Effects of Low Dose and Low Dose Rate Radiation

Oddvar F. Nygaard, Warren K. Sinclair, John T. Lett, Oddvar F. Nygaard, Warren K. Sinclair, John T. Lett

• Publisher: Academic Press
• Year: 2013
• ISBN:
  9781483281834
• Topic: Medicina
• Subtopic: Fisiología

Advances in Radiation Biology, Volume 6: Effects of Low Dose and Low Dose Rate Radiation examines the biological effects of low dose and low dose rate ionizing radiation on a broad scale, covering various articles from microdosimetry to analyses of human responses. Estimates of the effects on humans from low doses or from sustained exposures to low dose rates of ionizing radiations are of critical importance for the assessment of radiation risks under occupational and environmental conditions. This book consists of such knowledge that is essential for radiation protection and governmental regulatory activities pertaining to radiation exposure. This volume is intended for radiobiologists, radiation epidemiologists, radiation physicists, radiation safety personnel, health officials, and individuals involved in regulatory activities.

Track Structure Considerations in Low Dose and Low Dose Rate Effects of Ionizing Radiation

Dudley T. Goodhead, Medical Research Council Radiobiology Unit, Chilton, Didcot, Oxon Ox11 Ord, England

Publisher Summary

This chapter describes some characteristic features of radiation tracks in relation to cellular and subcellular structures. The main effects of potential concern to human populations exposed to low doses and low dose rates of ionizing radiations are the induction of tumors or of germ cell mutations. These effects arise in most cases from a stable, radiation-induced change in a single cell, probably from damage to a particular gene or genes within the DNA of the cell. Unlike most other DNA-damaging agents, for example, chemicals or ultraviolet light, the initial insult from ionizing radiation to the irradiated material is always in the form of highly structured tracks of atomic ionizations and excitations along the paths of the primary and secondary charged particles. As a result, the insult is always highly inhomogeneous in space and in time. These inhomogeneities become more apparent the smaller the target volumes under consideration and the lower the doses and dose rates.

I Introduction

The main effects of potential concern to human populations exposed to low doses and low dose rates of ionizing radiations are the induction of tumors or of germ cell mutations. These effects are believed to arise in most individual cases from a stable, radiation-induced change in a single cell, probably from damage to a particular gene or genes within the DNA of the cell. Unlike most other DNA-damaging agents, such as chemicals or ultraviolet light, the initial insult from ionizing radiation to the irradiated material is always in the form of highly structured “tracks” of atomic ionizations and excitations along the paths of the primary and secondary charged particles. Consequently, the insult is always highly inhomogeneous in space and in time. These inhomogeneities become more apparent the smaller the target volumes under consideration and especially at lower doses and dose rates.

In considering effects on single cells it is essential to include the numbers, distribution, and types of radiation tracks, and in considering the mechanisms within the cell it is essential to include the microscopic structures of the individual tracks themselves. Figure 1 illustrates the degree of spatial nonuniformity of radiation insult for different target volumes, from whole tissue down to a short segment of DNA, for three different types of radiation. The numerical values are in all cases for a macroscopic absorbed dose of about 10 mGy.¹

Fig 1 Diagrammatic Representation of Microscopic Patterns of Tracks in Tissue Irradiated with 10 mGy of Dose for Three Different Radiations (Low-LET γ Rays, High-LET α Particles, and Mixed-LET Fast Neutrons). For Each Radiation the Mean Numbers of Tracks are Given for Large Volumes, Corresponding to Whole Tissues, then for Cells, Chromatin, and Down to the Very Small Volumes of Segments of DNA. Also Given are the Dose Uniformity at these Four Levels of Resolution; These are Expressed as the Range of Doses Received by Individual Target Volumes. For Example, for the 2-nm Long Segments of DNA Only 1 Such Target Volume in 10⁸ Receives Any Energy Deposition at All; When This does Occur the Microscopic Dose to that Segment can have any Value up to about 10⁶ GY, Depending on the Chance Interactions of the Radiation Track as it Passes Through. The Last Column Illustrates how These Different Patterns of Local Energy Deposition (in all Cases for the Same Macroscopic Average Energy Deposition or Dose, 10 mGy) Affect the Probability of Permanent Biological Damage to the Cell. The Quoted Values are Typical for Cell Inactivation; Much Lower Probabilities are Found for Cell Transformation, or Mutation at a Single Locus, but the Sequence of Relative Effectiveness of the Radiations Persists. (From Goodhead, 1987b).

Reading the diagram from left to right for γ rays, which produce large numbers of sparsely ionizing tracks, it can be seen that the irradiation is essentially uniform at the level of whole tissues and even of individual cells. However, it is grossly nonuniform at the level of chromatin or DNA structures, for which most parts are totally unirradiated while a few parts may, by chance, have received quite substantial hits. Reading down the diagram, consideration is given also to two types of radiation that produce densely ionizing tracks. Because there are correspondingly fewer tracks for the same dose, the nonuniformities are in these cases strongly apparent at the cell and even whole-tissue levels.

This chapter will describe some characteristic features of radiation tracks in relation to cellular and subcellular structures of possible interest and then will consider what constraints these may place on extrapolation of available experimental and epidemiological data to the low doses and dose rates of main practical relevance. Only a few selected descriptions of microdosimetry will be used for this purpose. For alternative descriptions the reader is referred to broader reviews of the applications of microdosimetry (see, e.g., ICRU, 1983; Goodhead, 1987b).

II Features of Radiation Tracks

A rough classification of radiations is often given in terms of their linear energy transfer (LET). This is a measure of their average energy loss along the path of the track, ignoring track width and fluctuations along the track. The severe limitations of this description are well recognized (ICRU, 1983; Goodhead, 1987b), but nevertheless LET does provide a simple, semi-quantitative description for common usage.

When incident radiations interact with material, their energy is deposited predominantly by ionizations and excitations along the paths of the secondary charged particles that they produce. Figure 2 illustrates this for irradiation with neutral photons or neutrons or a charged heavy ion such as an α particle. X-Ray or γ-ray photons produce exclusively primary and secondary electrons, many of relatively high velocity and low average ionization density. These are low-LET radiations. Neutrons mostly produce proton and heavier recoil particles of intermediate to high LET, although they are usually also accompanied by a component of γ rays. α Particles from natural or artificial radionuclides are directly ionizing, as high-LET particles.

Fig 2 Diagrammatic Illustration of the Production of Tracks of Charged Particles in Material Irradiation with Neutral Particles (Photons, Neutrons) or Heavier Ions Such as α Particles or Accelerated Heavy Ions. (Reproduced with Permission from Paretzke, 1987.)

When the tracks are studied in more detail it immediately becomes apparent that there is great diversity within a given radiation itself. For example, Fig. 3 shows a small selection of simulated tracks of low-energy electrons, such as are responsible for a major part of the energy dep...