SU-E-T-117: Optically Stimulated Luminescence for Diagnostic and Therapeutic Low to Medium Energy X-Ray Beams Experimental Dosimetry

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Purpose: Optically stimulated luminescence (OSL) using aluminium oxide crystals (Al2O3:C) is a dosimetry technique (1), already validated in radiation therapy (2). The aim of this study was to characterize the OSL detectors in terms of energy dependence using monochromatic beams in the low-medium energy range (25-85 keV); and to evaluate their potential for experimental dosimetry in therapeutic applications at these energies. Methods: The OSL system is composed of a multichannel reader and alumina crystals (Al2O3:C, 0.09 mm3) that are remotely read by laser light. The OSL signal is directly linked to the absorbed dose. Irradiations in were performed at 8 energy points from 25 to 90 keV using a synchrotron source. Absolute OSL dose measurements were also performed at 80 keV at 2 cm depth in water equivalent phantoms (3) (1-8 Gy range). The detectors have been read over 300 s to collect all the light available from the trapped electrons. Results: The OSL detector show a lower energy dependence (< 10%) in the 25 to 50 keV range than at higher energies (∼ 45% from 50 to 90 keV). OSL is a reproducible absolute dosimetry technique at 80 keV (sigma<5 %). The field size has no other influence on the OSL signal than the one due to phantom scatter output factors (3). Conclusion: We demonstrated that the OSL energy dependance decreases with the beam energy. The conventional models (4) of OSL response in function of beam energy and delivered dose were confronted to our results. The next step of this study is to use OSL for beam penumbra caracterisation, where conventional detectors exhibit a severe energy dependance due to low energy spectra. In vivo dosimetry applications in low-medium enregy radiotherapy are foreseen due to the good reproducibility in OSL signal for absolute dose measurements.