Childhood Thyroid Carcinoma

The nuclear accident at Chernobyl in 1986 refocused interest on radiation as a factor in the development of thyroid carcinoma.113"122 Prior to 1950, irradiation was frequently used to treat acne, enlarged tonsils and adenoids, chronic sinusitis, and other benign conditions.115 External radiation was commonly used to irradiate enlarged thymuses in infants and young children. The latent period, the interval from exposure to the appearance of thyroid cancer, was assumed to be 10 years and increased for at least 3 decades. Childhood thyroid cancer appeared within 4 years after the Chernobyl accident.

Thyroid carcinomas in children and adolescents are rare and differ from adult carcinomas of the thyroid gland in the following ways113"118:

• More advanced disease in children: a high rate of extrathyroidal invasion, more frequent metastases to regional lymph nodes and lungs, larger tumors but a lower rate of nondiploid tumors

• Low mortality rates in spite of what may be more advanced disease; higher rates of recurrences in lymph nodes and lungs

The histologic categories for the thyroid tumors found in irradiated patients include113"121:

• Papillary carcinoma, including diffuse sclerosing and oxyphilic types

• Follicular carcinoma

• Medullary carcinoma

• Solid-follicular type

The papillary and follicular carcinomas are identical to those described in adults. Medullary carcinomas constituted a significant part (17%) of one report.121 However, the frequency of these tumors increased with the passage of time, with the highest number recorded in the last segment of one survey; hence, the increase is attributed to the development of screening programs.

The results of investigations of thyroid cancers occurring after the Chernobyl accident led to the consideration of these questions concerning postradiation cancers in children:

• Does the age of a patient affect the development of thyroid carcinomas after exposure?

• Is there a unique histologic variant of thyroid cancer in this group of children?

• Do such thyroid carcinomas behave more aggressively?

Studies related to thyroid carcinomas associated with the

Chernobyl accident have demonstrated that the thyroid glands of very young children are much more sensitive to the carcinogenic effect of radiation from fallout than the thyroid glands in older children.121 The relative risk of developing thyroid carcinomas in irradiated children younger than 1 year was 44 times more than the relative risk in nonirradi-ated patients (237:6).

The histology of the tumors was identical to that in postirradiated adults with the exception of a unique thyroid carcinoma, the solid-follicular type. A previous analysis of such tumors indicated that they may well be more aggressive than other histologic forms found in children.122

Microscopic examination of the tumors found in children exposed at Chernobyl showed a significant number of solid-follicular carcinomas, most prevalent in patients in whom the tumors developed earliest after exposure. However, can one conclude that these tumors are more aggressive than other forms of well-differentiated carcinomas? One probably cannot, and longer follow-up is necessary to answer this question.

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