Radiation Protection Dosimetry: A Radical Reappraisal

Author:  Jack Simmons & David Watt
ISBN:  9780944838877      ISBN10:  0944838871
Published:  1999 | 160 pp | 


OUT OF PRINT

  
  




Radiation Protection Dosimetry  |  Vol. 82, No. 3 (1999)


“The authors of this book state that, although the concepts of dosimetry developed by the International Commission on Radiation Units ad Measurements (ICRU) and the International Commission on Radiological Protection (ICRP) are widely accepted, there have been objections raised in ‘barroom discussions’ as well as scientific conferences and peer reviewed literature. Their stated aim is to present a ‘simpler, more logical system of radiation units and measurements’ based on relating biological response to fluence rather than absorbed dose.

“The book contains seven chapters, the first of which is entitled A Historical Precedent in Astronomy. This chapter describes the early efforts of astronomers to explain the movements of the planets. As is well known, the geocentric assumption of Ptolemy was accepted for more than 1500 years until the work of Copernicus provided a rationale for the heliocentric model of the solar system. The authors compare these developments in astronomy to the development of the present radiation protection dosimetry system and declare that the time is now right for a revolutionary change in the fundamental dosimetric quantity.

“Chapter 2, A Brief Early History of Radiation Units and Radiation Protection, summarises the work of scientists at the beginning of this century to measure radiation , and it details the efforts of the ICRP and the National Council on Radiation Protection and Measurements (NCRP) to develop a basis for radiation protection guidelines. The introduction of the concept of relative biological effectiveness (RBE), and changes made to values of RBE for various radiations over the years are mentioned. The simplifying assumption of a radiation effect relationship that was linear with no threshold was noted as providing a means for considering the dose to be ‘the mean dose over all the cells of uniform sensitivity in a particular tissue of organ’. This provided a practical advantage of defining the significant volume as the volume of the ‘tissue or organ under consideration’.

“Chapter 3 moves on to consider the 1990 Recommendations of The ICRP and Their Consequences. The authors indicate that a ‘tacit agreement between the ICRP and the ICRU...was rudely shattered in 1990...’. While it is certainly true that the ICRP Publication 60 introduced several new and controversial concepts, it is not at all obvious that the actions of the ICRP were ill mannered. The chapter contains a number of quotations from various authors that are critical of Publication 60. The ICRP ICRU Joint Task Group that developed ICRP Publication 74 and ICRU Report 57 considered many of the difficulties mentioned, and gave additional guidance for the use of the ICRU operational quantities to estimate the ICRP protection quantities. The authors mention the ‘Criticisms of the use of absorbed dose as a basis for assessing the effects of low levels of radiation...’ given by Bond and co-workers.

“Chapter 4, ‘Simulation Models of Biophysical Effectiveness’, summarises a number of the radiobiological models that have been developed to explain the action of ionising radiation. Most of the cited models are concerned with the cell killing effects of large doses of radiation, and the usefulness of these models for predicting the carcinogenic effects for low doses and dose rates exposures to ionising radiation remains an open question. In this chapter, the authors correctly point out that there is great difficulty in testing such models because of the low statistical power of radiobiological experiments.

“Although the chapter includes references to many useful publications, the authors’ discussion and model summaries are, in some cases, inconsistent and misleading. As an example, the authors suggest that the LPL model (p. 51) includes two classes of repairable lesions that are categorised by fast and slow rates of repair, εL and εL'. However, the LPL model as formulated by Curtis (1986) reserves the subscript L for lethal lesions that are not repaired. It is likely that the authors were referring to the ‘potentially lethal repair rate constants, εPL and εPL', the latter of which refers to lesions that can be ‘fixed’ by hypertonic treatment. Readers are cautioned to consult the original references describing these radiobiological models.

“Chapter 5 considers Radiation as a Probe for the Physical Investigation of Radiosensitive Structure in Biological Targets and deals with the implications of radiobiological experiments conducted using charged particles. The difference between the observed effects of densely ionising and sparsely ionising radiations are pointed out, and the relative merits of LET and z2 / β2 as appropriate quantities for characterising the quality of the radiation are discussed. The authors suggest that a more rigorously based system of dosimetry should make use of the ‘equilibrium charged particle fluence and the effect cross-section ration to provide’ a system that is applicable to both external and internal radiations.

“The authors’ proposed model of radiation is discussed in Chapter 6, Model of Radiation Bioeffectiveness and Its Application in a Fluence-based System. This chapter details a model that also seems to be applied to the inactivation of mammalian cells, but it is possible that some of the actions of radiation that are considered could result in carcinogenic transformations. This chapter also contains a discussion of the advantages of radiation therapy using heavy charged particles, as opposed to fast neutrons. The chapter concludes with a plot of the proposed effect cross section per unit kerma for several radiations (plotted as a function of photon or neutron energy). The effect function is shown as a smooth curve that seems similar in nature to the ICRP 60 histogram of radiation weighting factors.

“Conclusions are given in Chapter 7, which reiterates the statement that absorbed dose is not a useful predictor of the effects of ionising radiation. The biological model proposed in the previous chapter is offered as a means for predicting cell survival for any irradiation condition, and calculating the response function of radiation measuring instruments that would be used in the proposed fluence-based dosimetry system. It is the belief of the authors that such instruments could be developed and that their biological model ‘will be fully testable in the near future...’ The chapter finishes with several general statements, including: ‘To deny the existence of a threshold for the induction of cancer by radiation is to fly in the face of a large body of evidence supporting its existence’. This may be the opinion of the authors, but this text does not provide a lot of evidence to support their wide ranging statement. There may very well be thresholds for the induction of some forms of radiation-induced cancers, but it remains to be seen whether the radiobiological model and the instruments proposed by the authors will reveal the exact nature of low dose (or low fluence) radiation effects.

“This book should provide interesting reading for those who are following the continuing discussions regarding the linear non-threshold assumption of radiation action at low doses. These discussions are surrounded by controversy, and the statements made by the authors of this book might be considered to be at least as controversial as the statements attributed to the authors of ICRP and ICRU reports and publications. The readers must judge for themselves.”

J.C. McDonald

Pacific Northwest National Laboratories

Richland WA 99352 USA