A brief outline is given of the concepts of microdosimetry and the development of the theory of dual radiation action, which was proposed as the link between the physical properties of radiation (microdosimetry) and their biological effects. A good deal of experimental evidence was consistent with the theory in its original site model form, but there were also areas of significant disagreement. More recent experiments with ultrasoft x rays and beams of correlated ions have provided critical tests which have shown that the site model does not give an adequate description of the mechanism of radiation action. A large proportion, at least, of the radiation-induced lesions are produced by highly localized energy concentrations without the need for long-range interaction of sublesions. Attempts have been made to allow for such lesions within the original postulates of the theory of dual radiation action by development of the distance model or generalized formalism using a distance-dependent interaction probability. Some of the present limitations of this approach are discussed. An alternative hypothesis, involving dose-dependent repair acting directly upon the lesions, is described. A comparison is made between consequences of these two alternative approaches. The validity of the application of microdosimetric measurements to a varietymore » of problems is considered.« less

An assessment of the role of microdosimetry in radiobiology.

An experimental study of the relative biological effectiveness of neutrons of various energies for the inactivation of hamster cells has been reported previously; the present article contains a detailed statistical analysis of the data. Repeated determination of X-ray survival curves made it possible to assess the statistical fluctuations of survival rates. Within individual experiments the statistical variations are close to those expected on theoretical grounds. However, between experiments substantial variations in the reaction to X rays occur; these fluctuations can be approximately described by dose-modifying factors. An idealized survival curve for X-irradiation has been derived. The survival curves for neutrons have been compared to the linear-quadratic doseeffect relation and the two coefficients in this equation have been derived together with their joined confidence regions. The linear coefficient for neutrons reaches a maximum for neutron energies of several hundred thousand electron volts; the coefficient of the quadratic component is increased at neutron energies below 1 MeV. The dependence of RBE on neutron energy and on neutron dose is derived from the estimated dose-effect relations. In addition, the more rigorous nonparametric method is used to derive confidence intervals for the RBE of neutrons.

RBE as a function of neutron energy

RBE as a function of neutron energy. II. Statistical analysis.

As a result of recent radiobiological research it has become evident that the RBE of neutrons can at low absorbed doses reach values that are substantially higher than the values of the quality factor (Q) currently adopted for radiation protection. There is theoretical and observational evidence that such high RBE values are the rule rather than the exception and that they apply to at least one radiation effect on humans. New values as well as different formulations are proposed for the factor that is employed in radiation protection to weight absorbed doses for their biological effectiveness.

A proposal for revision of the quality factor.

Case studies were reviewed for 100 consecutive patients with carcinoma of the cervix, Stages I–III, who were treated prior to 1968 at a large municipal hospital in New York City. Treatments consisted of orthovoltage therapy prior to or following intracavitary radium. A 250 kV X-ray unit delivered 3000 rat tumor dose in 3 weeks through four oblique fields. Intracavitary radium delivered 6000–7200 mg. hr using the Ernst applicator. 6 The 9% incidence of fistulae was 4-fold higher than that found in subsequent years using Co teletherapy and the afterloading Fletcher-Suit applicator. A review of the dosimetry relating to the use of the Ernst applicator demonstrates inherent structural characteristics which lend themselves to such complications. Where less than an "ideal" application is possible, this rigid applicator compacts itself in accordion-Ilke fashion, producing a so-called short-system. Coupled with a reduced source to applicator-surface distance, such applications result in unacceptable dose anisotropy and excessive radiation of critical structures where a predetermined dose is to be delivered to anatomic Point A. Information gleaned from this study can be extrapolated to other rigid unprotected short-surface distance intra-vaginal applicators that have proliferated in recent years.

The intracavitary applicator in relation to complications of pelvic radiation—the ernst system

Neutron radiation quality

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Neutron radiation quality