Microdosimetric Measurements Research Articles

Generalization of the variance-covariance method for microdosimetric measurements

Generalization of the variance-covariance method for microdosimetric measurements

Generalization of the variance-covariance method for microdosimetric measurements. I. Basic equations and estimation formulae for constant and time-varying fields.

The variance method of microdosimetric measurements and its extension, the variance-covariance method, permit the determination of an essential parameter of radiation quality, the dose mean event size, y(d). The methods have--among other advantages--the feature that they permit measurements for smaller simulated sites than the conventional single-event technique. It is, therefore, desirable to employ them also for the determination of further moments of the distribution of y. The formulae for the first three moments are here derived both for the case of constant dose rate and of fluctuating dose rates. A second article will use the same mathematical approach to deduce formulae that remain valid even if there are slow changes of the ratio of dose rates in the two detectors for the variance-covariance method. A third article will explore--in terms of microdosimetric data--the applicability of the formulae.

Generalization of the variance-covariance method for microdosimetric measurements. II. Formulae for varying dose-rate ratio in the detectors and synopsis of results.

The variance method of microdosimetric measurements has been used to obtain the dose average event size, y(d), of a radiation from a series of repeated measurements at equal doses. Since it is not always possible to repeat the same dose in multiple measurements, a generalization, the variance-covariance method, has supplanted the earlier technique. It utilizes a pair of synchronized detectors and permits the derivation of y(d) from a series of measurements with doses that are allowed to fluctuate. In the preceding article, the two methods were extended to permit also the derivation of the second and third moments, y2 and y3, of the event sizes. The present communication carries the extension further by deriving formulae that remain valid when the ratio of the doses in the two detectors is subject to slow changes; this extends the applicability of the twin detector method to situations in which the detector system moves in a nonuniform field, or in which there is movement of shielding or scattering material near the detectors.

An estimation of the relative biological effectiveness of 50 MV bremsstrahlung beams by microdosimetric techniques

An efficient algorithm has been developed to estimate the relative biological effectiveness (RBE) from experimental RBE data and measured single-event energy deposition spectra. As benchmarks for the calculations well established RBE data for crypt cell survival and associated microdosimetric distributions from a number of therapeutic high- and low-LET beams was used. As a by-product of the calculations the RBE for this system was determined for the lineal energy range 0.1 - 5000 keV in very good agreement with experimental data. These data have been applied to estimate the biological effectiveness of therapeutic high-energy bremsstrahlung beams characterized by microdosimetric measurements with a wall-less proportional counter. The absorbed dose component due to photoneutrons and charged particles from photonuclear reactions in scanned 50 MV bremsstrahlung beams was measured to be about 2% of the total absorbed dose. The RBE of 50 MV scanned bremsstrahlung beams was estimated to be 1.09 for jejunum crypt cell survival as the biological endpoint at absorbed doses of the order of 10 Gy, in fair agreement with reported results based on radiobiological experiments. The RBE is fairly independent of the bremsstrahlung target used. Using the estimated increase in the RBE of neutrons compared to photons for absorbed dose levels applied in clinical fractionation schedules increases the RBE further to 1.13 at absorbed doses per fraction of the order of 2 Gy.

Effect of wire eccentricity on the performance of cylindrical proportional counters.

A theoretical and experimental investigation of the influence of eccentricity of the multiplication wire on the performance of cylindrical proportional counters is presented. The electric field in the counter is calculated by the method of images, and the Townsend formalism is used to derive the gas gain. The experimental determination of detector performance is carried out with 37Ar. The dependence of the gas gain and of the counter resolution on eccentricity is discussed, and it is shown that eccentricities up to 0.2 are of no concern in microdosimetric measurements with cylindrical proportional counters.

86 - Microdosimetric measurements in the high energy proton cancer therapy beams at Clatterbridge and Orsay

86 - Microdosimetric measurements in the high energy proton cancer therapy beams at Clatterbridge and Orsay

An Evaluation of Hydrogen as a TEPC Counting Gas in Radiation Protection Microdosimetry

Tissue-equivalent proportional counters (TEPC) filled with conventional tissue-equivalent gases have a significantly low neutron dose equivalent response in the neutron energy region between thermal and about 200 keV. Theoretical modelling for monoenergetic neutron fields suggests that using pure hydrogen as a microdosimetric counting gas would improve the dose equivalent response. Average neutron dose equivalent response calculations have been done for selected realistic broad energy neutron fields and are found generally to support this suggestion. Gas gain investigations performed with conventional tissue-equivalent gases and hydrogen indicate that hydrogen has only a limited region of applied voltage, up to a reduced electric field at the anode of about 600 V.cm −1 . torr −1 , where the counter operates in the proportional mode. Conventional tissue-equivalent gases can be used up to about 3000 V.cm −1 . torr −1 . Microdosimetric measurements performed with hydrogen as the counting gas nevertheless demonstrate that it is suitable for measuring event-size spectra and carrying out quantitative dosimetry. The non-tissue-equivalent nature of hydrogen, however, does present some difficulties in calibration and interpretation of the measured spectra. Taking into account the non-tissue-equivalence and the poor gas gain characteristics of hydrogen the advantage of using this counting gas, at least in its unadulterated form, appears marginal. However, if TEPC ambient dose equivalent responses are found to be lower than that predicted by the theoretical model used here for workplace neutron fields, then it may become an attractive option to use hydrogen. In this case effort will be required to improve the gas gain characteristic of hydrogen by investigating the effect of small admixtures of quenching gases

Variance-Covariance Measurements in Therapeutic and Diagnostic Radiation Beams

Abstract Cylindrical tissue-equivalent twin ionisation chambers have been developed based on experience with proportional counters for application in microdosimetric variance-covariance measurements of the dose mean lineal energy, yD, in therapeutic and diagnostic radiation beam qualities. It is concluded that the method is applicable but the size of the detector signals has to be optimised very carefully in respect to noise reduction and the total relative variance. It is proposed that results from the proportional mode and the ionisation mode are compared to obtain variance. It is proposed that results from the proportional mode and the ionisation mode are compared to obtain reliable values for the microdosimetric beam quality characterisation necessary to assess clinically relevant differences.

Radiobiological Effectiveness of Radiation Beams with Broad LET Spectra: Microdosimetric Analysis Using Biological Weighting Functions

The evaluation of averaged parameters from microdosimetric spectra, measured with proportional counters and biological weighting functions of the quantity lineal energy, provides a practical solution to the specification of radiation quality, especially for radiations with broad LET spectra, such as those used for radiation therapy. This approach has already proved satisfactory for fast neutron therapy beams of different energy using a biological weighting function numerically determined on empirical and statistical bases using the results of a RBE-microdosimetry intercomparison study. The paper discusses the applicability of this procedure by extending the range of radiations to fast protons. For this purpose, the earlier neutron data were combined with preliminary results from RBE experiments and microdosimetric measurements performed within the framework of two proton therapy programmes. Different weighting functions were derived by unfolding calculations applied to data sets combining high energy gamma rays, fast neutrons and fast protons.

Microdosimetric description of beam quality and biological effectiveness in radiation therapy.

Modern radiation therapy includes dosimetric and biological optimization methods aiming to improve the tumour control probability and reduce the unwanted reactions in healthy tissues. From this point of view the quality of the radiation beam is an important parameter that is not generally taken into account in clinical practice. The beam quality depends largely on the macroscopic absorption and scatter of the incident radiation but also on the microscopic fluctuations in the specific energy imparted within the cell nuclei in the patient. The radiation effect depends also on the complexity of the induced damage, the ability of the cells to control cell cycle progress and the efficiency and fidelity of the repair system. Therefore the conventional macrodosimetric quantities have to be complemented with microdosimetric quantities for an accurate description of the quality properties of the radiation beam. It is demonstrated that the DNA damage produced by sub keV electrons and high LET particles has a high probability to be lethal for the cell. These electrons may generate closely spaced double strand breaks or more general 'multiply damaged sites'. Such severely damaged sites may partly be due to the geometrical arrangement of double coiled DNA on the nucleosomes or triple coiled DNA in the cromatin fibre. This kind of damage has a large probability to be misrepaired when the DNA is opened up for repair and may therefore later result in cell death. Furthermore, it is shown that the reduced biological effect at ultra high LETs (> 200 eV/nm) and at ultra short pulses of high dose rate low LET radiation most likely is due to increased radical-radical recombination in the 10 nm-10 ns domain. Based on microdosimetric measurements, significant quality variations in conventional therapeutic beams are detected particularly close to inhomogeneities, in the build-up region or in the presence of high LET contamination. These variations influence the biological effectiveness of the radiation beam and the steepness of the dose-effect relation, thus affecting in some cases the clinical radiotherapeutic outcome. It is shown that such effects may also explain the limited success of most trials with high LET radiations. A serious consideration of beam quality variations in treatment planning algorithms combined with dosimetric and radiobiological optimization of the treatment techniques may increase the probability of tumour control and minimize the unwanted acute and late reactions in healthy normal tissues.

Microdosimetric measurements in the build-up region of very pure high-energy photon and electron beams

A microdosimetric system, specially designed to allow measurements in both the presence and the absence of secondary charged particle equilibrium, has been used to investigate the build-up region of a clean 15 MV bremsstrahlung beam and a 21 MeV almost monoenergetic electron beam. The use of quasi-broad-beam conditions with a split tissue-equivalent phantom and a wall-less chamber surrounded by a low-pressure tissue-equivalent gas minimizes the fluence perturbation, caused by the detector. Along with microdosimetric beam quality data, ordinary dosimetric information can be obtained, together allowing estimation of the properties of the beam with regard to radiation response in tissue. By applying an inversion algorithm on the measured microdosimetric distributions to correct for plural events, it is shown that already at a probability of coinciding pulses as low as 1% the dose mean value of the single-event distributions increases by 3%. The dose mean lineal energy for the pure bremsstrahlung beam is strongly affected by both photon contamination and in-phantom scattering thus being 55% higher close to the surface than near the dose maximum.

Specification of Fast Neutron Radiation Quality from Cell Transformation Data

Experimental data for neoplastic transformation of C3H 10 1 / 2 cells measured at Casaccia after neutron and X ray irradiation have been used to determine neutron RBE values for the RSV-Tapiro fast reactor energy spectrum and for monoenergetic neutrons of 0.5, 1, and 6 MeV. In parallel, microdosimetric measurements have provided the actual lineal energy distributions and related mean parameters for the reactor radiation. From these experiments, values of the neutron quality factor were derived for the reactor neutron energy spectrum and, in turn, for the other neutron energies tested. A mathematical expression giving a smooth dependence on neutron energy was also determined for the effective quality factor in the entire energy range examined, and the results have been compared with other proposals

Microdosimetry of Diagnostic X Rays: Applications of the Variance-Covariance Method

Microdosimetric measurements in beams of diagnostic X rays (between 30 and 125 kV) have been performed. In these pulsed radiation fields, microdosimetric measurements are possible only by application of the variance-covariance technique. The dose mean lineal energy, yD, is determined for various simulated diameters, at different depths in the absorber, and at different points within the pulse intervals. From the measured temporal dependences one can also obtain values of yD for different X-ray pulse generators. The results demonstrate the potential of the variance-covariance method for a diversity of microdosimetric measurements in radiation protection and in the quality control of radiation beams.

Microdosimetric measurements of heavy ion tracks.

The biological effectiveness of radiations depends on the spatial pattern of ionizations and excitations produced by the charged particle tracks involved. Ionizations produced by both the primary ion and by energetic delta rays may contribute to the production of biologically relevant damage and to the concentration of damage which may effect the probability of repair. Although average energy concentration (dose) can be calculated using homogeneous track models, the energy is actually concentrated in small volumes containing segments of the ion and delta ray tracks. These local concentrations are studied experimentally using low pressure proportional counters, and theoretically, using Monte Carlo methods. Small volumes near an ion track may be traversed by a delta ray. If they are, the energy deposited will be similar to that produced by a single electron track in a low-energy x-ray irradiation. The probability of a delta ray interaction occurring decreases with the square of the radial distance from the track. The average energy deposited is the product of this probability and the energy deposited in an interaction. Average energy deposited calculated from measured interaction probability is in good agreement with the results of homogeneous track models.

A Coaxial Double Cylindrical TEPC for the Microdosimetry of Selected Neutron Energy Bands in Mixed Fields of Fast Neutrons

Abstract A new low pressure tissue-equivalent proportional counter (TEPC) in a co axial double cylindrical form has been developed to measure separately the microdose spectrum from any desired energy band of neutrons in the presence of mixed fields of faster neutrons, by selecting the thickness of the common TE dividing wall to be equivalent to the corresponding maximum proton ranges and by appropriate use of coincedence/anti-coincidence pulse arrangements. This thickness ensures charged particle equilibrium for the relevant neutron energy. Event spectra due to recoils generated by faster neutrons which interact with both the counters are removed completely by anti-coincidence techniques, thereby optimising the sensitivity of the inner microdosemeter to the event spectra of interest. The ability of this counter to discriminate in favour of events due to neutrons of energy <850 keV was achieved in microdosimetric measurements from mixed fields of a nuclear reactor. Mean values of lineal energy and quality factor for neutrons of energy <850 keV from a nuclear reactor were determined from the anti-coincidence spectrum. Good discrimination against ? ray induced events is also achieved for the spectrum recorded in the anti-coincidence mode. This is an advantageous feature for other applications and requires further investigation.

A Coaxial Double Cylindrical TEPC for the Microdosimetry of Selected Neutron Energy Bands in Mixed Fields of Fast Neutrons

A new low pressure tissue-equivalent proportional counter (TEPC) in a co axial double cylindrical form has been developed to measure separately the microdose spectrum from any desired energy band of neutrons in the presence of mixed fields of faster neutrons, by selecting the thickness of the common TE dividing wall to be equivalent to the corresponding maximum proton ranges and by appropriate use of coincedence/anti-coincidence pulse arrangements. This thickness ensures charged particle equilibrium for the relevant neutron energy. Event spectra due to recoils generated by faster neutrons which interact with both the counters are removed completely by anti-coincidence techniques, thereby optimising the sensitivity of the inner microdosemeter to the event spectra of interest. The ability of this counter to discriminate in favour of events due to neutrons of energy <850 keV was achieved in microdosimetric measurements from mixed fields of a nuclear reactor. Mean values of lineal energy and quality factor for neutrons of energy <850 keV from a nuclear reactor were determined from the anti-coincidence spectrum. Good discrimination against ? ray induced events is also achieved for the spectrum recorded in the anti-coincidence mode. This is an advantageous feature for other applications and requires further investigation.

The Role of Microdosimetry in Radiotherapy

Abstract Some of the practical problems encountered in radiation therapy, and in which microdosimetry has contributed or may contribute towards solutions, are reviewed. Microdosimetric methods are especially useful for high LET radiation therapy due to the requirement of quantitative specification of radiation quality. In neutron therapy, a large number of microdosimetric measurements have been carried out including a systematic set of measurements at European treatment facilities. The correlation with available radiobiological data enabled a pragmatic solution for the specification of radiation quality for fast neutron therapy to be established. Heavy ion therapy requires several parameters to be optimised as a function of the individual clinical situation, the type of particles, the particle energy and the width of the spread-out Bragg peak. The experience gained in fast neutron therapy may provide a suitable basis for employing microdosimetric methods for the specification of radiation quality also in this field. Results of microdosimetric measurements at pi-meson therapy facilities are briefly reported. The measured spectra reveal the presence of a large low LET dose component and furthermore a marked dependence on irradiation parameters. Finally, microdosimetry provides useful information also in the field of low LET radiation therapy, for example in the context of selecting the clinical RBE of 200 kv X-rays and 125I relative to 60Co gamma rays.

The Effect of Geometrical Scaling on the Gas Gain of Proportional Counters Intended for Microdosimetric Measurements

Measurements of the relative gas gain of three cylindrical proportional counters covering two orders of magnitude in diameter have been carried out using two types of tissue-equivalent gas commonly employed in microdosimetry. Gas pressures have been used such that each counter simulated unit density volumes 0.5, 2 and 5 µm in diameter. In general, the results are consistent with the assumption that the scaling of proportional counters in terms of physical size and gas pressure does not significantly alter their operational characteristics. Actual gas gains were found to increase with decreasing counter size and hence increasing pressure for all simulated diameters. Other constructional factors, however, are likely to have more significance in counter design, particularly for small counters a few millimetres in size.

The Effect of Geometrical Scaling on the Gas Gain of Proportional Counters Intended for Microdosimetric Measurements

The Effect of Geometrical Scaling on the Gas Gain of Proportional Counters Intended for Microdosimetric Measurements

The Application of Microdosimetric Measurements to the Study of the Dosimetry of Low Energy X Rays

The Application of Microdosimetric Measurements to the Study of the Dosimetry of Low Energy X Rays