Paul Kliauga

Microdosimetric evaluation of relative biological effectiveness for 103Pd, 125I, 241Am, and 192Ir brachytherapy sources

PURPOSE To determine the microdosimetric-derived relative biological effectiveness (RBE) of 103Pd, 125I, 241Am, and 192Ir brachytherapy sources at low doses and/or low dose rates. METHODS AND MATERIALS The Theory of Dual Radiation Action can be used to predict expected RBE values based on the spatial distribution of energy deposition at microscopic levels from these sources. Single-event lineal energy spectra for these isotopes have been obtained both experimentally and theoretically. A grid-defined wall-less proportional counter was used to measure the lineal energy distributions. Unlike conventional Rossi proportional counters, the counter used in these measurements has a conducting nylon fiber as the central collecting anode and has no metal parts. Thus, the Z-dependence of the photoelectric effect is eliminated as a source of measurement error. Single-event spectra for these brachytherapy sources have been also calculated by: (a) the Monte Carlo code MCNP to generate the electron slowing down spectrum, (b) transport of monoenergetic electron tracks, event by event, with our Monte Carlo code DELTA, (c) using the concept of associated volume to obtain the lineal energy distribution f(y) for each monoenergetic electron, and (d) obtaining the composite lineal energy spectrum for a given brachytherapy source based on the electron spectrum calculated at step (a). RESULTS Relative to 60Co, the RBE values obtained from this study are: 2.3 for 103Pd, 2.1 for 125I, 2.1 for 241Am, and 1.3 for 192Ir. CONCLUSIONS These values are consistent with available data from in vitro cell survival experiments. We suggest that, at least for these brachytherapy sources, microdosimetry may be used as a credible alternative to time-consuming (and often uncertain) radiobiological experiments to obtain information on radiation quality and make reliable predictions of RBE in low dose rate brachytherapy.

Dose rate to the inner ear during Mossbauer 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.

The relative biological effectiveness of 160 MeV protons I. Microdosimetry

Abstract Measurements of the distribution of lineal energy density, the microdosimetric parameter analogous to linear energy transfer (LET), were performed on the 160 MeV proton beam at the Harvard Cyclotron. Measurements at various positions on the extended Bragg peak produced by a beam modulator are compared with each other and with measurements on an unmodulated beam. The results are compared with computer-generated spectra produced by using a simple model to simulate the action of the beam modulator. The spectra are found to be highly sensitive to position at the extreme distal portion of the extended Bragg peak. The effective dose mean lineal energy density (corrected for saturation) increases by ∼ 20% from the front to center of the extended peak region, and increases by a factor of two from center to rear. Thus, relative biological eflectiveness RBE may be a function of position, and biological effects in an extended specimen may be enhanced in a slab a few millimeters thick located at the distal portion of the extended Bragg peak.

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.

Microdosimetric analysis of radiation from a clinical mammography machine using realistic breast phantoms and a miniature proportional counter

We have measured the microdosimetric spectra of a Senographe 600T mammography machine employing an Mo target with 0.8 mm Be inherent filtration and 0.03 mm Mo added filtration, giving a half-value layer of 0.35 mm A1 at 28 kVp. In all of our measurements a large collimator producing a 24 cm x 30 cm field at 65 cm was used. Two different phantom compositions differing in the ratio of adipose to fibroglandular tissue were compared, using simulated breast material from Nuclear Associates. Spectra were taken at various depths and locations in simulated breasts of 3.4 and 5 cm thickness. The detector used was a miniature proportional counter having outer dimensions of 5 cm x 1.8 cm diameter, with a sensitive volume 0.5 mm x 0.5 mm. The small dimensions of the counter and the cavity allowed total embedding in the breast material with minimal disturbance of the photon and secondary electron spectrum. Our results show that there can be changes in the radiation quality amounting to as much as 17% (as measured by the dose mean lineal energy. yD) between breasts of different thickness, at the same relative position within the breast. There is little difference due to breast composition.

Measurements of Lineal Energy Spectra for Neutron Capture Therapy Using a Boron Doped Let Chamber

Preclinical studies for boron neutron capture therapy (BNCT) using epithermal neutrons are ongoing at several laboratories. The absorbed dose in tumor cells depends on the boron concentration, thermal neutron flux at depth, size of the cell, plus fast neutron and gamma contamination in the epithermal beam. Monte Carlo computer simulations can estimate the various dose components, but dosimetry and treatment planning for BNCT present unique difficulties. Dosimetry is complicated by the admixture of thermal, epithermal, and fast neutrons, plus gamma rays; and the array of secondary high linear energy transfer (LET) particles produced within the patient from neutron interactions. Absorbed dose and radiation quality will be difficult to determine, and microdosimetry may be a viable technique for determining these quantities. Only one set of microdosimetric data for BNCT has previously been reportedl. Spectra were measured in air for an epithermal beam, using a tissue equivalent (TE) proportional counter to assess the effects of beam filter designs. We report here the first set of in phantom microdosimetric measurements using paired TE and TE+boron chambers on an epithermal beam. Such measurements permit assessment of the dose enhancement factor, and lineal energy distributions of the boron capture reaction.

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.

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.

Use of the variance-covariance technique for determination of the radiation quality in a pulsed linear accelerator

Abstract Recent recommendations of a joint task group of the ICRU and ICRP have established the microdosimetric quantity “lineal energy” as the parameter of choice for determination of the radiation quality for purposes of radiation protection. Unlike linear energy transfer (LET or stopping power), lineal energy can be measured directly for tissue-equivalent volumes with dimensions of relevance to radiobiology (of the order of 1 μm). Conventional proportional-counter spectroscopy, the method most frequently utilized for determination of linear energy spectra, is impractical for short-duty cycle pulsed machines. A technique for determination of the dose mean lineal energy, utilizing the relative variance and covariance of two proportional counters operated in synchrony with the beam pulse, is described. Design and construction of a new ultraminiature proportional counter for use in high-dose-rate fields is described. This extremely small cross section (0.25 mm2) counter should make multi-event measurements practical in the beam of a pulsed linear electron accelerator at full dose rate and single-event measurements at slightly reduced intensities; this in turn should make possible the microdosimetric determination of photoneutron contamination in megavoltage photon beams.