A versatile plutonium-238 irradiator for radiobiological studies with alpha-particles.

A versatile irradiator has been constructed for in vitro irradiation of mammalian cells with alpha-particles of well-defined energy, LET, direction, dose and dose rate. It is based on approximately 1.2 x 10(9) Bq of 238Pu (on a platinum disc) contained in a He-filled chamber. In a standard configuration, monolayers of cells grown in 10 Hostaphan-based dishes are irradiated with 3.26 +/- 0.22 MeV alpha-particles (LET 121 keV microns-1) at selectable dose rates from approximately 2 Gy min-1 down to less than 10(-4) Gy min-1 (i.e. fluence rates of 1 x 10(7) cm-2 min-1 to 3 x 10(2) cm-2 min-1). Single dishes can be irradiated at dose rates up to 24 Gy min-1 (fluence rate 1 x 10(8) cm-2 min-1). Incident energy and LET can be varied from 0.8 to 4.2 MeV and 266 to 102 keV microns-1, respectively. The irradiator has full incubation and gassing facilities for protracted irradiations. The irradiator is particularly suitable for in vitro analytical studies of the biological effects of alpha-particles of energies and LETs similar to those which cells may receive in vivo from radionuclides such as radon and the actinides. It has been used successfully for investigations of a variety of alpha-particle-induced effects in different cell types irradiated either as attached monolayers or as very thin suspensions.

Biological effectiveness of isolated short electron tracks: V79-4 cell inactivation following low dose-rate irradiation with Al(K) ultrasoft X-rays.

PURPOSE: To investigate the biological effect of single, isolated, short electron tracks (<70 nm) relevant to practical human exposures to low-linear energy transfer radiation. MATERIALS AND METHODS: An irradiation rig was constructed that allowed environmentally controlled, protracted irradiations with an individually prescribed dose to up to 20 samples over a period of days. Inactivation of V79-4 mammalian cells by Al(K) ultrasoft X-rays was studied at high and low dose-rates with a maximum exposure time of 42 h. RESULTS: A significant increase in clonogenic survival was observed at the higher doses when the exposure time was increased from <6 min to 21 h, with no further increase observed for 42-h exposures. Despite the short range of the low-energy electrons produced (<70 nm), significant cell inactivation was observed for these low dose-rate exposures. CONCLUSIONS: The results are consistent with the hypothesis that even individual tracks can be biologically effective.

Direct comparison between protons and alpha-particles of the same LET: I. Irradiation methods and inactivation of asynchronous V79, HeLa and C3H 10T1/2 cells.

A direct comparison was carried out of the biological effectiveness of protons and alpha-particles of the same linear energy transfer (LET) under identical conditions with a variety of in vitro biological systems. Monolayers of mammalian cells were irradiated with accelerated beams of protons (1.2 and 1.4 MeV) and alpha-particles (30 and 35 MeV) corresponding to LETs of 23 and 20 keV microns-1 for each particle type. For V79-4 cells it was observed that the linear term of the dose-response for cell inactivation by protons was significantly greater than that for alpha-particles of the same LET. For HeLa and HeLa S3 cells, also, the linear term appeared to be greater for protons, but this was not observed with more limited data for C3H 10T1/2 cells. The result for V79 cells is in agreement with the report of Belli et al. (1989) who observed that the biological effectiveness of protons rose sharply between 17 and 30 keV microns-1 in strong contrast to alpha-particles which reached a peak effectiveness at greater than 100 keV microns-1. These results place new constraints on the biologically relevant features of the microscopic structure of radiation tracks, and have implications for the mechanistic and practical comparison between radiations.

Proton-induced characteristic x-rays: a versatile source of ultrasoft x-rays for biological and biochemical investigations.

Very low energy ( ultrasoft ) x-rays of 0.3-5 keV have provided a unique tool for investigation of mechanisms of radiation action, especially with respect to the energy and spatial properties of critical radiation damage in mammalian cells. Experimental investigations to date have been partially limited by the availability and characteristics of suitable ultrasoft x-ray sources. The suitability of small electrostatic proton accelerators, such as exist in many laboratories, have been investigated as a means of producing a secondary beam of ultrasoft x-rays suitable for irradiation of biological and biochemical systems. Results are presented on the physical characteristic of carbon K (0.28 keV) and aluminium K (1.5 keV) ultrasoft x-ray beams produced by bombardment of solid targets of carbon and aluminium with protons of energies up to 750 kV and currents up to 500 microA. These characteristics are compared with those of a cold cathode discharge ultrasoft x-ray tube previously used for mammalian cell investigations. It is seen that the proton accelerator produces much more versatile beams of characteristic ultrasoft x-rays which greatly extend the scope for future experiments on mammalian cells, micro-organisms and biochemical systems. Nevertheless there are situations in which the cold-cathode discharge tube will remain the source of choice and there are other situations, requiring for example energies between characteristic lines, where the greatly more complex synchrotron radiation sources are required.

Dosimetry comparison and characterisation of an Al K ultrasoft x-ray beam from an MRC cold-cathode source.

Ultrasoft x-rays of 0.3-5 keV have provided a unique tool for the investigation of intracellular mechanisms of radiation action in biological organisms, including mammalian cells. However, their use presents unique practical problems in dosimetry and experimental design. Detailed interpretation of the biological results requires reliable dosimetry and well characterised monoenergetic beams. This paper presents a comparison between two fundamentally different dosimetric techniques, namely the ionisation current in an extrapolation chamber and photon counts in a proportional counter. Agreement within 7% was obtained when these two methods were applied to an Al K x-ray beam (1.5 keV) from an MRC cold-cathode transmission target discharge tube as previously used in many biological experiments. Photographic film was calibrated as a relative dosimetric technique and used for investigation of the intensity uniformity of the radiation field. These techniques provide a comprehensive characterisation of the beam in the position of the biological cells, including photon flux (or absorbed dose rate), spectral purity (showing much less than 1% bremsstrahlung relative to characteristic Al x-rays) and uniformity over the irradiation area (within about 5% for mammalian cell irradiations).

Mutation and inactivation of cultured mammalian cells exposed to beams of accelerated heavy ions. I. Irradiation facilities and methods.

This paper, which is the first of four covering the inactivation of clonogenic capacity and induction of mutation in cultured mammalian cells, deals briefly with the general aims of the work and describes the irradiation techniques used. Human diploid fibroblasts and V79 Chinese hamster cells were irradiated as monolayers with ions of helium, boron or nitrogen at LET's in the range 20 to 479 keV micrometer-1 in H2O. The physical aspects of the irradiation including measurement of ion energies, dosimetry and uniformity of dose and also the methods of handling large numbers of samples are described in detail. Subsequent papers will present the biological methods and results and a biophysical analysis of the data.

High energy fast neutrons from the Harwell variable energy cyclotron. I. Physical characteristics.

A high energy fast neutron beam potentially suitable for radiotherapy was built at the Harwell variable energy cyclotron. The beam line is described and results are given of physical measurements on the fast neutron beams produced by 42 MeV deuterons on thick (4 mm) and thin (2 mm) beryllium targets. With 20 muA beam current the entrance dose rate in a phantom 150 cm from the target was about 130 rad min-1 with the thick target and about 60 rad min-1 with the thin target. Therefore, it is possible to use both the thin target and the relatively large target-skin distance of 150 cm to improve depth dose for radiotherapy or radiobiology. With this arrangement the dose rate decreased to 50% at depths in the phantom of 11.3-15.4 cm, depending on the field size. The use of primarily hydrogenous materials for shielding and collimation provided beam edge definition similar to that of 60Co teletherapy units, and off-axis radiation levels of approximately 1% which compare favorably with 14 MeV deuteron-tritium generators. The copper backing of the thin target became highly radioactive and an alterative material may be preferable. Biologic characteristics of the beam are described in a companion paper.

Effectiveness of 1.5 keV aluminium K and 0.3 keV carbon K characteristic X-rays at inducing DNA double-strand breaks in yeast cells.

Induction of DNA double-strand breaks in diploid wild-type yeast cells, and inactivation of diploid mutant cells (rad54-3) unable to repair DNA double-strand breaks, were studied with aluminium K (1.5 keV) and carbon K (0.278 keV) characteristic X-rays. The induction of DNA double-strand breaks was found to increase linearly with absorbed dose for both characteristic X-rays. Carbon K X-rays were more effective than aluminium K X-rays. Relative to 60Co gamma-rays the r.b.e.-values for the induction of DNA double-strand breaks were found to be 3.8 and 2.2 for carbon K and aluminium K X-rays respectively. The survival curves of the rad54-3 mutant cells were exponential for both ultrasoft X-rays. For inactivation of rad54-3 mutant cells, the r.b.e.-values relative to 60Co gamma-rays were 2.6 and 2.4 for carbon K and aluminium K X-rays, respectively. The DNA double-strand break data obtained with aluminium K and carbon K X-rays are in agreement with the data obtained for gene mutation, chromosome aberrations and inactivation of mammalian cells, suggesting that DNA double-strand breaks are the possible molecular lesions leading to these effects.