Dosimetry for absolute biological effectiveness of ionising radiations

Abstract

It was shown that the conventional radiation dosimetric system which is based on RBE and LET is incapable of determining the likely consequences of ionising radiation exposure. Analyses of data on the induction of the chromosome aberrations, mutations and transformation in mammalian cells by radiations of different types and energies has indicated that (a) the induction of double strand breaks (dsb) in the DNA is their common critical lesion, (b) Fast ions and neutrons radiations are by order of magnitude more damaging than photons and electrons of equal mean free path, (c) Damage is through intra track action of the charged particles. A new system of radiation dosimetry, which does not require a radiation quality parameter, was proposed. It was based on the observation that for each of the biological endpoints considered an Absolute Biological Effectiveness (ABE) for damage by the charged particles can be defined as the product of the charged particle fluence and the saturation effect cross section, scaled with the efficiency (e) of damage by radiation of mean free path (lambda). e is given by 1-exp-(lambda0/lambda), where lambda0), about 1.8nm, is the mean inter-strand distance of the DNA. The physical requirements for its instrumentation, basically the emulation and quantification of the induction of dsb in the DNA, were defined. The feasibility for its realisation using detectors based on gas ionisation, superconductivity, secondary electron emission, and semiconductivity was assessed. Ultrathin films of rectified, organic semiconductors appeared to have the best potential, but such materials are not yet available in the physically characterised form as may be required for detector construction; investigations were made with available films of plastic scintillators. Experimental investigations have shown that by using coincidence techniques, plastic scintillator films can be used as a single volume 'microdosimeter', that is as counters of single strand breaks. Its use as a single volume 'nanodosimeter' is handicapped by light losses in the detector assembly which reduces the detector's sensitivity, efficiency and resolution. Semi-empirical analysis showed that the production of light from a phosphor differs fundamentally from the induction of biological damage. It was inferred that the plastic scintillators are unsuitable for instrumentation of the proposed system of dosimetry. Suggestions for future investigations were made.