Characterisation of energy response of Al2O3:C optically stimulated luminescent dosemeters (OSLDs) using cavity theory

Abstract

Aluminium oxide (Al(2)O(3):C) is a common material used in optically stimulated luminescent dosemeters (OSLDs). OSLDs have a known energy dependence, which can impact on the accuracy of dose measurements, especially for lower photon energies, where the dosemeter can overrespond by a factor of 3-4. The purpose of this work was to characterise the response of Al(2)O(3):C using cavity theory and to evaluate the applicability of this approach for polyenergetic photon beams. The cavity theory energy response showed good agreement (within 2 %) with the corresponding measured values. A comparison with measured values reported in the literature for low-energy polyenergetic spectra showed more varied agreement (within 6 % on average). The discrepancy between these results is attributed to differences in the raw photon energy spectra used to calculate the energy response. Analysis of the impact of the photon energy spectra versus the mean photon energy showed improved accuracy if the energy response was determined using the entire photon spectrum rather than the mean photon energy. If not accounted for, the overresponse due to photon energy could introduce substantial inaccuracy in dose measurement using OSLDs, and the results of this study indicate that cavity theory may be used to determine the response with reasonable accuracy.

AAPM TG 191: Clinical use of luminescent dosimeters: TLDs and OSLDs

“…For kV photons, TLDs over‐respond by a factor of up to 1.4 relative to 60 Co. Of note, this beam quality dependence can vary with different sizes and shapes of TLD. The magnitude of over‐response to low‐energy beams is much greater for OSLDs, which over‐respond by a factor of as much as 3 to 4 relative to 60 Co . If the calibration is done in a kV environment, the beam quality correction factor will be less.…”

“…For kV photons, TLDs over-respond by a factor of up to 1.4 52,162 relative to 60 Co. Of note, this beam quality dependence can vary with different sizes and shapes of TLD 145 . The magnitude of over-response to low-energy beams is much greater for OSLDs, which over-respond by a factor of as much as 3 to 4 relative to 60 Co. 21,41,44,51,59,[163][164][165][166] If the calibration is done in a kV environment, the beam quality correction factor will be less. However, a beam quality correction factor may still be substantial (particularly for OSLDs) between the calibration and experimental irradiation conditions as the energy response changes substantially even with small changes in beam quality in the kV energy range.…”

Characterization of the nanoDot OSLD dosimeter in CT

“…A second major concern is that Al 2 O 3 over-responds to low energy photons (with respect to water or tissue) because of its relatively large effective atomic number. 10,14,16,17,24 Per milligray of dose delivered to water, the response of Al 2 O 3 can be larger by a factor of 3.5 or more due to increased photoelectric effect at low energies. While this issue has been identified in the literature, [24][25][26] and has been examined for some cone-beam CT scans, 20 a thorough study of appropriate correction factors in a diagnostic CT environment has not been done.…”

Characterization of nanoDot optically stimulated luminescence detectors and high‐sensitivity MCP‐N thermoluminescent detectors in the 40–300 kVp energy range

“…In comparison, the high‐sensitivity LiF:Mg,Cu,P (MCPN‐TLDs) have been the subject of fewer studies, but these detectors have shown a comparably lower energy sensitivity (~−20% relative to 60 Co, or ~50% that of MTS‐N TLDs) in the same kilovoltage x‐ray energy range . In contrast, Monte Carlo models and experimental investigations have shown that Al 2 O 3 :C OSLD‐absorbed dose sensitivities are up to 350–400% higher at x‐ray kilovoltage energies than at megavoltage x‐ray energies …”

“…Many other kilovoltage x‐ray applications, such as orthovoltage x‐ray therapy or animal research irradiation techniques, operate at much higher (150–300 kVp) energy ranges . Studies that investigate broadbands of energies tend to focus on Monte Carlo simulations for monoenergetic photons rather than simulating plausible polyenergetic spectra found in real x‐ray sources . Although OSLDs are increasingly being used for image‐guided animal irradiations at kilovoltage energies, a comprehensive, empirical, systematic review of the energy dependence of OSLDs has not yet been published.…”