Permissible Doses and Monitoring

Maximum Permissible Dose

Many people are killed or injured each year by automobiles, yet the benefits of the automobile are thought to outweigh the risks. To minimize the inherent risks, certain safety regulations have been applied to the use of automobiles. While radiation does not usually kill or maim people, there is a risk associated with its use, so the benefits can be optimized only by controlling the risks.

To maximize the benefits related to the use of radioisotopes/radioactive materials, the State of California has established regulations that govern the use of radioactive materials and/or radiation-producing machines. These requirements are enforced at the University/Medical Center by the Radiation Safety Committee through the Radiation Safety Office.

The maximum permissible dose equivalent in Rem for workers is as follows:

When reasonable, precautions are taken. Hospital workers seldom receive more than a small portion of the maximum allowed for the occupationally exposed.

Occupational workers are required to wear radiation monitoring devices that will measure the radiation exposure received, if they are likely to receive an exposure greater than or equal to 10% of the limit for an occupational worker. Nurses may be required to wear radiation monitors when caring for therapy patients with implanted radioactive materials, etc. This is a matter for the discretion of the Radiation Safety Committee.

Personnel Monitors

There are three common types of personnel monitors: film badges, TLDs, and ionization chambers. A film badge is a small device containing a piece of photographic film that is sensitive to radiation. The badge is worn by the worker and collected periodically for analysis. The degree of film darkening is a measure of the radiation exposure received. TLDs are small chips of material (usually LiF) that are placed on a holder and worn by the user, often on the finger. Periodically, these holders are collected for analysis to determine the dose received.

Ionization chambers, or pocket dosimeters, consist of a hollow tube with an electrically charged, insulated wire inside. As the air in the tube is exposed to radiation, the charge on the wire is gradually depleted. The degree of charge on the wire can be read by means of a scale on the inside of the tube. Pocket dosimeters can be read at any time, so the worker can know the amount of radiation exposure he/she has received by checking the scale periodically. Pocket dosimeters are usually used when the worker may be exposed to a relatively high exposure rate.

Area Monitors

Following the administration of therapeutic quantities of radioactive materials to patients, it is necessary to survey the patient's room to determine the radiation exposures at various locations around the patient's bed. If unsealed radioactive materials are used (e.g., I-131 therapy), dishes, linens, and utensils may become contaminated with radioactive material, so must be monitored before leaving the patient's room. These measurements are made using survey meters. Survey meters may be ionization chambers, Geiger-Muller (GM) detectors, or scintillation detectors. Ion chambers are usually used to measure exposure rates. GM and scintillation detectors are usually used to monitor for contamination since they tend to be more sensitive.

Reduction of Exposure/Shielding/Distance/Time

To prevent the harmful effects of sunlight, we must control the exposure to it. In the case of sunlight, we control exposure by limiting the amount of time we spend in the sun and by protective clothing (shielding). With a sun lamp, one can further control the exposure by changing the distance of the lamp from the skin.

The same factors (time, distance, shielding) are used to control radiation exposure. For gamma radiation, such as that emitted by I-131, dense materials such as lead are used for shielding because they absorb better than less dense materials. Beta radiation, on the other hand, is not nearly as penetrating, and must be shielded using low-density materials such as plastic or wood.

Time is the first factor used to control radiation exposure. Obviously, the less time a person spends in an area with radiation exposure, the less radiation is received.

Distance is the second method used to control radiation exposure. Radiation exposure is reduced as the distance from the source is increased according to the inverse square law. This means that if you stand 1 meter (~3 feet) from a source of radiation, and you receive an exposure of 16 mRem in one hour, you can reduce the dose to 1/4th of this value (4 mRem in one hour) by moving to a distance 2 meters from the source; or you can reduce the dose to 1/16th of the original value (1 mRem in one hour) if you move to a point 4 meters away from the same source.

Nurses in hospitals may be involved with patients with high-energy gamma-emitting radioactive materials such as I-131, Ra-226, and Cs-137 that require thick lead shielding. This is not always practical and it may be necessary to take advantage of the protection afforded by the judicious use of time and distance to minimize the radiation exposure.

Precautions for Nurses and Visitors

Since radiation exposure to nurses will be highest during the care of a patient with therapeutic quantities of radioactive materials, Figure l may be of value when planning work time with a patient or to the physician when allotting visiting time to relatives and friends of the patient. The curves are from actual measurements made at the bedside of a patient who has received 100 mCi of Ra-226 or I-131. At any point along a curve, the dose rate in milliroentgens per hour (mR/hr) is the same. Variations in distance occur because of the differential absorption of radiation in the various body tissues.

Fig. 1 Isodose rate curves around the bed of a patient during radioisotope therapy standardized at 100 mCi for illustrative purposes. Actual administered doses may differ substantially. Numbers represent milliroentgens per hour. Ra226.----- I131: ·····

The intensity of the radiation emitted from a patient can be divided roughly into 5 zones: Zone 0 which includes the patient's bed, and four zones each approximately 2 feet wide. It is obvious from the illustration that much of the nurse's work will be done within the first two zones that are within four feet of the patient's bed. If a nurse were to stand at three feet from the bed of a patient with 100 mCi of radium implanted, she/he would be exposed to approximately 30 mR/hour and receive a dose of approximately 30 mrem/hour. At this distance, the nurse would reach the annual maximum permissible limit in 167 hours. Radium does not decay appreciably during the treatment time, so the dose rates shown will be the same as long as the patient is undergoing treatment. Radioiodine may also be used in 100 mCi amounts, but the dose rate decreases over time since some of the I-131 is excreted and since some of the activity decays due to the shorter half-life.

The dose rates shown in Figure 1 are proportional to the activity, so if only 10 mCi are administered to the patient, the dose rate in each zone will be 10 percent of that shown without any supplemental shielding. Available portable lead shields can substantially reduce the dose to nurses and visitors. Leaded aprons used in radiography procedures are generally not recommended in this situation.

The maximum permissible exposure for non-radiation workers, e.g., visitors, is 100 mrem per year, and it is best to be conservative, so the visiting times for friends and relatives may be limited and a minimum safe distance from the patient may be specified by the physician. These limitations should be strictly adhered to. In all cases, follow the Medical Center policies related to the specific procedure.

Place and Record of Isotope Administration

Radioactive materials may be administered to the patient in the nuclear medicine department, in the patient's room, or in the operating room. In every case, the actual radioactive materials administered must be recorded on the patient's chart.

To notify personnel that patients have received therapeutic quantities of radioactive materials, a radioactive materials label is attached to the patient's wristband and chart noting the date, amount, and type of isotope administered. This label accompanies the patient at all times during his/her stay in the Medical Center.

Special information regarding the precautions advisable with each of the commonly used radioisotopes are detailed in specific Medical Center policies/procedures available on the unit or from the Radiation Medicine Department.

Control of Radiation Use

At LLUMC, the Radiation Safety Committee oversees the various uses of radioactive materials. The Radiation Safety Committee has adopted regulations to assure the safety of patients, personnel, and visitors. Details can be obtained from the Radiation Medicine Department or the Office of Radiation Safety, extension 44913.

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