Investigation of TL and OSL detectors in ultra-high dose rate electron beams

OBJECTIVE

This work aims at investigating the response of various thermally stimulated luminescence detectors (TLDs) and optically stimulated luminescence detectors (OSLDs) for dosimetry of ultra-high dose rate electron beams. The study was driven by the challenges of dosimetry at ultra-high dose rates and the importance of dosimetry for FLASH radiotherapy and radiobiology experiments.
Approach: Three types of TLDs (LiF:Mg,Ti, LiF:Mg,Cu,P, CaF₂:Tm) and one type of OSLD (Al₂O₃:C) were irradiated in a 15 MeV electron beam with instantaneous dose rates in the (1-324) kGys^-1range. 
Reference dosimetry was carried out with an integrating current transformer, which was calibrated in absorbed dose to water against a reference ionization chamber. Additionally, dose rate independent BeO OSLDs were employed as a reference. Beam non-uniformity was addressed by using a matrix of TLDs/OSLDs
Main results: The investigated TLDs were shown to be dose rate independent within the experimental uncertainties, which take into account the uncertainty of the dosimetry protocol and the irradiation uncertainty.
The relative deviation between the TLDs and the reference dose was lower than 4% for all dose rates. A decreasing response with the dose rate was observed for Al₂O₃:C OSLDs, but still within 10% from the reference dose. 
Significance: The precision of the investigated luminescence detectors make them suitable for dosimetry of ultra-high dose rate electron beams. Specifically, the dose rate independence of the TLDs can support the investigation of the beam uniformity as a function of the dose rate, which is one of the challenges of the employed beam. Al₂O₃:C OSLDs provided high precision measurements, but the decreasing response with the dose rate needs to be confirmed by additional experiments.

Radiation field and dose inhomogeneities using an X-ray cabinet in radiation biology research

PURPOSE

X-ray cabinets are replacing ¹³⁷ Cs/⁶⁰ Co sources in radiation biology research due to advantages in size, handling, and radiation protection. However, because of their different physical properties, X-ray cabinets are more susceptible to experimental influences than conventional sources. The aim of this study was to examine the variations related to the experimental setups typically used to investigate biological radiation effects with X-ray cabinets.

MATERIALS AND METHODS

A combined approach of physical dose measurements by thermoluminescence dosimetry and detection of biological effects by quantification of γH2AX and 53BP1 foci was used to analyze field inhomogeneity and evaluate the influence of the components of the experimental setup.

RESULTS

Irradiation was performed using an X-ray tube (195 kV, 10 mA, 0.5-mm-thick copper filter, dose rate of 0.59 Gy/min). Thermoluminescence dosimetry revealed inhomogeneity and a dose decrease of up to 42.3% within the beam area (diameter 31.1 cm) compared to the dose at the center. This dose decrease was consistent with the observed decline in the number of radiation-induced foci by up to 55.9 %. Uniform dose distribution was measured after reducing the size of the radiation field (diameter 12.5 cm). However, when using 15-ml test tubes placed at different positions within this field, the dose decreased by up to 17% in comparison to the central position. Analysis of foci number revealed significant differences between the tubes for γH2AX (1 h) and 53BP1 (4 h) at different time points after irradiation. Neither removal of some tubes nor of the caps improved the dose decrease significantly. By contrast, when using 1.5-ml tubes, dose differences were less than 4%, and no significant differences in foci number were detected.

CONCLUSION

X-ray cabinets are user-friendly irradiation units for investigating biological radiation effects. However, field inhomogeneities and experimental setup components considerably affect the delivered irradiation doses. For this reason, strict dosimetric monitoring of experimental irradiation setups is mandatory for reliable studies.

The correction factors for UD802 dosimeters irradiated with different radiation energies and at different incidence angles

Many laboratories around the world work on the issue of protection against ionising radiation. It is a very broad topic, covering both the protection of members of the public and workers exposed to ionising radiation, based on personal or environmental monitoring. Thermoluminescence detectors are commonly used for this purpose, which is accepted in most countries. The entire process of dose evaluation with such dosimeters can be divided into several steps. Undoubtedly, one of the most important steps is the calibration procedure. The calibration should be performed in conditions as similar as possible to those experienced during the exposure. Moreover, the calibration is performed using only the Cs-137 source. This article presents results of Panasonic UD-802 irradiation dosimeters in different radiation fields generated by Cs-137, Kr-85, Sr-90/Y-90 and an x-ray tube, series N, and at different angles such as 0°, 30°, 60°, 90°. On the basis of the obtained results, the calibration coefficients and correction factors were determined in relation to calibration based on Cs-137 or Sr-90/Y-90 irradiated at 0°.

Technical Note: On the feasibility of performing dosimetry in target and organ at risk using polymer dosimetry gel and thermoluminescence detectors in an anthropomorphic, deformable, and multimodal pelvis phantom

OBJECTIVE

To assess the feasibility of performing dose measurements in the target (prostate) and an adjacent organ at risk (rectum) using polymer dosimetry gel and thermoluminescence detectors (TLDs) in an anthropomorphic, deformable, and multimodal pelvis phantom (ADAM PETer).

METHODS

The 3D printed prostate organ surrogate of the ADAM PETer phantom was filled with polymer dosimetry gel. Nine TLD600 (LiF:Mg,Ti) were installed in 3 × 3 rows on a specifically designed 3D-printed TLD holder. The TLD holder was inserted into the rectum at the level of the prostate and fixed by a partially inflated endorectal balloon. Computed tomography (CT) images were taken and treatment planning was performed. A prescribed dose of 4.5 Gy was delivered to the planning target volume (PTV). The doses measured by the dosimetry gel in the prostate and the TLDs in the rectum ("measured dose") were compared to the doses calculated by the treatment planning system ("planned dose") on a voxel-by-voxel basis.

RESULTS

In the prostate organ surrogate, the 3D-γ-index was 97.7% for the 3% dose difference and 3 mm distance to agreement criterium. In the center of the prostate organ surrogate, measured and planned doses showed only minor deviations (<0.1 Gy, corresponding to a percentage error of 2.22%). On the edges of the prostate, slight differences between planned and measured doses were detected with a maximum deviation of 0.24 Gy, corresponding to 5.3% of the prescribed dose. The difference between planned and measured doses in the TLDs was on average 0.08 Gy (range: 0.02-0.21 Gy), corresponding to 1.78% of the prescribed dose (range: 0.44%-4.67%).

CONCLUSIONS

The present study demonstrates the feasibility of using polymer dosimetry gel and TLDs for 3D and 1D dose measurements in the prostate and the rectum organ surrogates in an anthropomorphic, deformable and multimodal phantom. The described methodology might offer new perspectives for end-to-end tests in image-guided adaptive radiotherapy workflows.

Radiation protection effect of mobile shield barrier for the medical personnel during endoscopic retrograde cholangiopancreatography: a quasi-experimental prospective study

OBJECTIVE

To investigate the effectiveness of radiation protection offered by a newly designed mobile shield barrier for medical personnel during endoscopic retrograde cholangiopancreatography (ERCP).

DESIGN

Quasi-experimental prospective study.

SETTING

ERCP procedures conducted between October 2016 and June 2017 at a single secondary referral hospital that performs approximately 250 therapeutic ERCP procedures annually.

INTERVENTIONS

The mobile shield barrier was a custom-made 2 mm Pb shielding plate (width: 120 cm, height: 190 cm) with a 0.5 mm Pb window (width: 115 cm, height: 60 cm) on its upper part was used. Four wheels were attached to the bottom to allow easy moving.

PRIMARY AND SECONDARY OUTCOME MEASURES

The radiation doses were measured during ERCP using personal thermoluminescence dosimetry (TLD) badges on both sides of the mobile shield barrier (patient's side: TLD1 and medical staff's side: TLD2). The radiation doses were also measured on the outer surface of the thyroid shield of the endoscopist (TLD3), and on the chest area inside the protective apron of the endoscopist (TLD4) and the main assistant (TLD5). The TLD was changed and reported once every 3 months. The radiation dose measured by TLD badges were compared.

RESULTS

During the study period, a total of 128 ERCP procedures were performed. The mean fluoroscopy time per procedure was 244.9±257.0 s and the mean number of digital radiographs per procedure was 3.7±1.0. TLD1 (outside the barrier) had a mean radiation dose of 26.85±3.47 mSv and all the other TLDs (inside the barrier) had less than 1 mSv (p<0.001). In the post hoc analysis, the difference between TLD1 and others showed a statistical significance; however, there were no significant differences between the TLDs inside the barrier.

CONCLUSION

Our mobile shield barrier was useful to reduce the radiation exposure of medical personnel during ERCP.