Antimicrobial susceptibility of staphylococci from bovine milk samples in routine microbiological mastitis analysis in Finland.

The most frequent reason for antimicrobial use in dairy herds is mastitis and knowledge about mastitis-causing pathogens and their antimicrobial susceptibility should guide treatment decisions. The overall objective of this study was to assess antimicrobial resistance (AMR) of staphylococci in mastitic milk samples in Finland. MALDI-ToF MS identified a total of 504 Staphylococcus isolates (260 S. aureus and 244 non-aureus staphylococci, NAS) originating from bovine mastitic milk samples. Phenotypic susceptibility against cefoxitin, ceftiofur, enrofloxacin, gentamycin, oxacillin, penicillin, and tetracycline was evaluated by disk diffusion method and the presence of blaZ, mecA, and mecC genes investigated by PCR. Nitrocefin test assessed these isolates' beta-lactamase production. The most common NAS species were S. simulans, S. epidermidis, S. chromogenes, and S. haemolyticus. In total, 26.6% of the isolates (18.5% of S. aureus and 35.2% of all NAS) carried the blaZ gene. Penicillin resistance, based on disk diffusion, was lower: 18.8% of all the isolates (9.3% of S. aureus and 28.9% of all NAS) were resistant. Based on the nitrocefin test, 21.5% of the isolates produced beta-lactamase (11.6% of S. aureus and 32.0% of all NAS). Between the Staphylococcus species, the proportion of penicillin-resistant isolates varied, being lowest in S. simulans and highest in S. epidermidis. Resistance to antimicrobials other than penicillin was rare. Of the eight NAS isolates carrying the mecA gene, six were S. epidermidis. One S. aureus isolate carried the mecC gene. Agreement beyond chance, assessed by kappa coefficient, between phenotypic and genotypic resistance tests, was moderate to substantial. Some phenotypically penicillin-susceptible staphylococci carried the blaZ gene but isolates without blaZ or mec genes rarely exhibited resistance, suggesting that the more reliable treatment choice may depend upon genotypic AMR testing. Our results support earlier findings that penicillin resistance is the only significant form of antimicrobial resistance among mastitis-causing staphylococci in Finland.

On the property of measurements with the PTW microLion chamber in continuous beams.

PURPOSE: The performance of liquid ionization chambers, which may prove to be useful tools in the field of radiation dosimetry, is based on several chamber and liquid specific characteristics. The present work investigates the performance of the PTW microLion liquid ionization chamber with respect to recombination losses and perturbations from ambient electric fields at various dose rates in continuous beams. METHODS: In the investigation, experiments were performed using two microLion chambers, containing isooctane (C(8)H(18)) and tetramethylsilane [Si(CH(3))(4)] as the sensitive media, and a NACP-02 monitor chamber. An initial activity of approximately 250 GBq (18)F was employed as the radiation source in the experiments. The initial dose rate in each measurement series was estimated to 1.0 Gy min(-1) by Monte Carlo simulations and the measurements were carried out during the decay of the radioactive source. In the investigation of general recombination losses, employing the two-dose-rate method for continuous beams, the liquid ionization chambers were operated at polarizing voltages 25, 50, 100, 150, 200, and 300 V. Furthermore, measurements were also performed at 500 V polarizing voltage in the investigation of the sensitivity of the microLion chamber to ambient electric fields. RESULTS: The measurement results from the liquid ionization chambers, corrected for general recombination losses according to the two-dose-rate method for continuous beams, had a good agreement with the signal to dose linearity from the NACP-02 monitor chamber for general collection efficiencies above 70%. The results also displayed an agreement with the theoretical collection efficiencies according to the Greening theory, except for the liquid ionization chamber containing isooctane operated at 25 V. At lower dose rates, perturbations from ambient electric fields were found in the microLion chamber measurement results. Due to the perturbations, measurement results below an estimated dose rate of 0.2 Gy min(-1) were excluded from the present investigation of the general collection efficiency. The perturbations were found to be more pronounced when the chamber polarizing voltage was increased. CONCLUSIONS: By using the two-dose-rate method for continuous beams, comparable corrected ionization currents from experiments in low and medium energy photon beams can be achieved. However, the valid range of general collection efficiencies has been found to vary in a comparison between experiments performed in continuous beams of 120 kVp x ray, and the present investigation of 511 keV annihilation photons. At very high dose rates in continuous beams, there are presently no methods that can be used to correct for general recombination losses and at low dose rates the microLion chamber may be perturbed by ambient electric fields. Increasing the chamber polarizing voltage, which diminishes the general recombination effect, was found to increase the microLion chamber sensitivity to ambient electric fields. Prudence is thus advised when employing the microLion chamber in radiation dosimetry, as ambient electric fields of the strength observed in the present work may be found in many common situations. Due to uncertainties in the theoretical basis for recombination losses in liquids, further studies on the underlying theories for the initial and general recombination effect are needed if liquid ionization chambers are to become a viable option in high precision radiation dosimetry.

Initial recombination in the track of heavy charged particles: numerical solution for air filled ionization chambers.

INTRODUCTION: Modern particle therapy facilities enable sub-millimeter precision in dose deposition. Here, also ionization chambers (ICs) are used, which requires knowledge of the recombination effects. Up to now, recombination is corrected using phenomenological approaches for practical reasons. In this study the effect of the underlying dose distribution on columnar recombination, a quantitative model for initial recombination, is investigated. MATERIAL AND METHODS: Jaffé's theory, formulated in 1913 quantifies initial recombination by elemental processes, providing an analytical (closed) solution. Here, we investigate the effect of the underlying charged carrier distribution around a carbon ion track. The fundamental partial differential equation, formulated by Jaffé, is solved numerically taking into account more realistic charge carrier distributions by the use of a computer program (Gascoigne 3D). The investigated charge carrier distributions are based on track structure models, which follow a 1/r(2) behavior at larger radii and show a constant value at small radii. The results of the calculations are compared to the initial formulation and to data obtained in experiments using carbon ion beams. RESULTS: The comparison between the experimental data and the calculations shows that the initial approach made by Jaffé is able to reproduce the effects of initial recombination. The amorphous track structure based charge carrier distribution does not reproduce the experimental data well. A small additional correction in the assessment of the saturation current or charge is suggested by the data. CONCLUSION: The established model of columnar recombination reproduces the experimental data well, whereas the extensions using track structure models do not show such an agreement. Additionally, the effect of initial recombination on the saturation curve (i.e. Jaffé plot) does not follow a linear behavior as suggested by current dosimetry protocols, therefore higher order corrections (such as the investigated ones) might be necessary.

Correction for volume recombination in liquid ionization chambers at high dose-per-pulse.

PURPOSE: To determine the volume recombination at high dose-per-pulse in liquid ionization chambers (LIC) and to ascertain whether existing calculation methods verified in air-filled chambers may be used to calculate a correction factor. METHODS: Two LICs, one filled with 2,2,4-trimethylpentane (isooctane) the other with tetramethylsilane (TMS), were irradiated in a pulsed, 20 MeV electron beam. Via reference measurements with a Faraday cup, the saturation correction for volume recombination was determined for dose-per-pulse values ranging from about 5 mGy to 1 Gy for both chambers at a pulse duration of 693 ns. In addition, the isooctane chamber was irradiated with pulses of varying duration, ranging from 5 ps to 10 ms, at a dose-per-pulse of about 76.5 mGy. The dose-per-pulse-dependent measurements were compared to calculations based on Boag's models (with and without a free electron fraction), the two-dose-rate method, and a numerical calculation. The pulse duration dependent measurements were compared only to a numerical calculation that iteratively calculates the charge transport and loss in a 1D model of an ionization chamber. RESULTS: In TMS only Boag's model with a free electron fraction and the numerical calculation are in good agreement with the experimental data. However, in isooctane, good agreement is observed between the experimental data, the numerical calculation as well as the two-dose-rate method, and Boag's model including a free electron fraction. Only Boag's model without a free electron fraction shows a good agreement with lesser extend. Furthermore, the pulse duration-dependent data for isooctane are well described by the numerical model. CONCLUSION: With isooctane as an active medium, a LIC could be directly used in a field with high dose-per-pulse utilizing the well-established two-dose-rate method to correct the volume recombination. In addition, pulsed fields with variable pulse duration are easily modeled for this medium using a numerical calculation. Other media, as exemplified by the TMS-filled chamber, might require additional considerations, such as including a fraction of free electrons in the consideration of volume recombination.

Modeling ion recombination in liquid ionization chambers - Improvement and analysis of the two-dose-rate method.

PURPOSE: The use of liquid ionization chambers can provide useful information to endeavors with radiation dosimetry for highly modulated beams. Liquid ionization chambers may be particularly suitable for computed tomography applications where conventional ionization chambers do not present a high enough sensitivity for the spatial resolution required to characterize common X-ray beams. Due to the sensitivity, which leads to high charge densities, liquid ionization chambers can suffer from large recombination losses leading to degradation in signal to dose rate linearity. To solve this problem, a two-dose-rate method for general recombination correction has been proposed for liquid ionization chambers. However, the valid range of recombination losses that the method can accurately account for has been found to vary depending on radiation quality. The present work provides an in-depth analysis of the performance of the two-dose-rate method. Furthermore, the soundness of applying gas theory to liquids is investigated by using the two-dose-rate method. METHODS: In the present work, the two-dose-rate method for general recombination correction of liquid ionization chambers used in continuous beams is studied by employing theory for gas-filled ionization chambers. An approximate relation for the general collection efficiency containing a material-specific parameter that is traceable to liquids has been derived for theoretical and experimental investigation alongside existing theory. Furthermore, the disassociation between initial and general recombination in the method is analyzed both theoretically and experimentally. RESULTS: The results indicate that liquids and gases share general recombination characteristics, where the liquids investigated (isooctane and tetramethylsilane) to a large extent mimic the behavior theoretically expected in gases. Furthermore, it is shown that the disassociation between initial and general recombination in the two-dose-rate method is an approximation that depends on the relation between initial recombination and the collecting electric field strength at the dose rates used. CONCLUSIONS: Due to the approximation used to separate initial and general recombination the valid range of collection efficiencies for the two-dose-rate method will not only depend on the model used to describe general recombination but also on the type of liquid and radiation beam quality. As there is no robust theory for initial recombination in liquids to apply, the valid range of general collection efficiencies for the two-dose-rate method should be experimentally evaluated for each radiation dosimetry application.

A general algorithm for calculation of recombination losses in ionization chambers exposed to ion beams.

PURPOSE: Dosimetry with ionization chambers in clinical ion beams for radiation therapy requires correction for recombination effects. However, common radiation protocols discriminate between initial and general recombination and provide no universal correction method for the presence of both recombination types in ion beams of charged particles heavier than protons. The advent of multiple field optimization in ion beams, allowing for complex patterns of dose delivery in both temporal and spatial domains, results in new challenges for recombination correction where the resulting recombination depends on the plan delivered. Here, the authors present the open source code IonTracks version 1.0, where the combined initial and general recombination effects in principle can be predicted for any ion beam with arbitrary particle-energy spectrum and temporal structure. METHODS: IonTracks uses track structure theory to distribute the charge carriers in ion tracks. The charge carrier movements are governed by a pair of coupled differential equations, based on fundamental physical properties as charge carrier drift, diffusion, and recombination, which are solved numerically while the initial and general charge carrier recombination is computed. A space charge screening of the electric field is taken into account and the algorithm furthermore allows an inclusion of a free-electron component. RESULTS: The algorithm is numerically stable and in accordance with experimentally validated theories for initial recombination in heavy ion tracks and general recombination in a proton beam. CONCLUSIONS: Given IonTracks' ability to handle arbitrary inputs, IonTracks can in principle be applied to any complex particle field in the spatial and temporal domain. IonTracks is validated against the Jaffé's and Boag's theory of recombination in pulsed beams of multiple ion species. IonTracks is able to calculate the correction factor for initial and general recombination losses in parallel-plate ionization chambers. Even if only few experimental data on recombination effects in ionization chambers are available today, the universal concept of IonTracks is not limited to the ions investigated here. Future experimental investigations of recombination in pulsed and possibly also continuous ion beams may be conducted with IonTracks, which ultimately may lead to a more precise prediction of recombination factors in complex radiation fields.

Liquid ionization chamber initial recombination dependence on LET for electrons and photons.

The possibility of indirect measurements of linear energy transfer (LET) with a liquid ionization chamber (LIC) has been investigated by studying initial recombination losses at different applied voltages. A linear fit is made to the voltage-signal curve and the intersection point of the fit and the voltage-axis is shown to correlate with LET. The LIC applied voltages were 100-700 V, which corresponds to electric field strengths between 0.3 and 2.0 MV m(-1). Several different photon and electron beams have been studied, and by using MCNPX™ the respective LET spectra have been determined. The beam qualities in this study were found to have a fluence averaged LET between 0.17 and 1.67 keV µm(-1) and a corresponding dose averaged LET between 0.97 and 4.62 keV µm(-1). For the experimental data in this study the linear fit method yields consistent results with respect to Monte Carlo simulated LET values. A calibration curve for LET determination is provided for the LIC used in the present work.

A comparison of different experimental methods for general recombination correction for liquid ionization chambers.

Radiation dosimetry of highly modulated dose distributions requires a detector with a high spatial resolution. Liquid filled ionization chambers (LICs) have the potential to become a valuable tool for the characterization of such radiation fields. However, the effect of an increased recombination of the charge carriers, as compared to using air as the sensitive medium has to be corrected for. Due to the presence of initial recombination in LICs, the correction for general recombination losses is more complicated than for air-filled ionization chambers. In the present work, recently published experimental methods for general recombination correction for LICs are compared and investigated for both pulsed and continuous beams. The experimental methods are all based on one of two approaches: either measurements at two different dose rates (two-dose-rate methods), or measurements at three different LIC polarizing voltages (three-voltage methods). In a comparison with the two-dose-rate methods, the three-voltage methods fail to achieve accurate corrections in several instances, predominantly at low polarizing voltages and dose rates. However, for continuous beams in the range of polarizing voltages recommended by the manufacturer of the LICs used, the agreement between the different methods is generally within the experimental uncertainties. For pulsed beams, the agreement between the methods is poor. The inaccuracies found in the results from the three-voltage methods are associated with numerical difficulties in solving the resulting equation systems, which also make these methods sensitive to small variations in the experimental data. These issues are more pronounced for the case of pulsed beams. Furthermore, the results suggest that the theoretical modelling of initial recombination used in the three-voltage methods may be a contributing factor to the deviating results observed.

Application of the two-dose-rate method for general recombination correction for liquid ionization chambers in continuous beams.

A method to correct for the general recombination losses for liquid ionization chambers in continuous beams has been developed. The proposed method has been derived from Greening's theory for continuous beams and is based on measuring the signal from a liquid ionization chamber and an air filled monitor ionization chamber at two different dose rates. The method has been tested with two plane parallel liquid ionization chambers in a continuous radiation x-ray beam with a tube voltage of 120 kV and with dose rates between 2 and 13 Gy min(-1). The liquids used as sensitive media in the chambers were isooctane (C(8)H(18)) and tetramethylsilane (Si(CH(3))(4)). The general recombination effect was studied using chamber polarizing voltages of 100, 300, 500, 700 and 900 V for both liquids. The relative standard deviation of the results for the collection efficiency with respect to general recombination was found to be a maximum of 0.7% for isooctane and 2.4% for tetramethylsilane. The results are in excellent agreement with Greening's theory for collection efficiencies over 90%. The measured and corrected signals from the liquid ionization chambers used in this work are in very good agreement with the air filled monitor chamber with respect to signal to dose linearity.

A two-dose-rate method for general recombination correction for liquid ionization chambers in pulsed beams.

The correction for general recombination losses in liquid ionization chambers (LICs) is more complex than that in air-filled ionization chambers. The reason for this is that the saturation charge in LICs, i.e. the charge that escapes initial recombination, depends on the applied voltage. This paper presents a method, based on measurements at two different dose rates in a pulsed beam, for general recombination correction in LICs. The Boag theory for pulsed beams is used and the collection efficiency is determined by numerical methods which are equivalent to the two-voltage method used in dosimetry with air-filled ionization chambers. The method has been tested in experiments in water in a 20 MeV electron beam using two LICs filled with isooctane and tetramethylsilane. The dose per pulse in the electron beam was varied between 0.1 mGy/pulse and 8 mGy/pulse. The relative standard deviations of the collection efficiencies determined with the two-dose-rate method ranged between 0.1% and 1.5%. The dose-rate variations of the general recombination corrected charge measured with the LICs are in excellent agreement with the corresponding values obtained with an air-filled plane parallel ionization chamber.