medical exposure to radiation and thyroid cancer pdf

Medical Exposure To Radiation And Thyroid Cancer Pdf

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Radiation-induced damage is a complex network of interlinked signaling pathways, which may result in apoptosis, cell cycle arrest, DNA repair, and cancer. The development of thyroid cancer in response to radiation, from nuclear catastrophes to chemotherapy, has long been an object of study. A basic overview of the ionizing and non-ionizing radiation effects of the sensitivity of the thyroid gland on radiation and cancer development has been provided. In this review, we focus our attention on experiments in cell cultures exposed to ionizing radiation, ultraviolet light, and proton beams.

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Environmental radioactive contamination caused by the Fukushima Dai-ichi Nuclear Power Plant accident has aroused great concern regarding a possible increase in the incidence of childhood thyroid cancer.

Some of their outcomes are reported regularly and made available to the public. We have detailed measurements of the air-dose rates and radioactive elements in soil in many places all over the Fukushima prefecture. To study the dose-response relationship, we begin with the assumption that the external and internal doses are correlated with the air-dose rate and the amount of I in soil, respectively.

Finally, we consider the potential mitigating effects of evacuation from highly contaminated areas in both external and internal exposure scenarios. Since then, there has been considerable debate over the possible effects of radiation exposure on human health.

Although much is known about immediate damage caused by acute exposure, we still need to make efforts to understand the long-term health risks of exposure to low-dose radiation 1. It is reported that low-dose radiation could raise the risk of thyroid cancer, especially for young children 1 , 2 , 3. A strong association between internal exposure from I and childhood thyroid cancer became widely known after the Chernobyl accident. It is important to note, however, that epidemiological studies on radiation-induced thyroid diseases predated the Chernobyl accident 4 , 5 , 6 , 7 and continued thereafter 8 , 9 , These early studies suggested that external medical exposure of the head or neck in childhood raised the risk of thyroid cancer.

Much of the data came from the period between the s and the s when it was rather common to apply radiotherapy to some types of diseases in childhood, such as enlarged thymus, tinea capitis and so on.

In , Ron et al. Their result, expressed in terms of the dose-response relationship, has shown that the risk of thyroid cancer due to external exposure was far more serious for children than for adults. The accident that occurred at the Chernobyl NPP in April in present-day Ukraine resulted in widespread radioactive contamination A few years later, the frequent incidence of thyroid cancer was reported among young children from heavily contaminated areas in Ukraine and Belarus 3 , A few more years had passed before the international community recognized this serious situation and initiated a systematic survey of childhood thyroid cancer along with dosimetry of radioactive iodine I focusing on fresh milk as a main route of exposure In the report of the World Health Organization WHO 14 , many experts still expressed their doubts about an alleged association between reported increases in thyroid cancer incidence and low-dose radiation exposure, citing as a reason the lack of a definitive study on the dose-response relationship.

Then, in , Cardis et al. This finding, among others, finally brought the international scientific community to the conclusion that exposure to I in childhood is associated with an increased risk of thyroid cancer Now the situation in Fukushima is said to be very different from the case of Chernobyl. The estimated radiation doses are much lower than those reported in Hiroshima, Nagasaki, and Chernobyl, namely less than 20 mSv per year in most of the municipalities except for those near the Fukushima NPP It must be noted that many people evacuated from some of the most contaminated areas shortly after the beginning of the accident.

It is also said that, due to the iodine-rich Japanese diet, people in Fukushima may have much lower intakes of I than those affected by the Chernobyl accident Nonetheless, it is important to obtain an accurate estimation of possible radiation-related health risks, especially thyroid gland anomaly and thyroid cancer. The first step toward this end was taken when the prefectural government of Fukushima launched the Fukushima Health Management Survey FHMS immediately after the nuclear event.

This survey includes, among others, the thyroid ultrasound examinations TUE of children at the age of 18 and below who resided in or visited the prefecture at the time of the accident 19 , The PBLS is essential because, at the time of the accident, there was no epidemiological data in Japan using ultrasonography. Studies have suggested that the minimum latency period for radiation-induced thyroid cancer is years 3 , The data from both surveys are regularly updated, and summaries of the data are made available to public through the bulletins of the advisory board for the FHMS committee.

Moreover, the Ministry of Environment sponsored a TUE program in three prefectures far from the disaster-stricken areas Aomori, Yamanashi and Nagasaki: AYN with the same screening and diagnostic protocols as those used in Fukushima 21 , This provides us with control data to be compared with that of the FHMS. Tsuda et al. These studies have drawn markedly different conclusions regarding the relationship between the incidence of childhood thyroid cancer reported in Fukushima and low-dose radiation exposure.

These different conclusions originate from the interpretation of the screening effect 26 , However, it is important to note that an important task ahead is to elucidate the dose-response relationship as suggested by the history of the scientific controversy over the health effects of the Chernobyl disaster. Without this, we cannot determine whether or not the observed excess risk is attributable to radiation exposure. Thanks to the nuclear physics community, we have detailed deposition maps of gamma-ray emitting radioactive nuclides in eastern Japan based on extensive soil sampling in addition to air dose measurements conducted shortly after the Fukushima accident.

The result of this soil survey was published by Saito, Tanihata and others, with help of over scientists and students mobilized through several organizations 28 , These large amounts of radiation datasets, combined with the epidemiological data from the PBLS and the FSS, provide us with a strong quantitative basis to investigate the relationship between radiation doses and the thyroid cancer cases detected in both the PBLS and FSS.

Although the TUE program is still underway, we believe that the data obtained so far allow us to make useful observation of such a relationship. In this paper, we investigate the possible effects of the Fukushima NPP accident on the incidence of childhood thyroid cancer in Fukushima. In Section 2, we describe the screening test of the TUE program conducted in Fukushima immediately after the accident.

In Section 3, we summarize the dose distribution based on the soil sampling and air dose survey conducted in Fukushima. We tabulate the air-dose rate and also the amounts of Cs, Cs, and I in soil for each municipality.

We examine the reported thyroid cancer incidence against the distribution of air-dose rates and also against the amounts of I. In Section 5, we study the dose-response relationship in the external and internal radiation exposure scenarios with the information currently available in the literature on 59 municipalities using the Poisson regression method with the standard R software.

Section 6 is devoted to the summary of the present study. The TUE program in Fukushima began on October 9, with the aim of screening children at the age of 18 and below who lived in or visited the prefecture at the time of the nuclear reactor accident. Although the participation was voluntary, over , children signed up for the program. The time schedule of the thyroid cancer screening program. The PBLS is divided into two parts. The primary examination started on October 9, in areas with the highest air doses and then moved to those with intermediate- and low-level contamination until it covered the entire prefecture by March 31, The numbers indicate the date in order of year-month-day.

In some cases with two numbers, the information on day is missing. The three regions classified for screening are indicated by different colors. The reexamination of those found to have some thyroid anomaly took place during the period from July to March 31, The FSS follows a similar two-stage schedule: the primary examination was conducted between April 2, and March 31, , and the second stage took place during the period from June to March 31, Each survey proceeded through two stages.

First, all participants were screened for any anomaly with the thyroid. To give priority to those presumably at the highest risk, the prefecture was divided into three zones with different air-dose rates, and the survey began with the most contaminated areas and then moved to those with lower doses.

The reexaminations of those diagnosed with some thyroid anomaly took place during the period starting from July until March 31, For the FSS, the primary examinations began on April 2, and continued through March 31, , and the reexamination phase starting from June up to March 31, To judge whether or not there are any health effects of the nuclear reactor accident, it is important to compare the results of these two surveys with information obtained from other parts of Japan located far away from the path of the radioactive plume arising from the destroyed reactors.

We note here that the measurements of air-dose rates to be used for the dose-response relationship in this paper were performed in June and July The further survey third survey, which is called the second FSS started on May 1, By the time of December 31, , about , children participated to the screening program. In this paper, we do not include the data from the third survey. To investigate the relationship between the amounts of radiation and the thyroid cancer cases in the PBLS and the FSS, we first offer an overview of the radiation data in Fukushima.

Shortly after the nuclear reactor accident, a thorough survey was conducted with regard to radioactive materials in soil and air-dose rates at 1, sites across Fukushima. The amounts of Cs, Cs and I are provided in the paper of Saito et al. It should be noted that the amounts of radioactive materials shown in these documents are calibrated to the date of June 14, , taking account of the half-lives of I 8. Of course, it would be ideal if we were to have information on the directly measured thyroid doses of all children who underwent the PBLS and FSS in order to discuss the dose-response relationship.

A dosimetry study with 62 evacuees conducted shortly after the Fukushima accident has shown that their estimated equivalent thyroid doses were much smaller than the mean thyroid dose in the Chernobyl accident However, these data are too few and uncertain to be extrapolated for direct comparison to the results of the ultrasound screening program that involved some hundreds of thousands of people from all municipalities.

Since a complete dataset of individual doses are currently unavailable and are unlikely to come in the foreseeable future, we proceed with the assumption that the external and the internal doses of the participants in the screening program are correlated with the air dose-rates and the amounts of I in soil in their original locations at the time of the accident, respectively. We then try to refine our dose estimates by considering the mitigating effects of evacuation and also on the basis of the existing studies with small numbers of samples.

We calculate the air-dose rate and the amounts of key radionuclides for each municipality village, town, and city in Fukushima prefecture using the data of the air-dose-rate measurements and the soil studies 28 , These tables show that radiation levels are relatively high in municipalities hosting or near the Fukushima Dai-ichi NPP, and that the levels are very low in those located far west.

For example, the air-dose rates in Okuma, Futaba and Namie are In contrast, those in Hinoemata, Shimogo and Nishiaizu are 0.

There are several municipalities with large numbers of children who participated in the health survey, for which the mean air-dose rates are low and moderate. Conversely, there are relatively small numbers of participants in the areas with high mean air-dose rates. On the other hand, the amounts of I seem to consist of two separate portions, which presumably came from different sources due to the multiplicity of explosions that occurred at the Fukushima Dai-ichi NPP To appraise the relative importance of I in the overall radiological situation in Fukushima, we calculate the integrated absorbed dose of each major radioactive element found in soil in the first year starting from March 15 or March 20, , considering the short half-life 8.

This comparison is meant only for the external dose. The integrated absorbed dose from I is about one-third for the March 15 assumption and one-fifth for March 20 assumption as compared to the sum of those from Cs and Cs, where the amount of Cs is 1. In most municipalities, the contribution of I is about one-third to one-fifth of that of the Cs isotopes.

It must be noted, however, that some municipalities like Iwaki have larger contributions of I than others. If we were to take the March 15 assumption, the amount of I is comparable to the radiation dose due to the Cs isotopes in Iwaki. As for the internal dose, we explore the correlation between the thyroid cancer cases and the amounts of I in municipalities. The airborne measurements are carried out occasionally and still continue to date The comparison of the results measured on the ground 28 against those of airborne measurements shows that radiation levels continued to decline after the soil survey was conducted.

We take the data of airborne measurements from June and compare them with those of the air-dose rates measured on the ground. In most places, the rates reported by the airborne sampling program are similar to those from the soil survey.

Radiation and Thyroid Cancer

Not a MyNAP member yet? Register for a free account to start saving and receiving special member only perks. Studies of patients irradiated for the treatment or diagnosis of diseases have provided considerable information for the understanding of radiation risks, particularly for specific cancer types, including thyroid and breast cancer IARC ; UNSCEAR b. Large cohorts of radiation-treated patients who have been followed for long periods are available, allowing evaluation of cancer and other late effects. Population-based cancer registries in many countries have been used to identify these patients and to facilitate patient enrollment, thus allowing investigators to determine the risks of a second primary cancer after treatment with radiation for a primary cancer Boice and others The characteristically detailed radiotherapy records for cancer patients and patients treated for nonmalignant conditions allow precise quantification of the doses to the organs of individuals, which in turn facilitates the evaluation of dose-response relationships. Frequently, patients with the same initial condition that receive treatments other than radiation are available for comparison, although the clinical indications for treatment may differ.

Thyroid Cancer Risk Factors

Radiation exposure of the thyroid at a young age is a recognized risk factor for the development of differentiated thyroid cancer lasting for four decades and probably for a lifetime after exposure. Medical radiation exposure, however, occurs frequently, including among the pediatric population, which is especially sensitive to the effects of radiation. In the past, the treatment of benign medical conditions with external radiation represented the most significant thyroid radiation exposures. Today, diagnostic medical radiation represents the largest source of man-made radiation exposure.

Different cancers have different risk factors. Some risk factors, like smoking, can be changed. Having a risk factor, or even several risk factors, does not mean that you will get the disease. And many people who get the disease may have few or no known risk factors. Even if a person with thyroid cancer has a risk factor, it is very hard to know how much that risk factor may have contributed to the cancer.

Thyroid cancer risks among medical radiation workers in South Korea, 1996–2015

Radiation Exposure and Thyroid Cancer

This is a preview of subscription content, access via your institution. Malignant and benign neoplasms of the thyroid in patients treated for hyperthyroidism, A report of the Cooperative Thyrotoxicosis Therapy Study. J Clin Endocrinol Metab ; Google Scholar. Cancer risk after iodine therapy for hyperthyroidism.

Skip to Content. Use the menu to see other pages. Although risk factors often influence the development of cancer, most do not directly cause cancer. Some people with several risk factors never develop cancer, while others with no known risk factors do.

Buenos Aires, Argentina. The association between radiation exposure and the occurrence of thyroid cancer has been well documented, and the two main risk factors for the development of a thyroid cancer are the radiation dose delivered to the thyroid gland and the age at exposure. The risk increases after exposure to a mean dose of more than 0. The risk is more important during childhood and decreases with increased age at exposure, being low in adults.

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Thyroid Cancer Following Exposure to Radioactive Iodine

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