RADIATION SAFETY, PRECAUTIONS AND ADVICE TO PATIENTS ADMINISTERED THERAPEUTIC DOSES OF I-131
The therapeutic use of 131I may be a potential radiation risk both for family members and individuals close to the patient, as well as, health workers and the environment. Therefore, it must be used according to strict safety measures in order to avoid unnecessary exposure to radiation. In places where there are not local regulations for patient release after the administration of 131I we recommended to adopt the criteria proposed by the United States Nuclear Regulatory Commission (NRC) published as 10 CFR 35.75 and the Regulatory Guide 8.39. This new rule changes a general limit to an individual based estimation of the radiation risk. According to this regulation the physician in charge may authorize the release of any individual who has been administered radiopharmaceuticals if the total effective dose equivalent to any other individual from exposure to the released individual is not likely to exceed 5 millisieverts (0.5 rem). In this paper we suggest some written instructions to the patients to ovoid the unnecessary radiation to the general public, health worker and the environment.
The present article is partially based on recommendations of a panel of experts invited by the Chilean Society of Endocrinology and Metabolism in order to achieve a "Consensus proposal for the use of 131-I in the treatment of the thyrotoxicosis and thyroid cancer" (1) and in the author's presentation during the International Seminar on Therapeutic Applications of Radiopharmaceuticals held in Hyderabad, India on January 1999 organized by the International Atomic Energy Agency (IAEA)
GENERAL RECOMMENDATIONS :
Therapeutic doses of 131I may be a potential radiation risk both for family members and individuals close to the patient, as well as, health workers and the environment. Therefore, it must be used according to strict safety measures, precautions and special instructions in order to avoid unnecessary exposure to radiation.
The administration of 131I must be done under the responsibility of a physician who must hold a license to manipulate radioactive materials. The reception, use and storage of radioactive material must be done at a medical institution which holds a radioactive installation license.
The physician administering the 131I dose shall be responsible for taking all the precautions to avoid unnecessary radiation to people close to the patient, health personnel and the general public. This professional must keep a logbook with all the radioactive quantities administered to each patient. As a rule, all reasonably acceptable measures should be taken to decrease radiation exposure to a minimum (criteria known as "ALARA" = As Low As Reasonably Achievable). In places where there is no written rules to release patients submitted to radioactive treatment, we recommend the adoption of the U.S.Nuclear Regulatory Commission (NRC) established in rule 10 CFR35.75 (2), which was revised and came into effect on May 29, 1997. This rule is in agreement with the dispositions of the International Commission for Radiation Protection (ICRP 60, 1990) and the National Council for Radiation Protection and Measurements of the USA (NCRP).
In summary, the new rule number 10 CFR 35.75 establishes the following:
This new rule allows for the establishment of criteria based on each individual and his or her own environment (3). Thus, more flexibility is allowed for a person who lives alone in a solid house and more strict measures must be taken when the patient lives with the family in a small room in a less solid construction. This new rule replaces the former that only allowed to send home individuals with dosages lower than 1110 MBq (30 mCi) or when radiation measured at 1 meter from the patient did not go over 0.05 mSv (5 mrem) per hour. However, this new regulation forces to dedicate more time to the patients to know their environment and customs in order to make a personalized calculation of the risk of the people potentially exposed. For this purpose there are special computer softwares to carry out the corresponding calculations. These are based on the following formula that is applicable to all radionuclides (4):
D (t) = Accumulated exposure at time t, in R (assuming that 1 R= 1 rem).
34.6 = Conversion factor of 24 hr. times the total integration decay (24* 1.44)
= Specific gamma ray constant of the radionuclide, R/mCi-hr at 1 cm.
Q0 = Initial activity of the source in mCi
Tp = Physical half life in days
r = Distance from the source to the point of interest in cm.
t = Exposure time in days
The factor (1- e-0.693t/Tp) represents the time of exposure or occupancy factor. With isotopes with a physical half life less than one day this value is substituted by 1 or 0.75 in the formula. If the radionuclide has a physical half life greater than one day an occupancy factor of 0.25 is generally used.
If, for example, we need to calculate the maximum likely dose to an individual exposed at 1 meter from a patient who has received 30 mCi of I-131 the following values should be used in the formula:
With this result we can observe the agreement of the new rule with the previous one to limit the ambulatory treatment to an activity not greater than 30 mCi since this dose produces a maximum exposure below 0.5 rem (5 millisieverts) at 1 meter to the source.
SUGGESTIONS FOR PATIENTS WRITTEN INSTRUCTIONS SHEET
Why are you going to receive radioactive treatment?
You are going to receive radioactive iodine treatment because together with your doctor has been decided that this is the best option for your disease. Most of the radiation emitted by the iodine will be absorbed by your thyroid gland, which is located in the anterior part of the neck. This radiation interferes with the function of your gland producing a desired and beneficial effect for your disease. However, small quantities of the radiation present in your body may reach people close to you exposing them to this radiation unnecessarily. Although there is no evidence that this radiation exposure has damaged other individuals, people should avoid exposure to any unnecessary radiation.
How is radioactive iodine administered and what sort of preparation is required?
Radioactive iodine is given orally in variable quantities according to the type of your disease. Your treating doctor together with the physician who will actually administer the treatment determined the dose. According to the administered dose and your condition, it is possible that you should remain hospitalized for some days. Women must be completely sure that they are not pregnant at the time they receive the treatment. Food should not be ingested in the 2 hours before receiving the treatment and in some cases, an iodine-low diet will be recommended for a few days. You should talk to your doctor to clarify all your doubts in order to organize the activities of you and your family.
For how long does iodine remain in my body?
Radioactive iodine remains in your body just for a few days. Mainly the urine eliminates most of the iodine not retained in your thyroid, within 48 hours. A small quantity will be present in the saliva, sweat and stools. The radioactive iodine that remains in your thyroid gland also decreases quickly. This means that the possibility of unnecessary radiation exposure to other people also decreases in a matter of days.
In which way other people may be exposed to my bodys radiation?
Radiation emitted by the radioactive iodine in your body is very similar to the x-rays used in radiological exams. For this reason, people who remain close to you and for prolonged times may be exposed to an unnecessary and avoidable radiation.
Besides the above mentioned radiation, there is the possibility that other people close to you may directly ingest small quantities of radioactive iodine eliminated by your body in the urine, saliva or sweat.
In which way can I reduce the risk of radiation exposure to other people?
Even though the amount of radioactive iodine present in your body is small, and there is no evidence that the radiation emitted by it may cause problems, anyway it is advisable to decrease the opportunities to exposure as much as possible. The three basic principles to avoid unnecessary radiation exposure are:
Michaud P. Proposición de consenso para el uso de 131-I en el tratamiento de la tirotoxicosis y el cancer del tiroides. Rev Med Chile 1998; 126: 855-865
U.S.Nuclear Regulatory Commission. Criteria for the Release of Individuals Administered Radiactive Material 10 CFR Parts 20 and 35, 1997.
U.S.Nuclear Regulatory Commission. Regulatory Guide 8.39. Release of Patients Administered Radiactive Materials. April 1997.
National Council on Radiation Protection and Measurements (NCRP), "Precautions in the Management of Patients Who Have Received Therapeutic Amounts of Radionuclides", NCRP Report No. 37, October 1, 1970
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