Last Updated: 2009-10-27 13:07:04 -0400 (Reuters Health)

NEW YORK (Reuters Health) - Childhood cancer survivors are 18 times more likely to develop thyroid cancer than the general population, UK researchers report.

"Survival after childhood cancer has greatly improved over the last three decades with 5-year survival 75% during the 1990s compared to 25% during the 1960s in the United Kingdom," Dr. Aliki J. Taylor, of the University of Birmingham, and colleagues write in the November 15th issue of the International Journal of Cancer. "Childhood cancer survivors are at an increased risk of late effects of treatment, including the development of second primary neoplasms."

To assess the risk of developing thyroid cancer, the researchers analyzed data from 17,980 patients who were enrolled in The British Childhood Cancer Survivor Study (BCCSS) and had survived at least 5 years after diagnosis with a childhood malignancy from 1940 to 1991.

During 340,202 person years and a median follow-up of 17.4 years per survivor, 50 cases of thyroid cancer were identified, compared to 2.8 expected in the general population (Standardized Incidence Ratio 18.0). Thirty-one (62%) were papillary carcinomas, 15 (30%) were follicular carcinomas, and 4 (8%) were of other histology.

Forty-four patients (88%) who developed thyroid cancer had been treated with radiotherapy in or around the thyroid gland.

The standardized incidence ratio "was highest after Hodgkin's disease (SIR 54.7) and non-Hodgkin's lymphoma (SIR 52.8) and lowest after leukemia (SIR 26.5), CNS primary (SIR 13.8), and 'other' childhood cancers (SIR 7.4)," the authors explain.

The SIR was higher among subjects who had radiotherapy compared to those who did not (SIR 25.4 vs. 3.9, p < 0.001). Patients treated with radiotherapy had a relative risk for thyroid cancer of 4.6 (p = 0.003).

"These results will be of use in counseling survivors of childhood cancer exposed to radiation in or around the thyroid area," the authors conclude.

Int J Cancer 2009;125:2400-2405.