Irradiation of the thyroid

Refetoff, S. 1990. A review of risks of external irradiation of the thyroid gland. Pages 101-103 in Iodine Prophylaxis Following Nuclear Accidents Proceedings of the Joint WHO/CEC Workshop. July 1988. Pergamon Press, New York. 

The International Commission on Radiological Protection (1977) introduced the concept of Effective Dose, which takes into account the probability of mortality from cancers in various tissues receiving irradiation. For irradiation of the thyroid, a weighting factor, wT, equal to 0.03... 

Note. Since the quality factor is unity for external gamma and for irradiation of the thyroid, Sieverts and Grays are equal numerically. 

Triggs SM, WiUiams ED. Irradiation of the thyroid as a canse of parathyroid adenoma. Lancet 1977 1(8011) 593. ... 

Prevention of disease by modification of risk factors is important in many areas of medicine. For optimal planning and execution of prevention programs it is important to know the dose—response relationship between the risk factor that is modified and the disease that is to be prevented. Such a relationship may take several forms (Rose, 1992 Laurberg et ai, 2001). For example, there seems to be a more or less linear association between irradiation of the thyroid in childhood and later development of thyroid cancer. Irradiation of the thyroid in childhood should be as low as possible. The relationship between iodine intake and the risk of disease is more complicated. Even if the most severe problems are seen when iodine intake is insufficient, a high iodine intake may also lead to disease. The relationship is U-formed (Laurberg et ai, 2001) (Figure 47.1). 

In the event of fallout from a nuclear incident there is a significant release of radionuclides that are sufficiently volatile to be dispersed, and these include radionuclides of iodine The most important of these firom the point of view of subsequent irradiation of the thyroid is l, although others may so be released in sufficient abundance to be important in special circumstances. However their physical properties are such that they are generally less important than 1, especially at a distance, and hence they will not be treated here Their impact is assumed to be included with the generalized external irradiadon associated with the inidal release, or to be otherwise relatively minor. 

This table moreover shows that adequate iodine supply or the introduction of programs of iodine prophylaxis reduce the thyroid uptake by a factor of 2 to 3. Adequate iodine supply may therefore reduce consistently the level of irradiation of the thyroid gland in the case of a 1311 fallout. 

It is cxmcduded that in a large number of European countries with a low iodine nutritional supply, the risk of irradiation of the thyroid gjand by I from acc idents in nucrlear plants is significantly increased. This situation justifies the prompt introduction of adecjuate programs of icxline prophylaxis in these cxiuntries. 

From 1905 to 1960, X-ray irradiation of the scalp for treating ringworm, T. capitis, was regularly performed on as many as 200000 children worldwide. For a typical series of X-ray treatments, doses of 220-540 rad were received by the scalp, 140 rad to the brain, 380 rad to the cranial marrow, and <100 rad to other organs and tissues of the head and neck. Cancers of the thyroid and skin (basal cell carcinoma) were the major consequences of irradiation. 

Cobalt-60 was a common isotope for external radiotherapy, but it has mostly been replaced by linear accelerators that provide high-energy electrons (P particles) without a dangerous isotope source. It is still used for food irradiation. Iodine-131 is used to combat diseases of the thyroid and of several types of cancer. A list of isotopes used in nuclear medicine may be found at http //www.cbvcp.com/nmrc/mia.html. 

In a study on the effects of lifetime feeding of I in rats, the only observed effect was a decreased I uptake by thyroid glands of I-fed rats (Book, 1983). That decrease reflected the blocking of the thyroid by the large quantities of iodine required because of the low specific activity of I. Lifetime ingestion of I in that study resulted in dose rates to the rodent thyroids of about 1 rad per day. There was no significant increase in thyroid tumors of I-exposed rats compared to controls, nor in other thyroid effects, nor was there any difference in longevity between I-irradiated and control rats. 

One lobe of the thyroid phantom was filled with a standard potassium iodide solution. The opposite lobe was initially kept empty. The count rate in the iodine K peak obtained by irradiating the filled-in lobe was recorded for varying standard potassium iodide concentrations ranging from O.Ol to 10 mg/ml. Fig. 7 shows the dose-response curve with the thyroid phantom in air and then in the neck phantom. A linear relationship was maintained but as expected the count rate is reduced with the thyroid in the neck phantom due to absorption of the incident and emitted photons by the overlying thickness of wax. Addition of potassium iodide solution in the opposite lobe made no significant difference to the dose response curve for the left lobe. 

The use of the thyroid inhibitors to induce considerable weight gain through augmented water retention is both fraudulent and undesirable for safety reasons. The thiouracils may be detected at the 1 mg/kg level by UV irradiation after TLC. More sensitive methods involving derivatisation to form fluorescent compounds followed by TLC or HPLC are also available with limits of detection of the order of 10 fig/kg. The main problem in the field of hormone residue analysis is the lack of general procedures for multiple residue detection the demands placed on the analyst by EEC proposals provide a stimulus towards further development. 

The severity of induced irradiation is related to the efficiency of the iodide trapping mechanism in the thyroid gland. Subsequent deleterious effects of thyroidal irradiation are the development of thyroid cancer, thyroid nodules and hypothyroidism (1-2). Radiobiological aspects of the problem have been reviewed by Dr. Malone. The present paper deals with the conditions which are responsable for the irradiation level of the thyroid gland by 1311 jn a given population. 

As a consequence, in the case of a nuclear accident, the irradiation level of the thyroid gland by vvill mainly depend on the value of thyroid uptake and in turn on the size of the iodine supply of the population. Regarding the low iodine supply observed in a series of European countries (4), an increased risk of thyroid irradiation by 1311 could be ejqiected. 

Introduction of iodine prophylaxis programs results in a reduction of the thyroid irradiation by a factor of 2 to 3. A generalized iodine prophylaxis is justified regarding this problem, beciause of ... 

I therapy as absolutely safe at each age (31 ). Indeed the actual total follow-up period in children and adolescents is still much lower than the average time interval (25 years) between X-ray irradiation of the neck and thyroid cancer. Although the occurrence of hypothyroidism after I therapy is usually considered as an unwanted side effect, intentional thyroid destruction has in some cases, when follow-up is likely to be difficult, been successfully( ) attained... 

PNP-DTP has been used to photoaffinity label the thyroid hormone nuclear receptors in intact cells by preparing a derivative of 3,5,3 -triiodo-L-thyronine with the crosslinker (Pascual et al., 1982 Casanova et al., 1984). Effective photoreactive conjugation was found to occur after irradiation with UV light at 254 or 310 nm. 

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