py is that hypothyroidism may occur almost any time after the initial response.
HAZARDS OF 131-I TREATMENT
In the early days of RAI treatment for Graves’ disease, only patients over 45 years of age were selected for treatment because of the fear of ill effects of radiation. This age limit was gradually lowered, and some clinics, after experience extending over nearly 40 years, have now abandoned most age limitation. The major fear has been concern for induction of neoplasia, as well as the possibility that 131-I might induce undesirable mutations in the germ cells that would appear in later generations.
|Table 8. Gonadal Radiation Dose (in Rads) From Diagnostic Procedures and 131-I Therapy|
|131-I-therapy, 5mCi||usually <1.6||usually <1.6|
|Adapted from Robertson and Gorman |
Radiation is known to induce tumor formation in many kinds of tissues and to potentiate the carcinogenic properties of many chemical substances. Radiation therapy to the thymus or nasopharyngeal structures plays an etiologic role in thyroid carcinoma both in children and in adults[ 83- 85]. 131-I radiation to the animal thyroid can produce tumors, especially if followed by PTU therapy . Cancer of the thyroid has appeared more frequently in survivors of the atomic explosions at Hiroshima and Nagasaki than in control populations . Thyroid nodules, some malignant, have appeared in the natives of Rongelap Island as the result of fallout after a nuclear test explosion in which the radiation cloud unexpectedly passed over the island .
Thyroid cancer following 131-I treatment?
The experience at 26 medical centers with thyroid carcinoma after 131-I therapy was collected in a comprehensive study of the problem. A total of 34,684 patients treated in various ways were included. Beginning more than one year after 131-I therapy, 19 malignant neoplasms were found; this result did not differ significantly from the frequency after subtotal thyroidectomy. Thyroid adenomas occurred with increased frequency in the 131-I treated group, and the frequency was greatest when the patients were treated in the first two decades of life . Holm et al  have thoroughly examined the history of a large cohort of 131-I-treated patients in Sweden and similarly found no evidence for an increased incidence of thyroid carcinoma or other tumors. For reasons that are not clear, the injury caused by 131-I therapy for Graves’ disease seems to induce malignant changes infrequently.. This result may occur because the treatment has largely been given to adults with glands less sensitive to radiation, because damage from 131-I therapy is so severe that the irradiated cells are unable to undergo malignant transformations, or because all cells are destroyed, or possibly because of the slow rate at which the dose is delivered . In up to one-half of patients followed for 5-10 years, there may be no viable thyroid cells remaining. We note that two studies reported above extend through an average follow-up period of 15 years.
As described above [44.1], a recent report by Franklyn and coworkers indicated that there is an increased (3.25 fold) risk of mortality from cancer of the thyroid (and also bowel) after RAI, detected in along term follow up of a very large patient cohort. However it remains uncertain that this is related to hyperthyroidism per se, or radioiodine therapy.
While these data are reassuring in regard to 131-I use in adults, Chernoby made it clear that its use in children can not be considered safe. Children in the area surrounding Chernobyl have developed a hugely increased incidence of thyroid carcinoma predominately due to ingestion of iodine-131 [89.1]. The latency has been about 5 years, and younger children are most affected. Risk is probably linearly related to dose. It is apparent that low doses, possibly down to 20 rads, produce malignan change in children(89.2).Risk of carcinogenesis decreases with increasing age at exposure, and is much less common after age 12. However some data indicates that an increased incidence of thyroid carcinoma is seen even among adults exposed at