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Biomedical subjects

P Kliauga

Publications and source records attributed to P Kliauga.

13 recordsLinked to original sources
PubMed1995

Photoneutrons from high energy medical linear accelerators: measurement of the spectrum and dose using a miniature proportional counter.

PURPOSE: A new method of measuring photoneutron dose to the patient during treatment with high energy photon or electron beams is presented. This method has the advantage of providing not only the dose, but the microdosimetric spectrum at the same time. METHODS AND MATERIALS: A miniature cylindrical gas proportional counter (0.5 mm diameter by 0.5 mm height) has been used to measure scatter radiation from a 20 MV teletherapy photon beam. At atmospheric pressure, filled with propane base tissue equivalent gas, this counter simulates a unit density tissue region of approximately 0.9 microns. We present here single event microdosimetric spectra measured outside the primary beam 1.4 m from the target. This technique allows a single measurement to determine the scattered dose due to gammas and photoneutron contamination, as well as the quality factor of the photoneutrons. RESULTS: Spectral components from scattered photons and the photoneutrons are easily separated, and dose contributions can be estimated. The ratio of photoneutron dose measured by the proportional counter to photon dose at isocenter is 0.75 x 10(-4). CONCLUSIONS: Neutron dose was also measured using a bubble neutrometer. The proportional counter and neutrometer agree within experimental errors. This type of instrument is shown to be a viable technique for determination of exposure of patient and also personnel to photoneutrons, providing not only a dose determination, but also a spectrum that can be used to estimate quality factors for equivalent dose. Its main drawback is that it requires a lengthy (several hours) measurement because of low count rate of the neutrons.

Humans↗
PubMed1992

Microdosimetry for boron neutron capture therapy.

Preclinical studies for boron neutron capture therapy (BNCT) using epithermal neutrons are ongoing at several laboratories. The absorbed dose in tumor cells is a function of the thermal neutron flux at depth, the microscopic boron concentration, and the size of the cell. Dosimetry is therefore complicated by the admixture of thermal, epithermal, and fast neutrons, plus gamma rays, and the array of secondary high-linear-energy-transfer particles produced within the patient from neutron interactions. Microdosimetry can be a viable technique for determining absorbed dose and radiation quality. A 2.5-cm-diameter tissue-equivalent gas proportional counter has been built with 50 parts per million (ppm) 10B incorporated into the walls and counting gas to simulate the boron uptake anticipated in tumors. Measurements of lineal energy (y) spectra for BNCT in simulated volumes of 1-10 microns diameter show a dose enhancement factor of 4.3 for 30 ppm boron, and a "y" of 250 keV/microns for the boron capture process. Chamber design plus details of experimental and calculated linear energy spectra will be presented.

Boron↗
PubMed1990

Microdosimetry at middle age: some old experimental problems and new aspirations.

The increasingly wider use of microdosimetry in fields other than pure radiobiological research, especially in "practical" domains such as radiation protection and medical physics, has created a number of new practitioners who are not fully acquainted with many of the experimental pitfalls which beset the aspiring microdosimetrist. This paper attempts to review some of the lesser known experimental obstacles. It also presents first results of measurements of single-event spectra at nanometer site sizes, based on the use of a small (0.5 x 0.5 mm) counter. Some observations and a critique of the operation of this counter are also presented. It is shown that the counter produces spectra at 5 nm which can be compared with theoretical predictions grounded on fundamental avalanche theory for a cylindrical counter.

Radiometry↗
PubMed1989

Microdosimetric measurements and the variance-covariance method. Some experimental experience.

Systematic and statistical uncertainties in the variance-covariance method have been investigated. Two spherical wall-less detectors have been used to determine the dose mean lineal energy (yD) in a neutron beam of 5.7 MeV produced by a Van de Graaff accelerator. It is shown that certain systematic uncertainties influenced the mean yD of the two detectors much less than yD from only one of them. A statistical uncertainty of 6% (95% confidence level) was achieved if yD was calculated from 2000 measurements. In this particular experiment insufficient shieldings of the preamplifiers positioned in the beam turned out to limit the possibility to measure below 20 nm.

Analysis of Variance↗
PubMed1989

Microdosimetry of a 42 MeV therapy neutron beam.

Radiation quality of fast neutron therapy beams can change with depth. For a neutron beam generated by p----Be, the primary effect is a "hardening" of the neutron beam produced by the scattering of slow neutron components. A 42 MeV neutron therapy beam was investigated using microdosimetric techniques. We report measurements of dose mean and saturation-corrected lineal energy in water phantom at build-up depth, 5, 15, and 30 cm, and show representative logarithmic dose distributions in lineal energy. RBE is also calculated using assumptions based on the Theory of Dual Radiation Action and presented as a function of dose and depth.

Humans↗
PubMed1989

A multi-element proportional counter for radiation protection measurements.

A detector incorporating about 300 individual counting volumes is described, and the results of performance tests are reported. The device can be employed for a direct measurement of the dose equivalent in an unspecified radiation field on the basis of the lineal energy spectrum in 1-micron diameter tissue regions. It is substantially smaller than a conventional tissue equivalent proportional counter yielding the same counting rate and may be useful for measurements in phantoms.

Radiometry↗
PubMed1986

Microdosimetry of pulsed radiation fields employing the variance method.

The relationship between dose mean lineal energy and relative variance has been exploited previously to derive yD from the calculated variance in current measurements in steady and uniform radiation fields. Recently Kellerer and Rossi made the observation that utilization of two detectors can make the variance technique practicable in time-varying fields. We report here the first measurements of yD for 10 MeV X rays and 9 and 18 MeV electrons from a pulsed linear accelerator using the variance method. Two independent analog-to-digital converters were used to obtain data from two spherical proportional counters in synchrony with the beam pulse. The method is described in detail and results are reported for site diameters of 1/2, 1, and 2 microns. Data for an accurate determination of yD can be obtained with this technique in less than 1 min, making possible an essentially "on line" determination of yD or zD in a clinical situation.

Electrons↗
PubMed1984

Microdosimetry and thermoluminescence.

The mechanism for production of light from a thermoluminescent phosphor requires the combination of a trapped electron and hole at a luminescent center. The kinetics is that of a second-order process. Accordingly one may expect a similarity in the formalism for describing thermoluminescent emission (TL) and the theory of dual radiation action in radiobiology (TDRA). The TDRA was developed to provide a framework for describing the effects of ionizing radiation on biological systems. It requires the interaction of two primary units of radiation damage called "sublesions" to form a "lesion" which may then be expressed as a biological effect. The physical parameters used in the TDRA come from microdosimetry, which deals with the deposition of energy by ionizing radiation in domains whose dimensions are of the order of micrometers. This paper explores the relationships between microdosimetric concepts and a description of TL properties, such as TL dose-response curves and TL LET dependence. This is then compared with similar quantities in biology to determine the possible relevance of TL as a biological model.

Cell Survival↗
PubMed1983

Dose rate to the inner ear during Mössbauer experiments.

The most widely used technique for studying vibrations of the inner ear utilises the Mössbauer effect; this requires placement of a radioactive source on the basilar membrane. This source, although small in size and less than 37 MBq (1 mCi) in strength, is placed in close proximity to sensitive receptor cells. Using a series solution for the radiation field of a rectangular source the absorbed dose rate delivered to receptor cells at various depths and at points off-axis from the centre of the source is calculated. It is concluded that the dose delivered during the course of a Mössbauer experiment may well be sufficient to damage receptor cells and cause a loss of response.

Basilar Membrane↗
PubMedDate not supplied

Measurement of the dose equivalent of leakage radiation through an isocentric gantry used for neutron therapy.

The leakage radiation through the shielding on an isocentric gantry of a neutron therapy machine was measured with a Rossi-type proportional counter. The dose equivalent of the leakage radiation was determined at two positions: (1) in the plane of the patient and (2) in the plane of the target. The dose equivalent of the leakage radiation is approximately the same as the leakage of a high-energy x-ray linac.

Humans↗
PubMedDate not supplied

Evaluation of an iron-filtered epithermal neutron beam for neutron-capture therapy.

An epithermal neutron filter using iron, aluminum, and sulfur was evaluated to determine if the therapeutic performance could be improved with respect to aluminum-sulfur-based filters. An empirically optimized filter was developed that delivered a 93% pure beam of 24-keV epithermal neutrons. It was expected that a thick filter using iron with a density thickness greater than 200 g/cm2 would eliminate the excess gamma contamination found in Al-S filters. This research showed that prompt gamma production from neutron interactions in iron was the dominant dose component. Dosimetric parameters of the beam were determined from the measurement of absorbed dose in air, thermal neutron flux in a head phantom, neutron and gamma spectroscopy, and microdosimetry.

Brain Neoplasms↗