TABLE 1998 Commonly Treated Forms of Internal Contamination

RADIOIODINE Inhalation or ingestion of radioiodine is particularly hazardous to the thyroid with a potential risk of causing hypothyroidism or thyroid cancer. I-131 is the predominant internal contaminate resulting from incidents that involve the release of nuclear fission products such as a nuclear reactor accident or nuclear weapons test. Studies on the health effects of the Chernobyl accident have shown that populations in heavily contaminated areas have an increase in thyroid cancer presumed to have resulted from radioiodine exposure. The number of thyroid cancers reported in these areas continues to increase, with the highest prevalence in individuals who were under the age of 10 years at the time of the accident.4

Rapid detection of radioiodine uptake is essential because treatment is most beneficial when administered within 12 hours. Radioiodine is a soluble nuclide that is detected in the urine immediately after exposure. Thus urine excretion measurements are used to provide early identification of radioiodine uptake. A more rapid, but less sensitive, method is direct measurement with a survey meter held over the thyroid gland.

Early administration of stable potassium iodine (KI) is recommended for high levels of accidental radioiodine exposure. Oral administration of 300 mg of stable iodine (390 mg of KI) within one hour of exposure is approximately 90 percent effective at blocking the thyroid uptake of radioiodine. KI administered six hours after exposure reduces thyroid uptake by approximately 50 percent. Little protective effect is seen when KI is given after 12 h of exposure. KI should be continued daily for 7 to 14 days to prevent recycling of radioiodine.5 Administration of antithyroid medications, such as propylthiouracil or methimazole, may be considered if the time since exposure is more than 12 h.6 In the event of widespread release of radioiodine, the FDA has approved the use of nonprescription KI. State and local officials are responsible for the procurement and distribution of KI to the general public.

ALPHA CONTAMINATION Early treatment is recommended for contamination with alpha-emitting radionuclides such as Pu-239. Alpha radiation has a high LET with potential for significant damage when internally deposited. This internal contamination hazard is a result of alpha's deposition of large amounts of energy in a short range and thus causing intense ionization in a concentrated area. This potential damage is made worse by the fact that many of the radioisotopes that emit alpha radiation have very long half-lives.

Chelating agents such as calcium and zinc salts of diethylenetriamine pentaacetic acid (Ca-DTPA and Zn-DTPA, respectively) are effective treatments for contamination with heavy metals and rare earths that emit alpha radiation. If alpha-emitting contamination is detected in wounds or in the nares or oropharynx, treatment with DTPA should be initiated promptly, ideally within 1 to 2 h after contamination has occurred. Potential contraindications for the use of DTPA are severe renal dysfunction, thrombocytopenia, or leukopenia. DTPA is administered systemically by slow IV push or by aerosol administration. Ca-DTPA has been shown to be more effective in animal studies and is the preferred form of drug for the initial one to two days of treatment. Zn-DTPA is less toxic and recommended for treatments of longer duration and of pregnant females. For aerosol administration, Ca-DTPA is preferred because of the metallic taste associated with Zn-DTPA. Most DTPA solutions in nuclear medicine departments are a too dilute concentration for effective decorporation. DTPA for this purpose can be obtained from Oak Ridge Associated Universities, REACTS, Oak Ridge, TN 37831.

The success of decorporation techniques depends on early administration. The risks associated with therapy must be weighed against the risk of untreated internal radiation exposure. Assistance from expert consultants should be requested prior to beginning decorporation therapy.

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