rs occasionally, and rarely 4 or 5 therapies are given. The reason for this failure is usually not clear. The radiation effect may occur slowly. A large store of hormone in a large gland may be one cause of a slow response. Occasional glands having an extremely rapid turnover of 131-I require such high doses of the isotope that surgery is preferable to continued 131-I therapy and its attendant whole body radiation. If a patient fails to respond to one or two doses of 131-I, it is important to consider that rapid turnover may reduce the effective radiation dose. Turnover can easily be estimated by measuring RAIU at 4, 12, 24, and 48 hours, or longer. The usual combined physical and biological half-time of 131-I retention is about 6 days. This may be reduced to 1 or 2 days in some cases, especially in patients who have had prior therapy or subtotal thyroidectomy. If this rapid release of 131-I is found, and 131-I therapy is desired, the total dose given must be increased to compensate for rapid release. A rough guide to this increment is as follows: Increased dose = usual dose X ( (usual half time of 6 days) / (observed half time of “X” days) ) Most successfully treated glands return to a normal or cosmetically satisfactory size. Some large glands remain large, and in that sense may constitute a treatment failure. In such a situation secondary thyroidectomy could be done, but it is rarely required in practice. Long term care- Patients who have been treated with RAI should continue under the care of a physician who is interested in their thyroid problem for the remainder of their lives. The first follow-up visit should be made six to eight weeks after therapy. By this time, it will often be found that the patient has already experienced considerable improvement and has begun to gain weight. The frequency of subsequent visits will depend on the progress of the patient. Symptoms of hypothyroidism, if they develop, are usually not encountered until after two to four months, but one of the unfortunate facts of RAI therapy 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 gre