IRPA Regional Symposium
Radiation Protection in Neighbouring Countries of Central Europe. Prague. 8-12 September 199
A SCINTILLATION METHOD FOR MEASUREMENT OF THE BIO-
EFFECTIVENESS OF IONIZING RADIATION BY SIMULATION OF THE
CELLULAR RESPONSE
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CZ9928577
I.C. McDougall1, A.S. Alkharam1, G.E. Thomas2 and D.E. Watt1
'School of Physics and Astronomy, University of
St.
Andrews, St. Andrews, Fife KYI 6 9SS, Scotland, UK.
2 Department of Mathematics, University of Dundee DD1 4HN, Scotland, UK.
1.
Introduction
Practical implementation of our system of dosimetry depends on the construction of detection devices
which have an appropriate response function. Useful guidance for designing the function can be ob-
tained from the generalised relationship between the instrumental response and the output signal as
stated formally by Ritchie(1) in 1967. Ideally, for radiation protection, the detector output signal should
be directly relatable to the biological effect of interest, independently of the energy, type and intensity
of the radiations. This has not yet been achieved. Microdosimetric devices*-2-1 probably come closest to
the requirements as they aim to simulate the radiation response of mammalian cell nuclei and, for their
operation, information on the radiation type is not necessary. Nevertheless, microdosimeters can only
provide a measure of the physical quality of the radiation field. They cannot yet give a measurement of
the biological effectiveness^ although there is provision in the theory to accommodate nanometre-
sized sensitive volumes which may make this possible depending on the damage mechanism(s) in-
volved. If the corresponding radiosensitive targets can be unambiguously identified and if the mecha-
nism of damage can be better specified, then it should be possible to design instrumentation which has
a radiation response equivalent to that of the critical targets in the mammalian cell nucleus^1. Such de-
tectors would, in principle, give an absolute measure of bio-effectiveness for any ionizing radiation
field. They would form a key part of a fluence-based system for radiation protection. Absorbed dose
would not be relevant and quality weighting factors would become redundant along with many of the
other dose-based units and quantities^5-1. The work reported here is aimed at producing such detectors.
During the past ten years biological and biophysical evidence has emerged which supports the conten-
tion that double-stranded breaks induced by ionizing radiation in segments of the DNA in mammalian
cell nuclei are the dominant critical lesions which are precursors to various biological endpoints such
as inactivation, chromosome aberrations, mutations and oncogenic transformations *•' . The biophysi-
cal evidence demonstrates that the radiation damage can be correlated uniquely for any radiation type if
the 'effect cross-section for production of the initial damage' is expressed as a function of the mean
spacing between ionizations produced by the charged particle tracks^4' 6\ The effect cross-section is an
absolute measure of the bio-effectiveness. Cross-sections for induction of oncogenic transformations,
of interest for protection, can be scaled directly from the data for inactivation(7).
From the foregoing considerations, the observed response curve for damage to the nuclei of mammal-
ian cells can be represented by a plot of the cumulative probability for inactivation (as deduced from
the ratio of the observed bio-effect cross-section to the saturation value) against the 'mean spacing of
primary ionizations1 for the relevant charged particles in the equilibrium spectrum generated by the ra-
diation field(8). This results in a unified curve which accommodates all radiation types. It gives a
measure of the absolute bio-effectiveness of the radiation field as a function of the fluence-weighted
mean free path for linear primary ionizations by the equilibrium charged particle track. In other words,
this curve represents the radiation response function for the initial production of double-stranded
breaks in the intanuclear DNA of a mammalian cell. The response for other end-points such as onco-
genic transformations can be obtained by appropriate scaling as demonstrated in reference 7.
548 Session 5
