Benign thyroid adenomas among persons X-irradiated in infancy for enlarged thymus glands.

Thyroid adenoma incidence in a cohort of 2657 infants given X-ray treatment for a supposedly enlarged thymus gland, along with 4833 unirradiated siblings, has been ascertained for an average of 37 years since irradiation. Estimated thyroid doses ranged from 0.03 to > 8 Gy, with 62% receiving < 0.5 Gy. After excluding 4 adenoma cases with concurrent or previous thyroid cancer, there were 86 cases with pathologically diagnosed thyroid adenomas in the irradiated group and 11 in the sibling controls. The estimated excess relative risk (ERR) was 6.3 per gray (90% CI = 3.7, 11.2). Once the dose group with > or = 6 Gy, which was producing downward curvature in the dose-response function, was removed, the curve was compatible with linearity and the ERR was 7.8 per gray. Thyroid adenoma rates were elevated even at low doses: the lowest dose group (< 0.25 Gy) showed a significant elevation in risk. The relative risk appeared to be constant over time and was comparable for both sexes. Excess adenoma risk was observed in the irradiated group to the maximum follow-up interval of about 50 years. A number of potential risk factors for thyroid adenoma were examined both as risk factors in their own right and as modifiers of the radiogenic risk. Parity and use of hormones in relation to menopause were significantly associated with thyroid adenoma risk in women, while education, Jewish origin, history of hyperthyroidism or hypothyroidism, and family history of cancer were also adenoma risk factors in both sexes. An examination of interactions between possible risk factors and radiation suggested that women with a history of oral contraceptive use or hysterectomy and persons with a family history of cancer may have greater risk (per unit dose) of radiogenic thyroid adenomas than their counterparts.