Radiobiology of ultrasoft X rays. I. Cultured hamster cells (V79).

Ultrasoft X rays (approximately less than keV) provide a useful probe for the study of the physical parameters associated with the induction of biological lesions because the spatial scale of their energy depositions is of nanometer dimensions, comparable to that of critical structures within the cell. We report on cell-killing experiments using cultured hamster cells (V79) exposed to carbon K (0.28 keV), aluminum K (1.5 keV), copper K (8.0 keV), and 250 kVp X rays, under oxic and hypoxic conditions, and as a function of cell-cycle phase. Our principal results are: RBE increases with decreasing X-ray energy; OER decreases with decreasing X-ray energy; and cell-cycle response is similar for all X-ray energies. Our RBE results confirm earlier observations using ultrasoft X rays on mammalian cells. The shapes of fitted curves through the data for each energy are statistically indistinguishable from one another, implying that the enhanced effectiveness is purely dose modifying. The results reported herein generally support the view that single-track effects of radiation are predominantly due to very local energy depositions on the nanometer scale, which are principally responsible for observed radiobiological effects.

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).