Table 3 Classification of thyroid cancer

Percentage

Follicular-cell derived:

differentiated carcinoma

. papillary and papillary-follicular . follicular

. Hurthle cell (oncocytic) poorly differentiated carcinoma undifferentiated (anaplastic) carcinoma

Parafollicular-cell derived medullary carcinoma of the thyroid

Nonepithelial tumors: Lymphoma Sarcoma

Epidermoid carcinoma

Metastatic

Adapted from: Biddinger P and Nikiforov YE. Pathologic features of thyroid tumors. In: Thyroid Cancer. Fagin JA (ed.); Kluwer Academic Publishers. 1998.

large component of radioiodine isotopes, in the Marshall Islands following atmospheric nuclear weapons testing was that exposed persons developed thyroid nodules and an increased incidence of cancers after several decades. It was anticipated at the time of the Chernobyl nuclear reactor accident, which also released a large amount of iodine isotopes, that there would be an increase in the number of thyroid cancers but the rising incidence within only five years was surprising at the time. In retrospect, this early appearance was predictable as it merely reflects the progressive broadening of the Gaussian distribution of risk on account of the very large exposed population to that event. In the 15 years since the Chernobyl accident, it appears that the population at highest risk of developing thyroid cancers is the group that ranged from the second trimester in utero to about five years in age at the time of exposure and that the annual incidence of new cases may now have peaked.

In the United States, the National Cancer Institute estimated that radiation exposure of Americans from weapons testing fallout may have been responsible for 7,000-70,000 thyroid cancers. This estimation assumes that the linear, no threshold hypothesis holds at the doses and dose rates in question. The question then arises whether the diagnostic use of x-rays and isotopes also contributes to the incidence of thyroid cancers. It appears, from the Swedish Cancer Registry, that patients who underwent 131I imaging in the past do not have an increased incidence of malignancy compared to those who had similar radiation exposures from x-rays. The difference has been attributed to difference in dose rates.

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