Scintillation properties of the YVO4:Eu3+ compound in powder form: its application to dosimetry in radiation fields produced by pulsed mega-voltage photon beams

Investigation of the Effect of Sb3+ Ion on the Spectroscopic Properties of Eu3+-Doped YVO4 Nanosized Materials

This paper discusses the origin of emission quenching in yttrium orthovanadate codoped with Eu3+ and Sb3+ ions. Highly crystalline yttrium orthovanadate nanoparticles with chemical composition Y1-x-yEuxSbyVO4 (x = 0-5.4 mol %, y = 0-2.1 mol %) were prepared using the modified Pechini's method. The particles' structural and morphological properties were studied using X-ray powder diffraction and scanning electron microscopy techniques. Moreover, X-ray photoelectron spectroscopy was used to determine the oxidation state and surface bonding of individual ions in the obtained materials. The photoluminescence spectral and dynamical properties were investigated in detail. The observed Eu3+ luminescence depreciation is ascribed to reabsorption mediated by a low-lying Sb3+-to-V5+ metal-to-metal charge transfer state in conjunction with lattice defects absorbing in the visible spectral region.

Advances on inorganic scintillator-based optic fiber dosimeters

This article presents a new perspective on the development of inorganic scintillator-based fiber dosimeters (IOSFDs) for medical radiotherapy dosimetry (RTD) focusing on real-time in vivo dosimetry. The scintillator-based optical fiber dosimeters (SFD) are compact, free of electromagnetic interference, radiation-resistant, and robust. They have shown great potential for real-time in vivo RTD. Compared with organic scintillators (OSs), inorganic scintillators (IOSs) have larger X-ray absorption and higher light output. Variable IOSs with maximum emission peaks in the red part of the spectrum offer convenient stem effect removal. This article outlines the main advantages and disadvantages of utilizing IOSs for SFD fabrication. IOSFDs with different configurations are presented, and their use for dosimetry in X-ray RT, brachytherapy (BT), proton therapy (PT), and boron neutron capture therapy (BNCT) is reviewed. Challenges including the percentage depth dose (PDD) deviation from the standard ion chamber (IC) measurement, the angular dependence, and the Cherenkov effect are discussed in detail; methods to overcome these problems are also presented.

Characterization of an inorganic scintillator for small-field dosimetry in MR-guided radiotherapy

INTRODUCTION

Aim of this study is to dosimetrically characterize a new inorganic scintillator designed for magnetic resonance-guided radiotherapy (MRgRT) in the presence of 0.35 tesla magnetic field (B).

METHODS

The detector was characterized in terms of signal to noise ratio (SNR), reproducibility, dose linearity, angular response, and dependence by energy, field size, and B orientation using a 6 MV magnetic resonance (MR)-Linac and a water tank. Field size dependence was investigated by measuring the output factor (OF) at 1.5 cm. The results were compared with those measured using other detectors (ion chamber and synthetic diamond) and those calculated using a Monte Carlo (MC) algorithm. Energy dependence was investigated by acquiring a percentage depth dose (PDD) curve at two field sizes (3.32 × 3.32 and 9.96 × 9.96 cm2 ) and repeating the OF measurements at 5 and 10 cm depths.

RESULTS

The mean SNR was 116.3 ± 0.6. Detector repeatability was within 1%, angular dependence was <2% and its response variation based on the orientation with respect to the B lines was <1%. The detector has a temporal resolution of 10 Hz and it showed a linear response (R2  = 1) in the dose range investigated. All the OF values measured at 1.5 cm depth using the scintillator are in accordance within 1% with those measured with other detectors and are calculated using the MC algorithm. PDD values are in accordance with MC algorithm only for 3.32 × 3.32 cm2 field. Numerical models can be applied to compensate for energy dependence in case of larger fields.

CONCLUSION

The inorganic scintillator in the present form can represent a valuable detector for small-field dosimetry and periodic quality controls at MR-Linacs such as dose stability, OFs, and dose linearity. In particular, the detector can be effectively used for small-field dosimetry at 1.5 cm depth and for PDD measurements if the field dimension of 3.32 × 3.32 cm2 is not exceeded.

Real-time in vivo dosimetry system based on an optical fiber-coupled microsized photostimulable phosphor for stereotactic body radiation therapy

PURPOSE

To develop an in vivo dosimeter system for stereotactic body radiation therapy (SBRT) that can perform accurate and precise real-time measurements, using a microsized amount of a photostimulable phosphor (PSP), BaFBr:Eu²⁺ .

METHODS

The sensitive volume of the PSP was 1.26 × 10^-5  cm³ . The dosimeter system was designed to apply photostimulation to the PSP after the decay of noise signals, in synchronization with the photon beam pulse of a linear accelerator (LINAC), to eliminate the noise signals completely using a time separation technique. The noise signals included stem signals, and radioluminescence signals generated by the PSP. In addition, the dosimeter system was built on a storage-type dosimeter that could read out a signal after an arbitrary preset number of photon beam pulses were incident. First, the noise and photostimulated luminescence (PSL) signal decay times were measured. Subsequently, we confirmed that the PSL signals could be exclusively read out within the photon beam pulse interval. Finally, using a water phantom, the basic characteristics of the dosimeter system were demonstrated under SBRT conditions, and the feasibility for clinical application was investigated. The reproducibility, dose linearity, dose-rate dependence, temperature dependence, and angular dependence were evaluated. The feasibility was confirmed by measurements at various dose gradients and using a representative treatment plan for a metastatic liver tumor. A clinical plan was created with a two-arc beam volumetric modulated arc therapy using a 10 MV flattening filter-free photon beam. For the water phantom measurements, the clinical plan was compiled into a plan with a fixed gantry angle of 0°. To evaluate the energy dependence during SBRT, the percent depth dose (PDD) was measured and compared with those calculated via Monte Carlo (MC) simulations.

RESULTS

All the PSL signals could be read out while eliminating the noise signals within the minimum pulse interval of the LINAC. Stable real-time measurements could be performed with a time resolution of 56 ms (i.e., number of pulses = 20). The dose linearity was good in the dose range of 0.01-100 Gy. The measurements agreed within 1% at dose rates of 40-2400 cGy/min. The temperature and angular dependence were also acceptable since these dependencies had only a negligible effect on the measurements in SBRT. At a dose gradient of 2.21 Gy/mm, the measured dose agreed with that calculated using a treatment planning system (TPS) within the measurement uncertainties due to the probe position. For measurements using a representative treatment plan, the measured dose agreed with that calculated using the TPS within 0.5% at the center of the beam axis. The PDD measurements agreed with the MC calculations to within 1% for field sizes <5 × 5 cm² .

CONCLUSION

The in vivo dosimeter system developed using BaFBr:Eu²⁺ is capable of real-time, accurate, and precise measurement under SBRT conditions. The probe is smaller than a conventional dosimeter, has excellent spatial resolution, and can be valuable in SBRT with a steep dose distribution over a small field. The developed PSP dosimeter system appears to be suitable for in vivo SBRT dosimetry.

Inorganic scintillation detectors for 192Ir brachytherapy

Many brachytherapy (BT) errors could be detected with real-time in vivo dosimetry technology. Inorganic scintillation detectors (ISDs) have demonstrated promising capabilities for BT, because some ISD materials can generate scintillation signals large enough that (a) the background signal emitted in the fiber-optic cable (stem signal) is insignificant, and (b) small detector volumes can be used to avoid volume averaging effects in steep dose gradients near BT sources. We investigated the characteristics of five ISD materials to identify one that is appropriate for BT. ISDs consisting of a 0.26 to 1.0 mm³ volume of ruby (Al₂O₃:Cr), a mixture of Y₂O₃:Eu and YVO⁴:Eu, ZnSe:O, or CsI:Tl coupled to a fiber-optic cable were irradiated in a water-equivalent phantom using a high-dose-rate ¹⁹²Ir BT source. Detectors based on plastic scintillators BCF-12 and BCF-60 (0.8 mm³ volume) were used as a reference. Measurements demonstrated that the ruby, Y₂O₃:Eu+YVO₄:Eu, ZnSe:O, and CsI:Tl ISDs emitted scintillation signals that were up to 19, 19, 250, and 880 times greater, respectively, than that of the BCF-12 detector. While the total signals of the plastic scintillation detectors were dominated by the stem signal for source positions 0.5 cm from the fiber-optic cable and  >3.5 cm from the scintillator volume, the stem signal for the ruby and Y₂O₃:Eu+YVO₄:Eu ISDs were  <1% of the total signal for source positions  <3.4 and  <4.4 cm from the scintillator, respectively, and  <0.7% and  <0.5% for the ZnSe:O and CsI:Tl ISDs, respectively, for positions  ⩽8.0 cm. In contrast to the other ISDs, the Y₂O₃:Eu+YVO₄:Eu ISD exhibited unstable scintillation and significant afterglow. All ISDs exhibited significant energy dependence, i.e. their dose response to distance-dependent ¹⁹²Ir energy spectra differed significantly from the absorbed dose in water. Provided that energy dependence is accounted for, ZnSe:O ISDs are promising for use in error detection and patient safety monitoring during BT.

New model for explaining the over-response phenomenon in percentage of depth dose curve measured using inorganic scintillating materials for optical fiber radiation sensors

Inorganic scintillating material used in optical fibre sensors (OFS) when used as dosimeters for measuring percentage depth dose (PDD) characteristics have exhibited significant differences when compared to those measured using an ionization chamber (IC), which is the clinical gold standard for quality assurance (QA) assessments. The percentage difference between the two measurements is as high as 16.5% for a 10 × 10 cm² field at 10 cm depth below the surface. Two reasons have been suggested for this: the presence of an energy effect and Cerenkov radiation. These two factors are analysed in detail and evaluated quantitatively. It is established that the influence of the energy effect is only a maximum of 2.5% difference for a beam size 10 × 10 cm² compared with the measured ionization chamber values. And the influence of the Cerenkov radiation is less than 0.14% in an inorganic scintillating material in the case of OFS when using Gd₂O₂S:Tb as the luminescent material. Therefore, there must be other mechanisms leading to over-response. The luminescence mechanism of inorganic scintillating material is theoretically analysed and a new model is proposed and validated that helps explain the over-response phenomenon.

Inorganic scintillation detectors based on Eu-activated phosphors for 192Ir brachytherapy

The availability of real-time treatment verification during high-dose-rate (HDR) brachytherapy is currently limited. Therefore, we studied the luminescence properties of the widely commercially available scintillators using the inorganic materials Eu-activated phosphors Y₂O₃:Eu, YVO₄:Eu, Y₂O₂S:Eu, and Gd₂O₂S:Eu to determine whether they could be used to accurately and precisely verify HDR brachytherapy doses in real time. The suitability for HDR brachytherapy of inorganic scintillation detectors (ISDs) based on the 4 Eu-activated phosphors in powder form was determined based on experiments with a ¹⁹²Ir HDR brachytherapy source. The scintillation intensities of the phosphors were 16-134 times greater than that of the commonly used organic plastic scintillator BCF-12. High signal intensities were achieved with an optimized packing density of the phosphor mixture and with a shortened fiber-optic cable. The influence of contaminating Cerenkov and fluorescence light induced in the fiber-optic cable (stem signal) was adequately suppressed by inserting between the fiber-optic cable and the photodetector a 25 nm band-pass filter centered at the emission peak. The spurious photoluminescence signal induced by the stem signal was suppressed by placing a long-pass filter between the scintillation detector volume and the fiber-optic cable. The time-dependent luminescence properties of the phosphors were quantified by measuring the non-constant scintillation during irradiation and the afterglow after the brachytherapy source had retracted. We demonstrated that a mixture of Y₂O₃:Eu and YVO₄:Eu suppressed the time-dependence of the ISDs and that the time-dependence of Y₂O₂S:Eu and Gd₂O₂S:Eu introduced large measurement inaccuracies. We conclude that ISDs based on a mixture of Y₂O₃:Eu and YVO₄:Eu are promising candidates for accurate and precise real-time verification technology for HDR BT that is cost effective and straightforward to manufacture. Widespread dissemination of this technology could lead to an improved understanding of error types and frequencies during BT and to improved patient safety during treatment.

Ruby-based inorganic scintillation detectors for 192Ir brachytherapy

We tested the potential of ruby inorganic scintillation detectors (ISDs) for use in brachytherapy and investigated various unwanted luminescence properties that may compromise their accuracy. The ISDs were composed of a ruby crystal coupled to a poly(methyl methacrylate) fiber-optic cable and a charge-coupled device camera. The ISD also included a long-pass filter that was sandwiched between the ruby crystal and the fiber-optic cable. The long-pass filter prevented the Cerenkov and fluorescence background light (stem signal) induced in the fiber-optic cable from striking the ruby crystal, which generates unwanted photoluminescence rather than the desired radioluminescence. The relative contributions of the radioluminescence signal and the stem signal were quantified by exposing the ruby detectors to a high-dose-rate brachytherapy source. The photoluminescence signal was quantified by irradiating the fiber-optic cable with the detector volume shielded. Other experiments addressed time-dependent luminescence properties and compared the ISDs to commonly used organic scintillator detectors (BCF-12, BCF-60). When the brachytherapy source dwelled 0.5 cm away from the fiber-optic cable, the unwanted photoluminescence was reduced from  >5% to  <1% of the total signal as long as the ISD incorporated the long-pass filter. The stem signal was suppressed with a band-pass filter and was  <3% as long as the source distance from the scintillator was  <7 cm. Some ruby crystals exhibited time-dependent luminescence properties that altered the ruby signal by  >5% within 10 s from the onset of irradiation and after the source had retracted. The ruby-based ISDs generated signals of up to 20 times that of BCF-12-based detectors. The study presents solutions to unwanted luminescence properties of ruby-based ISDs for high-dose-rate brachytherapy. An optic filter should be sandwiched between the ruby crystal and the fiber-optic cable to suppress the photoluminescence. Furthermore, we recommend avoiding ruby crystals that exhibit significant time-dependent luminescence.

Performance of ZnSe(Te) as fiberoptic dosimetry detector

Fiberoptic dosimetry (FOD) is an experimental technique suitable for in-vivo, real time dosimetry in radiotherapy treatments. FOD relies on using a small scintillator coupled to one end of a long optical fiber. The scintillator is placed at the point where the dose rate is to be determined whereas a light detector at the other end of the fiber measures the intensity of the radioluminescence emitted by the scintillator. One of the problems hampering the straightforward application of this technique in clinics is the presence of Cherenkov radiation generated in the fiber by the ionizing radiation, which adds to the scintillating light and introduces a bias in the dose measurement. Since Cherenkov radiation is more important in short wavelength range of the visible spectrum, using red-emitting scintillators as FOD detectors permits to reduce the Cherenkov contribution by using optical filters. In this work, the performance of red-emitting tellurium-doped zinc selenide crystal as FOD detector is evaluated and compared to the response of an ion-chamber.