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

Abstract

To investigate empirically the energy dependence of the detector response of two invivo luminescence detectors, LiF:Mg,Cu,P (MCP-N) high-sensitivity TLDs and Al2 O3 :C OSLDs, in the 40-300-kVp energy range in the context of invivo surface dose measurement. As these detectors become more prevalent in clinical and preclinical invivo measurements, knowledge of the variation in the empirical dependence of the measured response of these detectors across a wide spectrum of beam qualities is important. We characterized a large range of beam qualities of three different kilovoltage x-ray units: an Xstrahl 300 Orthovoltage unit, a Precision x-Ray X-RAD 320ix biological irradiator, and a Varian On-Board Imaging x-ray unit. The dose to water was measured in air according to the AAPM's Task Group 61 protocol. The OSLDs and TLDs were irradiated under reference conditions on the surface of a water phantom to provide full backscatter conditions. To assess the change in sensitivity in the long term, we separated the invivo dosimeters of each type into an experimental and a reference group. The experimental dosimeters were irradiated using the kilovoltage x-ray units at each beam quality used in this investigation, while the reference group received a constant 10cGy irradiation at 6MV from a Varian clinical linear accelerator. The individual calibration of each detector was verified in cycles where both groups received a 10cGy irradiation at 6MV. The nanoDot OSLDs were highly reproducible, with ±1.5% variation in response following >40 measurement cycles. The TLDs lost ~20% of their signal sensitivity over the course of the study. The relative light output per unit dose to water of the MCP-N TLDs did not vary with beam quality for beam qualities with effective energies <50keV (~150kVp/6mmAl). At higher energies, they showed a reduced (~75-85%) light output per unit dose relative to 6MV x rays. The nanoDot OSLDs exhibited a very strong (120-408%) dependency of the light output relative to 6 MV x rays. Variations up to 15% between different x-ray units with equivalent effective energies were also observed. While convenient for clinical use, nanoDot OSLDs exhibit a strong variation in their measured light output per unit dose relative to 6MV in the 40-300kV x-ray range. This variability differs unit-to-unit, limiting their effective use for invivo dosimetry applications in the kilovoltage x-ray energy range. MCP-N TLDs offer a much more stable response, but suffer from variations in sensitivity over time dependent on radiation history, which requires careful experimental handling.