relative composition in fluorescence and Cerenkov of the stem effect light has been determined for all irradiations.
Results: The total stem effect spectra can be represented by a linear superposition of the fluorescence and Cerenkov spectra. The fluorescence contribution was shown to strongly differ between the superficial therapy unit (99%±1%), the Ir-192 HDR source (25%±3%) and higher energy irradiations (3%±2%). Variations within each energy regime (kV, HDR brachytherapy and MV) were small at 3% or lower. These were observed for irradiations at angle or when the fiber was near the surface. This study suggests it is better to calibrate the stem effect of a scintillation detector using the same irradiation modality.
Conclusions: Stem effect light was shown to be composed of fluorescence and Cerenkov light in different proportions depending on the geometry of the experimental setup, nature of the irradiation, and irradiation energy. Calibrating detectors separately for fluorescence and Cerenkov may lead to better performance of the stem effect removal technique.