Microdosimetric spectra of the THOR neutron beam for boron neutron capture therapy.

A primary objective of the BNCT project in Taiwan, involving THOR (Tsing Hua Open Pool Reactor), was to examine the potential treatment of hepatoma. To characterise the epithermal neutron beam in THOR, the microdosimetry distributions in lineal energy were determined using paired tissue-equivalent proportional counters with and without boron microfoils. Microdosimetry results were obtained in free-air and at various depths in a PMMA phantom near the exit of the beam port. A biological weighting function, dependent on lineal energy, was used to estimate the relative biological effectiveness of the beam. An effective RBE of 2.7 was found at several depths in the phantom.

Damage induced by monochromatic synchrotron x-rays at the resonance absorption energy of intracellular phosphorous in yeast cells.

Two key experiments are analysed in terms of track structure parameters to obtain information on the excessive damage observed upon the production of inner shell vacancies at the resonance energy of bound phosphorus (Kobayashi et al., 1991) and for bromine incorporated into DNA (Usami et al., 1991). The relative damage enhancement factor due to resonance in phosphorus is 1.32. Additional Auger electron cascade events induced at resonance contribute about 35% to the total effect cross-section. In the bromine experiments, the intrinsic effectiveness is shown to be less than that for phosphorus. The effects can be quantified by the equilibrium electron fluences generated, combined with a quality specification determined from the net mean free path for ionization.

The quality of ionising radiations emitted by radionuclides incorporated into mammalian cells.

Radionuclides which decay by electron capture accompanied by Auger electron cascades, or by beta emission are thought to be excessively damaging when incorporated into the molecular structure of mammalian cell nuclei and thereby pose a radiation hazard which is not assessable by conventional dosimetry. Survival data, from the literature, for 125I, 77Br, 3H and 131I have been re-analysed to extract cross sections for inactivation, by the slowing down charged particle fluence, as these are absolute specifications of the radiation quality. When comparison is made with results similarly expressed for external irradiation with heavy particle and photon beams, the qualities of the low-energy emitters 125I, 77Br and 3H are found to approach those for heavy particles. An explanation for the damage mechanism is sought in a recently developed model for radiation action. The results are consistent with the interpretation that electron damage is caused predominantly at the end of the tracks, and the actual incorporation simply ensures that the slowing down fluence of low-energy tracks interacts preferentially in the vicinity of the radiosensitive sites. The absolute biological effectiveness of these radiations can be expressed quantitatively in terms of the model parameters.

Interpretation of the increase in the frequency of neoplastic transformations observed for some ionising radiations at low dose rates.

The anomalous increase of transformation frequency with decreasing dose rate observed by Hill et al. (1982, 1984b) for mouse fibroblast cells irradiated with fission neutrons cannot be satisfactorily explained by current models of radiation action. Recently a new model has been proposed which predicts the enhancement of damage with prolongation of irradiation, for equal doses. This is applied to the transformation studies in an attempt to interpret the enhancement observed for some radiations but not for others. Evidence is presented which suggests that repaired double-strand breaks in the DNA of cells which survive are the precursors of transformation. A critical physical factor is the total irradiation time rather than the dose rate. Approximately 1 per cent of repaired surviving cells go on to transform. From the results an explanation emerges of why transformation enhancement at low dose rates is not observed for natural alpha radiation and for photons or electrons, but is observed for fission neutrons and fast iron ions.

Physical mechanism for inactivation of metallo-enzymes by characteristic X-rays.

Measurements have been made of the inactivation of the metallo-enzyme dihydro-oratic dehydrogenase in solution by characteristic X-rays at energies above and below the K absorption edge of the constituent iron atom. From the dose-survival curves and knowledge of the equilibrium electron spectrum generated by the X-ray 'field', inactivation cross-sections are deduced and expressed in terms of intrinsic efficiencies for the various proposed direct and indirect mechanisms of inactivation. It is concluded that the inactivation is caused by direct X-ray interaction in an area equivalent to about 30 per cent of the mean geometrical cross-section of the molecule, and is independent of whether the target is wet or dry. The contribution from Auger electron cascades, Coulomb charges etc. initiated by the inner-shell vacancy in the metal atom is negligible--possibly due to saturation effects. It seems that the presence of the metal atom simply serves to enhance the overall interaction probability with the molecule in a manner consistent with expectations from the photon absorption coefficients. No anomalously large damage is detected. These conclusions are supported by comparison with published results for other metallo-enzymes and bromine-loaded bacteria.

Biophysical mechanism of radiation damage to mammalian cells by X- and gamma-rays.

Published information on the reproductive death in mammalian cells irradiated by a wide range of X- and gamma-ray energies has been re-analysed to extract intrinsic efficiencies of damage for the secondary electrons in transient equilibrium. On a log-log plot, a linear dependence on the track average l.e.t. and the average specific primary ionization is found, indicating that either serves as a good quality parameter. The soft X-ray data are consistent with this conclusion. Upon comparison with data for fast heavy ion irradiations, the average specific primary ionization is shown to be applicable independently of radiation type whereas track average l.e.t. is not. Furthermore it is revealed that electrons are most damaging near the end of their range but their efficiency is only about 10-20 per cent of that of fast ions at the same quality, possibly due to the influence of multiple scatter on the electron penetration depth. It is deduced that, for the dose rates involved, the damage by electrons is predominantly by intra-track action and not inter-track action. The results are consistent with the suggestion that optimum damage occurs when the mean free path between ionizations is equivalent to the strand separation in the double-stranded DNA.

Analysis of minor and trace elements in gallstones by induction of characteristic ionising radiation.

In an investigation of the role of trace elements in gallstone formation and of minor and trace elements in gallstone and bile, samples from five patients were analysed using three different techniques for cross-verification: NAA, PIXE, and XRF. In addition, certified standards of bovine liver and oyster tissue from the NBS, Washington, were re-analysed using the different techniques to confirm the accuracy of the experimental procedures. NAA of five gallstones was used for the quantitative analysis of 19 elements with Z greater than 11. The concentrations of Cl, K + Ca, Mn, Fe and Cu + Zn were determined by PIXE and those of Mg, P, S, K and Ca by XRF. In most cases the concentrations were in the range of a few PPB to a few hundred PPM. The spatial distribution of the component elements was obtained using XRF with a scanning electron microscope and the results demonstrated that calcium was present centrally in all the stones. The analysis of gallbladder bile from four patients showed that the calcium concentration was much greater than normal, indicating that calcium and other elements play an important role in stone formation. Infrared spectroscopy showed that the calcium salts were present in gallstones in three compound forms: calcium carbonate, calcium phosphate and calcium bilirubinate. Reasons for the cause of calcium precipitation are discussed.

Similarity treatment of phase effects in stopping power of low energy heavy charged particles.

Bragg's additivity rule is the basis for calculating stopping powers of compounds from data on their constituent elements. Major assumptions in applying it are that chemical binding and the physical state of the stopping material have negligible influence on the slowing down process. The fragmentary and, at times conflicting evidence for phase effects for low energy heavy charged particles is summarised. A detailed survey of published stopping powers has been made and the information grouped according to target medium and phase. Each group has been fitted to universal curves of Lindhard type with the object of separating phase contributions to stopping. Least square fits have been made to each set of data. From these a quantitative measure of the effect is obtained and presented as stopping power ratios (vapour/condensed phase). From the experimental evidence available, it is concluded that phase effects do occur, decreasing the stopping power in solids and liquids as compared to the corresponding vapours, and are thought to be due largely to changes in electronic excitation levels as the phase is changed, combined with a physical-state dependence of the effective ion change at low energies. Magnitudes of the stopping power ratio deviate from unity by up to approximately 25%, but have significant associated errors, pointing to need for more accurate stopping power measurements.

Hit cross-sections in single target theory.

A theoretical single target, single or multi-hit model for the biological effect cross-section for charged particles and gamma rays is expressed in terms of the fundamental physical interaction cross-sections. The model allows for the chemical action of the fragments and includes correction for saturation effects. An interesting conclusion, based on very limited experimental evidence, is that there are prospects of constructing a new system of radiation dosimetry with only two significant parameters; the effective volume of the target and the concentration of events. No energy parameters are necessary.

Cross-sections for the inactivation of ribonuclease by slow heavy ions.

Average cross-sections for inactivation of dry ribonuclease by H, He and N projectiles with energies between 100 eV and 10 keV are presented. The trend of the damage cross-sections with particle energy is consistent with the trend of the molecular quasi-elastic scattering and ionization cross-sections. A tentative radiation damage model is proposed in which single hit damage attributed to direct physical interactions by the primary and secondary radiations is supplemented by chemical action of the elastically scattered knock-on atoms which inactivate additional targets with an efficiency of 50-100%. Further experiments at energies below 100 eV are required for positive confirmation of the chemical efficiency. Free electrons (sub-ionizing) are chemically relatively innocuous (less than 10% efficiency). Saturation effects are satisfactorily accounted for in this model. Inactivation penetration depths measured in the microcrystalline enzyme agree with the theoretical predictions if a residual range of about 8 mug cm-2 is included. The mean energy expenditure to produce inactivation of a ribonuclease molecule is found to vary from 30 eV for 100 eV protons to 220 eV He and N ions.

Optimum photon energies for the measurement of bone mineral and fat fractions.

Relative error functions have been calculated for the various single and double photon energy techniques used for the measurement of linear bone mineral and fat mass in vivo. By considering only those errors arising from statistical fluctuations in photon counting useful information for guidance in the most efficient operation of the method is obtained. The results indicate that statistically the single energy method is always significantly better than the double energy method for measurement of both bone and fat fractions in soft tissue at total thicknesses up to 15 g/cm-2. In those circumstances where it may be necessary to use the double-photon energy technique there is an optimum sample thickness of 5 g/cm-2 for minimum error. Values of photon energy which permit measurement with the maximum precision in the minimum count time are deduced for a wide range of practical conditions and are presented along with a table of potentially suitable radioisotopes.