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Relative Dose Measurements
Published in W. P. M. Mayles, A. E. Nahum, J.-C. Rosenwald, Handbook of Radiotherapy Physics, 2021
For Type (b) measurements, one solution would be to use the same method, changing the position of the detector between consecutive measurements. However, this would be extremely time consuming, and as explained in Section 20.1.3.1, using water phantoms is the standard solution. They are supplied with their own electrometer connected to a PC. They can work either with step-by-step movements, with a short time integration of the signal at each position, or continuously. In the latter case, the electrometer is measuring the dose rate (intensity mode). For scanning beams (e.g. protons), using the intensity mode is not possible, and enough time must be allowed at each fixed position to integrate the contribution from several ‘frames'. As for Type (a) measurements, it is recommended to check the absence of drift by repeating a reference measurement at the beginning and the end of a series.
Radiotherapy Physics
Published in Debbie Peet, Emma Chung, Practical Medical Physics, 2021
Andrea Wynn-Jones, Caroline Reddy, John Gittins, Philip Baker, Anna Mason, Greg Jolliffe
Once the secondary standards are received back in the radiotherapy centre, Clinical Scientists use these to calibrate other ionisation chamber and electrometer calibrations. The procedures to calibrate these tertiary standards (field instruments) for use in MV photon and electron beams are documented in Codes of Practice (Eaton et al. (2020) and McEwen et al. (2003), respectively). This is an important process as a systematic error could be introduced which would affect all patients subsequently treated and therefore it is essential that an MPE is involved in all aspects of the procedure.
Ionization Chamber Instrumentation
Published in Arash Darafsheh, Radiation Therapy Dosimetry: A Practical Handbook, 2021
Larry A. DeWerd, Blake R. Smith
The current produced by ionization chambers for medical physics applications is typically between 0.5 pA and 800 nA. To measure and display such a low-magnitude current, or to integrate the current over time to display a charge collected, requires an instrument with an extremely high input impedance. Starting in the 1970s, electrometers began to be manufactured to meet the requirements of the medical physicist and have been refined to include more capabilities and features. As mentioned, an electrometer is a high impedance measuring device for which low currents or charges are detected such that the device itself does not affect the measurement. There are two basic circuits, one or both of which are found in the preamplifier (front end) of every electrometer. Both circuits are impedance converters in that a high-impedance input is converted to a low-impedance output. Low-level measuring techniques are described in the Low Level Handbook [12]. To measure charge, an operational amplifier is configured as an inverting amplifier with a feedback capacitor, as illustrated in Figure 2.2. The amplifier output voltage is a measure of charge transferred from the ionization process to the feedback capacitor and is related by the capacitance definition shown in Equation 2.1:
Determination of paraffin oil mist penetration at high flow rates through air-purifying respirators
Published in International Journal of Occupational Safety and Ergonomics, 2022
Agnieszka Brochocka, Małgorzata Okrasa
Huang et al. [7] studied the penetration of monodisperse liquid aerosols through polysulfone materials used in membrane filters. Those materials were characterized by good physicochemical properties, including thermal and chemical stability, mechanical strength and excellent oxidative resistance. The experimental system used in that study consisted of an aerosol generator, electrostatic particle classifier, neutralizer, mixing column, filter holder, aerosol electrometer, condensation particle counter, pressure gauge and flow meter. The liquid test aerosol was single-charged monodisperse DOP with a concentration of 5 × 103 particles/cm3 and a submicrometer-sized range from 0.03 to 0.05 µm with a geometric standard deviation of 1.10–1.15. Tests were conducted at different linear flow rates (5, 10 and 20 cm/s). It was found that the most penetrating particle size for polysulfone membrane filters with different morphological structures was 0.05 µm.
Role of blood derived cell fractions, temperature and sample transport on gene expression-based biological dosimetry
Published in International Journal of Radiation Biology, 2021
Farah Nasser, Lourdes Cruz-Garcia, Grainne O’Brien, Christophe Badie
Whole blood, PBMCs and WBCs were exposed to 2 Gy of x-rays at room temperature (22 °C) in air on top of a Perspex platform, with a dose rate of 0.5 Gy/min, using self-contained 250 kVp X-ray unit (CD160/1, AGO X-Ray Ltd., Aldermaston, Reading RG7 4PW, UK) with aluminum and copper filtration (∼1 mm) containing a Varian NDI-320 source (output 13 mA, 250 kV peak, 0.2 mA). Dosimetry was performed with a calibrated reference ionization chamber for the exact exposure setup used. Exposures were always monitored using a calibrated UNIDOS E electrometer and ‘in-beam’ monitor ionization chamber (all from PTW, Germany) located at source. Correction factors are used to calculate exact dose. Spatial dose uniformity was checked using Gafchromic EBT2 films (Vertec Scientific Ltd., UK) to ensure a homogenous dose was delivered. Once irradiated, the samples were incubated at 37 °C with 5% CO2, 4 °C or left at room temperature (22 °C). Following incubation, whole blood was mixed with 1 ml of RNA later Stabilization Solution; PBMCs and WBCs were mixed with 700 µl of QIAzol Lysis Reagent (miRNeasy kit, Qiagen Ltd, Crawley, UK) and stored at −80 °C (Figure 2).
Monte Carlo dosimetry using Fluka code and experimental dosimetry with Gafchromic EBT2 and XR-RV3 of self-built experimental setup for radiobiological studies with low-energy X-rays
Published in International Journal of Radiation Biology, 2020
Joanna Czub, Janusz Braziewicz, Adam Wasilewski, Anna Wysocka-Rabin, Paweł Wołowiec, Andrzej Wójcik
Supplementary Table 4 shows the values that were read from the four detectors after using the specified thickness of aluminum filter and calculated HVL values. The X-ray generator was set as 60 kV and 50 mA. The functions of the normalized dose rate and the thickness of aluminum filter are presented in Supplementary Figure 14. The HVL1 values were estimated using three detectors that were irradiated by an X-ray beam that was emitted through a beryllium window (14 mm in diameter, 300 μm in thickness) at voltages equal to 30, 40, 50 and 60 kV without additional filtration and 30 kV filtered by a V filter (see Supplementary Table 5). Supplementary Figure 15 presents the function between the normalized dose rate and the thickness of the aluminum filters. Supplementary Table 6 shows the HVL1 values, effective energies calculated for voltage 60 kV and different filters that were set on an X-ray tube using data read from ionization chamber Farmer 30013 without a build-up cap. Supplementary Figure 16 shows the function between the normalized dose rate and the thickness of the aluminum filters for these settings. Supplementary Table 7 shows the dose rates read with the help of the Farmer 30013 ionization chamber connected to the Unidoswebline electrometer, thicknesses of Al filters, and calculated HVL values with combined standard uncertainties. The combined standard uncertainties were calculated according to instructions in JCGM (2008).