Radiation-Related Thyroid Cancer

Abstract

Radiation is an environmental factor that elevates the risk of developing thyroid cancer. Actual and possible scenarios of exposures to external and internal radiation are multiple and diverse. This article reviews radiation doses to the thyroid and corresponding cancer risks due to planned, existing, and emergency exposure situations, and medical, public, and occupational categories of exposures. Any exposure scenario may deliver a range of doses to the thyroid, and the risk for cancer is addressed along with modifying factors. The consequences of the Chornobyl and Fukushima nuclear power plant accidents are described, summarizing the information on thyroid cancer epidemiology, treatment, and prognosis, clinicopathological characteristics, and genetic alterations. The Chornobyl thyroid cancers have evolved in time: becoming less aggressive and driver shifting from fusions to point mutations. A comparison of thyroid cancers from the 2 areas reveals numerous differences that cumulatively suggest the low probability of the radiogenic nature of thyroid cancers in Fukushima. In view of continuing usage of different sources of radiation in various settings, the possible ways of reducing thyroid cancer risk from exposures are considered. For external exposures, reasonable measures are generally in line with the As Low As Reasonably Achievable principle, while for internal irradiation from radioactive iodine, thyroid blocking with stable iodine may be recommended in addition to other measures in case of anticipated exposures from a nuclear reactor accident. Finally, the perspectives of studies of radiation effects on the thyroid are discussed from the epidemiological, basic science, and clinical points of view.

Keywords: Chornobyl; Fukushima; radiation; radioiodine; thyroid cancer.

Graphical abstract

Figure 1.

Thyroid doses from diagnostic radiological and nuclear medicine procedures. (A) Estimates of doses from radiological examinations for a reference adult; the doses from CT are averaged for males and females (18, 19); mean thyroid doses for newborn, 5-year-old and 15-year-old children (the star, triangle and square, respectively) from head and chest CT scans (20), and for children with average age of 8-year-old (the diamond) from neck CT (21). (B) Estimates of weighted average thyroid doses from nuclear medicine examinations in the first decade of 2000s (18).

Figure 2.

Thyroid dose ranges from different exposure categories and situations. NB, natural background; M, mammography; XR, conventional X-ray radiography; F, fluoroscopy; CT, computed tomography; NM, diagnostic nuclear medicine procedures; EBRT, external beam radiotherapy; RIT, radioiodine therapy; PP, planned public; PO, planned occupational; EP, existing public; EMRG, emergency exposures.

Figure 3.

Relative risk of thyroid cancer from external exposures due to radiotherapy (A) in a wide dose range and (B) in a low-dose range. (A) Different symbols correspond to different studies in the pooled analysis. CCSS, the Childhood Cancer Survival Study. Reprinted and modified with permission from Veiga et al Radiat Res, 2016; 185(5):473-84. © Radiation Research Society.

Figure 4.

Incidence of thyroid cancer (per 100 000 individuals from corresponding age groups) in Belarus in individuals aged <18 years at the time of the Chornobyl accident by age groups at diagnosis. Data from (171) are reproduced under the Creative Commons Attribution-NonCommercial 4.0 International Public License. The lines were spline-smoothed for clarity.

Figure 5.

The major changes of the Chornobyl PTC in time. Red designates the higher frequencies or greater values, and green corresponds to the lower frequencies or smaller values. With increasing patient age and longer tumor latency, the frequencies of PTCs with papillary dominant growth pattern, oncocytic changes, and multifocal growth are increasing; and this is paralleled by the increasing frequency of the BRAF^V600E-positive tumors. In contrast, the M:F ratio, radiation dose, probability of tumor causation due to radiation, tumor size, the frequencies of tumors driven by fusion genes, follicular or solid-trabecular growth patterns, and tumor invasive features are declining.