Definitions of physical and biological low dose.

The concept of 'low dose' may be understood to refer to an average frequency of microdosimetric events (charged-particle traversals) that is substantially less than unity in cells or their nuclei. An important consequence is that in this case the probability of any effect on autonomous cells must be proportional to the absorbed dose and independent of dose rate. However, this definition may be unnecessarily restrictive because--especially in the case of low-LET radiation--only a small fraction of events may cause the effects under consideration (e.g. cell lethality). This results in larger 'biological' rather than 'physical' 'small doses'. From a pragmatic viewpoint, one may consider the fact that in the linear-quadratic model deviations from proportionality between effect probability and absorbed dose are attributed to a term that depends on the square of the absorbed dose. This permits the selection of a criterion which establishes as 'small doses' those in which such deviations are less than a chosen value which in the examples given here is 10%. Different applications of this criterion to the inactivation of V 79 hamster cells are considered.

Radiogenic lung cancer: the effects of low doses of low linear energy transfer (LET) radiation.

A critical review of the literature leads to the conclusion that at the radiation doses generally of concern in radiation protection (< 2 Gy), protracted exposure to low linear energy transfer (LET) radiation (x- or gamma-rays) does not appear to cause lung cancer. There is, in fact, indication of a reduction of the natural incidence.

Radiation physics and radiobiology.

Three quite general rules link radiation physics to radiobiology. They concern the dependence on linear energy transfer of relative biological effectiveness and of the cross section for cell killing, as well as the dependence of relative biological effectiveness on absorbed dose. These rules are accounted for in compound dual radiation action according to which damage in the nanometer domain depends linearly on dose with no dose rate dependence and on relative biological effectiveness that is limited to low values because of saturation. Energy concentration in the micrometer domain can cause large relative biological effectiveness in processes in which pairs of DNA lesions interact with quadratic dose dependence and dose rate dependence for low linear energy transfer radiations. Damage at both the nanometer and the micrometer level can cause observed effects and their relative contributions determine the maximum relative biological effectiveness at very low doses.

On the revised concept of linear energy transfer.

The quantities linear energy transfer or restricted linear energy transfer are utilized in calculations that link absorbed dose to the fluence distribution of a radiation field. The computations provide approximations to absorbed dose in terms of the intermediate quantity cema or reduced cema. With the definition of the restricted linear energy transfer, L delta, given in ICRU Report 33, the approximation remains imperfect. This study deals with the resulting need for a modified definition of L delta, as proposed in a draft report of ICRU. Essential differences between the old and the new definitions are demonstrated. The changed definition permits a rigorous formulation of the dependence between fluence and absorbed dose.

Radially restricted linear energy transfer for high-energy protons: a new analytical approach.

Radially restricted linear energy transfer (LET) is a basic physical parameter relevant to radiation biology and radiation protection. In this report a convenient method is presented for the analytical computation of this quantity without the need for complicated simulation. The method uses the energy-restricted LET L delta, as recently redefined in a 1993 ICRU draft document and supplements it by a relatively simple term that represents the energy of fast delta rays lost within distance r from the track core. The method provides a better fit than other models and is valid over the entire range of radial distance from track center to the maximum radial distance traveled by the most energetic secondary electrons. Lr computed by this approach differs only a few percent from the values obtained from explicit Monte Carlo simulations. The concept applies equally to heavy ions and to electrons.

Compound dual radiation action. I. General aspects.

The theory of dual radiation action (A. M. Kellerer and H. H. Rossi, Curr. Top. Radiat. Res. Q. 8, 85-158, 1972) has attributed the effects of ionizing radiation on eukaryotes to the production of molecular changes (sublesions) that combine pairwise to produce injury (lesions) responsible for radiation effects. If the yield of sublesions is independent of radiation quality (as is currently assumed), dual radiation action results in the well-known proportionality between the average yield of lesions and alpha D+beta D2, where beta is a radiation-independent quantity. It has, however, been observed that beta changes with radiation type. In this paper we propose an explanation of this discrepancy. Specifically, we suggest that dual radiation action-type processes where beta is variable are the result of a mechanism--termed compound dual radiation action--which consists of a sequence of simple dual radiation action processes, each process being the causative agent for the next one. The sequence, single-strand DNA breaks, double-strand DNA breaks (chromosome breaks), and exchange-type chromosomal aberrations, is one such example examined in the paper.

A comment on the 1990 recommendations of the ICRP.

ICRP Publication 60 recommends a change relating to the numerical assessment of radiation quality in radiation protection. The quality factor, Q, is to be replaced by "radiation weighting factors," WR, and the quantity "effective dose equivalent" is to be supplanted by "effective dose." Reasons are given why it is virtually impossible to measure this quantity and why it appears unavoidable that practical measurements will continue to be based on the current system. No sensible justification was provided for the proposed change, which is likely to cause confusion.

On the question of RBE reversal at high doses.

We present theoretical arguments to explain observations of a "reversal" of the RBE at relatively large doses; that is, the RBE of high-LET vs low-LET radiation is less than one. Numerical examples are given and the results of Bogo et al. (Radiat. Res. 118, 341-352, 1989) are discussed qualitatively.

Intermediate dosimetric quantities.

The transfer of energy from ionizing radiation to matter involves a series of steps. In wide ranges of their energy spectra photons and neutrons transfer energy to an irradiated medium almost exclusively by the production of charged particles which ionize and thereby produce electrons that can ionize in turn. The examination of these processes leads to a series of intermediate quantities. One of these is kerma, which has long been employed as a measure of the energy imparted in the first of the interactions. It depends only on the fluence of uncharged particles and is therefore--unlike absorbed dose and electron fluence--insensitive to local differences of receptor geometry and composition. An analogous quantity for charged-particle fields, cema (converted energy per unit mass), is defined, which quantifies the energy imparted in terms of the interactions of charged particles, disregarding energy dissipation by secondary electrons. Cema can be expressed as an integral over the fluence of ions times their stopping power. However, complications arise when the charged particles are electrons, and when their fluence cannot be separated from that of the secondaries. The resulting difficulty can be circumvented by the definition of reduced cema. This quantity corresponds largely to the concept employed in the cavity theory of Spencer and Attix. In reduced cema not all secondary electrons but all electrons below a chosen cutoff energy, delta, are considered to be absorbed locally. When the cutoff energy is reduced, cema approaches absorbed dose and thereby becomes sensitive to highly local differences in geometry or composition. With larger values of delta, reduced cema is a useful parameter to specify the dose-generating potential of a charged-particle field 'free in air' or in vacuo. It is nearly equal to the mean absorbed dose in a sphere with radius equal to the range of electrons of energy delta. Reduced cema is a function of the fluence at the specified location at and above the chosen cutoff energy. Its definition requires a modification of restricted linear collision stopping power, L delta, and it is recommended that the definition of L delta be so changed.

Elements of microdosimetry.

This is an introductory tutorial to the field of microdosimetry. Following a brief historical outline, we review the physics of experimental and theoretical microdosimetry and the application of microdosimetry to radiation biology within the so-called "site" approximation. Modern interpretations of microdosimetry that make use of methods developed in integral geometry are described under the heading "structural microdosimetry;" this also illustrates the conceptual basis of dual radiation action. A discussion concerning some future prospects of this field concludes the review.