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Problems of radiation protection
Radiation hazards in 131I-treated patients have to be considered at 2 different
levels, i.e. the patient her/himself and the environment.
For the patient, somatic and genetic consequences of radiation therapy represent the potential risks to be evaluated. As previously discussed, late hypothyroidism is the true risk of radioiodine therapy but its consequences can be easely prevented by a yearly screening test based on serum TSH determination. The question of carcinogenesis has been examined in great details, particularly in the United States and in Sweden(6-9). These studies include several hundreds thousands patient-years: no significant excess of malignant diseases, particularly leukemia and thyroid carcinoma, have been demonstrated in the 131I-treated patients as compared to the control groups. Therapeutic radioiodine should not be given to premenopausal women whose last period finished more than 10 days previously. Pregnancy is to be avoided for 4 months after radioiodine therapy. An absolute contraindication for the administration of radioiodine is pregnancy or the possibility of pregnancy and breast-feeding. For these reasons, it is important to obtain the patient's informed consent. Theoretically, the risk of inducing a genetic abnormality by radioiodine treatment has been estimated at 0.003%(16). Such a risk has not been identified in treated thyrotoxic patients, even in large studies. Any theoretical increase should be evaluated in the light of the spontaneous incidence of (partly) genetic disorders. A somewhat similar conclusion arises from the follow-up of a fixed cohort of approximately 13,000 children born to parents exposed to the Hiroshima and Nagasaki bombs(17). Several reports indicate that deliveries in women treated with 131I below the age of 20 years have not demonstrated any particular genetic abnormalities. The same is true for the observation of the progeny of thyroid cancer women treated with much higher dose of 131I (17,18). The gonadal dose after radioiodine treatment for thyrotoxicosis is usually less than 30 mSv, thus much less than what is delivered by many routine radiological procedures. Table 1: Activities carrying a one in a million risk of causing death
Table 2: Estimated loss of life expectancy from various health risks
The hazards related to the pollution of our environment include the potential risks related to radiation. In order to properly evaluate the risk of radioactivity, a comparison with human activities is essential. Examples are given in Table 1. The mortality risk can also be translated into loss of life expectancy (Table 2). It is important not only to consider the sizes of the risks but also the differences in types of risks. In terms of life expectancy, an acute death from a car accident cannot be easily compared with death caused by cancer appearing after a long latency period. Although there are significant statistical problems with the epidemiological studies in this field, there are indications that the actual risk of inducing fatal cancer at low doses of ionizing radiation is below that predicted on the basis of extrapolation of data obtained at high levels of radiation (19). The general incidence of cancer has even been observed to be definitely lower in regions with higher doses of background radiation (20). Such observations are probably related to a radiation-induced stimulation of the DNA repair system ("hormesis”). Thus, the possibility of a beneficial effect of ionizing radiation is not at all to be rejected and requires further studies. The International Commission on Radiological Protection (ICRP) has recently recommended to lower the annual effective dose for members of the public to 1 mSv. Actually, the regulations regarding the administration of radioiodine (131I) on an ambulatory basis vary from country to country. The maximum limits for 131I activity permitted for out-patient treatment is 75 MBq (2 mCi) in Germany and 1110 MBq (30 mCi) in the United States; the average limit in the European countries is 555-800 MBq (15-20 mCi). A reasonable consensus has obviously to be met to protect the patient and the general public aswell. In this respect, the Committee on Radioiodine (131I) Therapy in Thyrotoxicosis of the European Thyroid Association has issued recommendations for the 131I-treated thyrotoxic patients (Table 3). These recommendations have to be used as guidelines, taking into account each patient with her/his own environment. The effectiveness of these recommendations is based on the cooperation and the willingness of the thyrotoxic patient her/himself. If these recommendations cannot be followed, the only alternative is hospitalization for an appropriate period of time, a decision which requires the allocation of high expenses. At all levels, an effort has to be maintained to reduce the radiation hazards but it is essential to keep in mind that there are many other sources of background radiation - not necessarily deleterious - like living in Cornwall (0,1 mSv/week) or in a radon-build house for years or generations (1.2 mSv/year). Such radiation hazards are only a small part of the general risks of living, taking into account those related to chemical pollution of our environment. In 1994, the United Nations Scientific Committee on the Effects of Atomic Radiations (UNSCEAR) published its report on radiation hormesis, entitled "Adaptative responses to radiation in cells and organisms” (21). In living cells, several adaptive processes to low-level radiation have been observed, including the key role of the DNA repair enzymes. Such mechanisms may apply not only to ionizing radiation but also to chemicals including pesticides and carcinogenic agents. A reexamination of the "no-treshold hypothesis” based upon the hypothetical linear extrapolation of high-doses effects to effects of very low doses approaching zero are urgently needed. Pollycove concludes in an editorial: "...Instead of wasting many billions of dollars annually for protection against hypothetical risks of low-level radiation exposure, the resources could be used productively for effective health measures...” (22). Adaptive processes to low-level radiation allows to observe beneficial effects at dose levels between 1 and 50 cGy (1-50 rads) (23). It should be remembered here that the concept of hormesis is very broad. As an example comparing with the radiation risks, one may mention vitamins which are beneficial at low concentrations but which may become toxic at higher doses. The concept of radiation hormesis has undoubtly to face the difficult goal of the analysis of the long-term effects of low-dose radiation (23). The continuity of the living process is in harmony with the concept of "good sense” of life. This is so for cosmic and terrestrial radiations that stimulated the evolution of life in our world. Mother Nature has teached us that too much of anything may be dangerous but a touch of something can be most beneficial. It is our responsibility to define the reasonable limits to protect a safe environment but at the same time, to avoid disturbing the members of the public without appropriate justification. Table 4: Recommenations after 131I therapy (European Thyroid Association 1994)
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