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

Abstract

Purpose: To investigate empirically the energy dependence of the detector response of two in vivo luminescence detectors, LiF:Mg,Cu,P (MCP-N) high-sensitivity TLDs and Al 2 O 3 :C OSLDs, in the 40-300-kVp energy range in the context of in vivo surface dose measurement. As these detectors become more prevalent in clinical and preclinical in vivo 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. Method: 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 in vivo 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 10 cGy irradiation at 6 MV from a Varian clinical linear accelerator. The individual calibration of each detector was verified in cycles where both groups received a 10 cGy irradiation at 6 MV. Results: The nanoDot OSLDs were highly reproducible, with AE1.5% 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 <50 keV (~150 kVp/6 mm Al). At higher energies, they showed a reduced (~75-85%) light output per unit dose relative to 6 MV 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. Conclusions: While convenient for clinical use, nanoDot OSLDs exhibit a strong variation in their measured light output per unit dose relative to 6 MV in the 40-300 kV x-ray range. This variability differs unit-to-unit, limiting their effective use for in vivo 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.

Preliminary Investigation into the regeneration of luminescent signal in nanoDot OSLDs

“…5 and Tables 1 17 relative to water, thereby leading to a greater incidence of photoelectric interactions in the nanoDot. 18 A hypothesis as to why this may be the case can be attributed to differences in photon interactions and photon cross quantities of free electrons to be captured by the shallow, intermediate, and deep dosimetric traps relative to nanoDots exposed to the same dose but from photons in the MV energy range that experience predominantly Compton scattering interactions. This may in turn allow for more electrons to be captured in the deep dosimetric traps that will migrate to shallower dosimetric traps following their optical exposure to blue light.…”

“…5 and Tables 1–2 indicate that the phototransfer of charge carriers from deep dosimetric traps not only have an accumulated dose and time dependence, but also an energy dependence as examined in the differences in measured signal of OSL nanoDots with prior irradiation history from MV and kV photons. The differences in measured signal growth, with regards to magnitude and growth rate, observed in optically bleached OSL nanoDots with prior irradiation history to MV and kV x rays may in part be attributed to the over‐response in Al 2 O 3 at low energies due to its greater effective atomic number (Z eff ), 17 relative to water, thereby leading to a greater incidence of photoelectric interactions in the nanoDot 18 . A hypothesis as to why this may be the case can be attributed to differences in photon interactions and photon cross sections in Al 2 O 3 at kV and MV energies.…”

Ultra‐high dose rate dosimetry: Challenges and opportunities for FLASH radiation therapy

“…OSLDs have been shown to be linear over five orders of magnitude in radiation dose response up to 10 Gy in megavoltage photon fields, 117,124 with some supralinearity reported in kilovoltage X-ray fields that is more pronounced as the photon energy is lowered. 125,126 OSLDs have also been utilized in particle therapy with encouraging results. 114,127,128 Recently, the response of OSLD in a pencil beam proton field for a wide range of dose rates up to 9000 Gy/s has been studied by Christensen et al 129 It was found that the OSLD response is dose-rate independent (see Figure 4).…”

“…The lower sensitivity of OSLDs to radiation was considered a disadvantage, however this may be useful in FLASH radiation environments. OSLDs have been shown to be linear over five orders of magnitude in radiation dose response up to 10 Gy in megavoltage photon fields, 117,124 with some supralinearity reported in kilovoltage X‐ray fields that is more pronounced as the photon energy is lowered 125,126 . OSLDs have also been utilized in particle therapy with encouraging results 114,127,128 …”

Development and Characterization of Silver Containing Free Standing Polymer FILMS for Dosimetry Applications

“…However, registration and monitoring of the occupational exposure to the staff dealing with ionizing radiation is another important issue. Due to this dosimetry and related materials, instruments and methods have become a prominent research field in physics and chemistry as it involves different radiation-induced processes and phenomena in irradiated materials: thermoluminescence (TLD) [1][2][3], optically stimulated luminescence (OSL) [3][4][5][6], electron spin resonance (ESR) [7,8], radioluminescence (RL) [9], and others. Dosimetry methods utilizing the above-mentioned phenomena are well established, when speaking about high-energy and high-dose irradiation; however, dose registration in the interval 0-0.1 Gy might be challenging.…”