Radiation protection in humans: extending the concept of as low as reasonably achievable (ALARA) from dose to biological damage.

Abstract

Ionizing radiation has proven to be a double-edged sword since its discovery by Dr Roentgen in 1895. Radiation is a potent mutagen and carcinogen; however, it is also used in the diagnosis and treatment of human diseases. Background radiation consists of cosmic radiation and radiation emitted from radioactive substances present in the ground or commercial sources. Thus, all living organisms have been exposed to background radiation since their appearance on Earth. The appearance of oxygen some 4.5 billion years ago, allowed the appearance of aerobic organisms that used oxygen for their survival. The usage of oxygen generated toxic chemical species, free radicals, as byproducts. The interaction of radiation with oxygen may have caused increased production of free radicals. In order to cope with these adverse conditions, aerobic organisms that survived have antioxidant defence systems to quench excessive levels of free radicals and repair systems that can correct mutagenic changes. Today, humans, well equipped with antioxidant systems (from dietary and endogenous sources) and efficient repair mechanisms, are constantly exposed to varying levels of background radiation, depending upon the region and altitude where they live. In the USA, the average annual effective dose equivalent from natural sources to a member in the population is about 3.0 mSv (2.0 mSv from radon and 1 mSv from cosmic, terrestrial and internal) [1]. Medical exposure, consumer products, occupational, fallout, nuclear fuel cycle and miscellaneous contribute to about 0.6 mSv [1]. Patients can receive medical radiation exposure of varying levels up to 100 mSv, generally about 20 mSv or less, for diagnostic purposes. The use of radiation in medicine has always been rationalized on the basis of risk vs benefit. Owing to the growing use of nuclear energy in the world, especially in developed countries for civilian and military purposes, the concept of risk was applied to all personnel involved with radiation. In order to further safeguard against potential radiation damage, the concept of maximum permissible dose (MPD) was developed for the two major population groups. The annual MPD value for the general population for stochasticeffects, where the probability of the biological effect is proportional to the dose (linear no-threshold effect) is about 50 times less (1 mSv) than that recommended for radiation workers (50 mSv). In the UK, the radiation worker standard is 20 mSv as effective dose [2]. Although the value of MPD for radiation workers for the above effect has remained fairly constant during the last several decades, this value for the US population was recommended to be reduced by a factor of 5 in 1991 [3] and made law in 1993. This is due to the emergence of new data on low doses of radiation on somatic and heritable mutations in mammals and in mammalian cells in culture [4–7], the synergistic effect of the interaction of radiation with other chemical and biological carcinogens and tumour promoters on cancer incidence and mutations, and epidemiological studies in humans. To address the growing concerns of radiation-induced somatic and heritable mutations, the concept of ALARA (as low as reasonably achievable) with respect to dose was recommended by national and international radiation protection agencies for radiation workers [2, 3]. Efforts to protect normal tissues were started soon after the discovery of X-rays. However, the observation by Dr Muller of Columbia University in 1927, that radiation causes gene mutations inDrosophila melanogaster (common fruit fly) provided new impetus to reduce exposures [5]. The initial concept of radiation protection involved three physical principles: (a) shielding (usually by lead) of unexposed areas, especially radiosensitive organs such as bone marrow, gonads and thyroid; (b) increased distance between the radiation source and radiation workers or patients; and (c) reduction of exposure time. Each of these factors has been very useful, but they have limitations. For example, during fluoroscopy, it may not be possible to protect the gastrointestinal tract (one of the most radiosensitive organs) against radiation damage by lead shielding. Increasing the distance between the radiation source and exposed individuals may not be practical for many radiation workers, patients, civilian or military personnel. Reducing exposure time may also not be pertinent to all populations, except those that are involved in taking care of patients who have received gammaemitting radioisotopes for medical purposes or who are responsible for radioactive decontamination as a result of accidents or attack. Nevertheless, radiation protection based on physical factors has served a useful purpose and has been successful in reducing the level of unnecessary medical exposure to patients and to radiation workers. In order to protect normal tissues from potential radiation Received 31 March 2003 and in final form 1 September 2003, accepted 22 September 2003. The British Journal of Radiology, 77 (2004), 97–99 E 2004 The British Institute of Radiology DOI: 10.1259/bjr/88081058