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11.1 Radiation Exposures

When a person works with radioactive material, he or she may receive both external and internal exposure from the material. External exposure occurs when x-rays, gamma rays or beta particles emitted from radioactive material external to the body strike the body. Internal exposure occurs after the radioactive material has entered the body through inhalation, ingestion, or absorption through the skin.

Internal exposures in staff working with large amounts of radioactive iodine are monitored by thyroid bioassay. Internal exposure in staff working with >100 mCi 3H is monitored by urine bioassay. External exposure to radiation is measured with dosimeters worn on the person. These dosimeters can measure both the penetrating (whole body) dose and the shallow, non-penetrating (skin) dose to the entire body, lens of the eye and extremities (hands).

State and federal regulatory agencies set maximum occupational radiation limits for radiation workers and members of the public, as listed below. The Fred Hutchinson Cancer Research Center's goal is to keep all exposures as low as reasonably achievable (ALARA).

Table 11.1.1: Radiation Dose Limits

Group

Dose Limit

Occupational worker

5 rem per year total effective dose equivalent

15 rem per year to the lens of the eyes

50 rem per year to the skin, organ or any extremity

Declared pregnant occupational worker

0.5 rem (entire gestation period)

Minors (under age 18)

10% of occupational worker limits

Members of the public

100 millirem in a year

Unrestricted areas

2 millirem in any one hour

11.2 Whole-body and Ring Dosimeters

Dosimeters (often called radiation badges) are used to measure an individual's external occupational radiation exposure over the period of wear. Reports of dosimeter readings are kept in permanent files in EH&S to provide a record of occupational radiation exposure.

11.2.1 Who Wears a Whole-body Dosimeter?

Staff who could receive over 10% of a legal limit must be monitored. At the Hutchinson Center, staff who meet the following descriptions must wear a whole-body dosimeter:

  1. Lab staff using 32P,51Cr,125I,131I or other high energy beta or gamma emitters greater than 100 μCi at one time (note that dosimeters cannot measure the dose from 3H);
  2. Maintenance staff working on an irradiator;
  3. Operators of non-self shielded x-ray machines or LinAc (linear accelerator);
  4. Anyone likely to receive occupational exposure in excess of 10% of the values in Table 11.1.1; or
  5. Anyone entering a high radiation area.

11.2.2 Who Wears a Ring Dosimeter?

Ring dosimeters should be worn by those staff who handle >10 mCi amounts of high energy beta or gamma emitters (>300 keV), such as 32P,131I, or 51Cr.

11.2.3 How to Wear and Care for Dosimeters

Wear a whole-body dosimeter at waist or chest level, external to clothing. It may be worn under the disposable aprons or smocks used in AHR or DE 750 but should not be worn under lab coats.

Wear a ring dosimeter on your right hand if you are right-handed, on your left if you are left-handed, or on the hand which is expected to receive the highest dose. Wear a ring dosimeter under gloves to prevent its contamination. Caution must be used when removing gloves to avoid removing the ring dosimeter with them.

A dosimeter should only be used to measure occupational radiation exposure. Do not use it to measure medical x-rays or intentionally expose a dosimeter to radiation sources not associated with your actual occupational exposure. When not worn, store dosimeters in an area with low background radiation. The dosimeters are water-resistant, but avoid conditions of high heat such as near a radiator, in a car, or in the hot water wash cycle of a clothes washer.

11.2.4 How to Order Dosimeters

Call EH&S to receive a Dosimeter & Radiation Worker Application Form.

Please indicate on the form when you will require the dosimeter. Normally, upon returning the application you should receive your whole-body or ring dosimeter within two weeks. If requested, temporary dosimeters can be issued during the current quarter.

A dosimeter should only be used to measure occupational radiation exposure. Do not use it to measure medical x-rays or intentionally expose a dosimeter to radiation sources not associated with your actual occupational exposure. When not worn, store dosimeters in an area with low background radiation. The dosimeters are water-resistant, but avoid conditions of high heat such as near a radiator, in a car, or in the hot water wash cycle of a clothes washer.

11.2.5 Returning Dosimeters

Regular radiation worker dosimeters are exchanged once a quarter. Return the dosimeter to the dosimeter coordinator for your group, or return it directly to EH&S no later than the tenthday of the first month of the quarter. EH&S sends the dosimeters to the processing company for analysis. If dosimeters are not returned by the tenth of the month, analysis of the dosimeters will be delayed.

The RSO may request more frequent exchanges for high-dose experiments or work, or if a worker is approaching a limiting dose.

If your dosimeter is not returned, it becomes an ineffective tool for dose monitoring. We are required by the Center's radioactive materials license to have a working dosimetry program, and unreturned dosimeters place our permission to use radioactive material at risk.

11.2.6 Records

All dosimetry reports are reviewed upon receipt by the RSO. Any individual who exceeds the ALARA levels or has an unusual dose will be contacted by the RSO. All exposures greater than 10% of the dose limits in Table 11.1.1 are investigated. Investigations may include visiting the lab and requesting an explanation from the person about the exposure. Records of these investigations are presented to the RSC.

Annual dose reports are available for each individual monitored that year and will be sent to all workers who are required to be monitored per WAC 246-221. A worker may request his or her dose and dose history at any time by contacting the RSO.

11.2.7 Area Monitors

Some areas which receive exposure to radiation are monitored continuously with a dosimeter to be sure no excessive levels occur. This is for tracking purposes or to determine regulatory compliance for public dose limits. If you would like an area monitored with a dosimeter, contact the RSO.

11.3 Policy on Declared Pregnancy

Radiation protection regulations specify a lower dose limit for women who formally declare their pregnancy to reduce the risk to the developing embryo or fetus (see Table 11.1.1 above). Female workers occupationally exposed to radiation who become pregnant may declare their pregnancy to take advantage of the lower dose limit, additional monitoring and counseling. Declaration of pregnancy is voluntary. The lower dose limit does not apply if the pregnancy is not formally declared in writing, although counseling is always available from the RSO. The pregnancy may be undeclared at any time.

Declaration of pregnancy must be in writing and include the estimated date (month and year) of conception. The worker may contact EH&S for a Declaration of Pregnancy form letter, or she may write her own letter. The RSO will review the worker's previous radiation exposure, order a fetal monitoring dosimeter and suggest ALARA techniques, provide counseling or additional information as requested.

The worker should schedule an appointment with the RSO after the declaration of pregnancy. The RSO will review and provide radiation protection guidelines during the appointment.

Declared pregnant radiation workers are issued a fetal monitoring dosimeter. The fetal monitoring dosimeter maybel be used to evaluate the worker's monthly dose and will be exchanged monthly. The worker's regular whole body dosimeter, and ring dosimeter(s) (if applicable) will remain on a quarterly exchange cycle and will be used as the dose of record.

11.4 Thyroid Bioassays

When working with radioactive iodines 125I,131I, and 123I, there is a chance the iodine may enter the body via inhalation, ingestion, or absorption through the skin. When this occurs, approximately 30% of the iodine accumulates in the thyroid gland, taking several hours to reach maximum levels (normally within 24 hours). The effective half-lives in the thyroid gland for iodines are 40 days for 125I, eight days for 131I, and 13 hours for 123I. The iodine that does not become incorporated in the thyroid is eliminated from the body, usually within three days.

The dose limit for the thyroid (dose equivalent) is 50 rem per year. To receive 1/1000 of this dose, a thyroid burden of approximately 12 nCi of 125I or eight nCi of 131I is required.

Note that the thyroid gland varies in activity, shape, and size; therefore this is only an estimate for an average thyroid.

The RSO performs thyroid bioassays routinely to verify that proper safety precautions are taken by those in the thyroid bioassay program, and that those who work with radioiodines do not receive unmonitored doses. Results of the thyroid bioassays are kept in EH&S.

11.4.1 Who Receives Routine Thyroid Bioassays?

Individuals who receive thyroid bioassays include:

  1. All persons working with more than five mCi of 125I or 131I over a period of one month are required to receive monthly thyroid bioassays.
  2. All persons working directly with more than five mCi of 125I or 131I are required to receive thyroid bioassays within 16 working days of the use, but no sooner than 24 hours after use.
  3. All persons working directly with more than 30 mCi of 125I or 131I are required to receive thyroid bioassays within three days of the work, but no sooner than 24 hours after use.
  4. Any individual involved in a spill of more than one mCi of any radioiodine outside of a fume hood, or who has radioiodine contamination of the face, nose or a wound, must receive a thyroid bioassay within three days, but no sooner than eight hours after the spill.

Follow-up bioassays may be required based on results from the above monitoring.

Everyone in the thyroid bioassay program should have a baseline bioassay before beginning work with radioiodine and upon termination of work with radioiodine or of employment.

11.4.2 How to Obtain a Thyroid Bioassay

As part of the RMUA process, workers who are required to have bioassays are identified. In addition, EH&S reviews inventory records and all radioactive material orders to identify labs that may require bioassays. EH&S will routinely contact these labs/individuals to schedule thyroid bioassays.

Anyone may request a radioactive materials bioassay from EH&S at any time.

Workers may perform bioassay screenings on themselves on a frequent, routine basis. Noticeable activity (twice above background) levels in the thyroid should be brought to the attention of the RSO for further evaluation.

Everyone in the thyroid bioassay program should have a baseline bioassay before beginning work with radioiodine and upon termination of work with radioiodine or of employment.

11.4.3 Action Levels for Thyroid Burdens

When a thyroid bioassay exceeds 12 nCi of 125I or eight nCi of 131I (1/1000 of annual limit), or the burden is unexpected, EH&S will conduct an informal investigation. The investigation attempts to find the cause of the burden and to identify methods of prevention.

When a thyroid bioassay at the time of measurement exceeds 50 nCi, EH&S conducts a formal investigation. EH&S writes a report presenting the suspected cause and date of the uptake, and suggesting actions which may be taken to reduce the chance of recurrence. The report is sent to the person with the burden, the PI, and the members of the RSC.

When the calculated up-take is over 100 nCi, the RSO calculates the committed effective dose equivalent (CEDE) and effective dose equivalent (EDE) from the up-take. This dose is added to the annual dose report for that individual. If the up-take results in a dose exceeding 50 rem CEDE or 5 rem total effective dose equivalent (TEDE), the state DOH is informed.

11.4.4 How to Perform a Thyroid Bioassay Self-screening Procedure

Workers may perform their own self-screening thyroid bioassays by using the following procedure. Any questions should be addressed to the RSO or the radiation safety staff.

  1. Find the thyroid gland, situated in the neck below the "Adam's apple" (laryngeal prominence) and above the center of the bones in the upper chest.
  2. Using a radiation detector containing a solid sodium iodide (NaI) probe calibrated to measure the iodine with which you have been working, hold the probe area adjacent to the thyroid, flush with the neck.
  3. Hold the probe in this position for 10 to 20 seconds and record the highest count rate (cpm). Subtract the normal background count rate from this reading. This reading is the net count rate of iodine in the thyroid. The detector's efficiency for this type of measurement is usually only 2-5%. That is, of all the iodine incorporated in the thyroid, only 2-5% of the radiation emitted reaches the detector probe and is recorded.
  4. Using the net count rate (total count rate less the normal background count rate) of iodine measured in the thyroid, and the nominal 2% efficiency, the results can be converted into nanocuries with the following equations:
     
Equation for Disintegrations


 

Equation for nanocuries

Example:
Total count rate in thyroid = 650 cpm
Normal background count rate = 200 cpm
Efficiency = 2% or 0.02
Therefore: [(650-200)/0.02] = 22,500 dpm of iodine in thyroid
22,500 dpm/2,220 dpm per nCi = 10 nCi in thyroid

Any self-screening that indicates greater than 8 nCi for 131I, 10 nCi for 125I, or two times background should be reported to the RSO for further evaluation.

11.5 Hazards of Tritium Exposure

Radioactive hydrogen, usually called tritium or 3H, is a low energy (18.6 keV maximum) beta emitter. The farthest this beta particle can travel in air is only 4.7 millimeters (0.19 inches). Since the beta particle is unable to penetrate the outer dead layer of skin, working with millicurie quantities of tritium does not pose an external radiation hazard.

However, when tritiated compounds are ingested, injected, inhaled or absorbed through the skin, they mix with water in the body and permeate all tissue within a few hours, irradiating the body internally in a fairly uniform manner. In water, the beta particle from tritium only travels six microns, but the diameter of a cell nucleus may be on the order of eight microns. Therefore, when tritium releases the beta particle while in the nucleus of a cell, almost all of the energy is deposited in the cell and could lead to damage.

The majority of tritium used is labeled to organic compounds. When tritiated organic compounds are ingested, the majority is catabolized (broken down) in the gastrointestinal tract and tritiated water is produced. The tritiated water is then dispersed throughout the body.

Injection, inhalation, or absorption through the skin of tritiated organic compounds results in less of the compound being broken down to tritiated water. A portion of the tritiated organic product is not broken down into water and may concentrate in certain tissues. The exact location and amount absorbed depends upon the type of compound and the route of entry into the body.

The tritiated compound that does concentrate in a specific cell is thymidine. If not catabolized, tritiated thymidine can be taken up by the nuclei of cells synthesizing DNA. Some of the dividing cells that take up the tritiated thymidine may renew themselves and propagate the damage to another cell.

It is assumed, therefore, that tritiated organic compounds pose a higher risk (up to 50 times greater) than that of tritiated water.

11.6 Performing Wipe Surveys and Urine Bioassays

All workers whose jobs involve tritium must perform wipe surveys to monitor the potential spread of tritium in the lab. The wipes must be counted in a liquid scintillation counter. In addition, since the majority of tritium absorbed into the body is excreted in the urine with a half-life in the body water of approximately twelve days, EH&S performs urine bioassays on workers who work with more than 100 mCi in any one month.

11.6.1 Who Receives 3H Urine Bioassays?

EH&S keeps an inventory of tritium in each lab. If the tritium inventory exceeds 100 mCi, EH&S notifies the lab that they may be required to perform urine bioassays on staff involved with the tritium work between four hours and one week after use. The RSO may request more frequent bioassays or large quantity samples based on review of the initial bioassay results. A person using more than 100 mCi of tritium during a month must have a urine bioassay performed.

11.6.2 How to Perform a Urine Bioassay

Use one milliliter of urine and at least 10 milliliters of water-miscible scintillation fluid. Count for at least one minute, using a window set on the LSC to count the tritium beta radiation energy level. Required bioassays for workers are done by EH&S. Labs may do their own self-checks.

11.6.3 Action Levels

When the lab analyzes a urine sample and the level is above eight nCi/ml (approximately 8000 cpm above background) urine, EH&S must be notified. Note that an initial level of eight nCi/ml in urine represents less than 0.1% of the annual dose limit.

For samples greater than 30 nCi/ml, urine bioassays must continue to be performed weekly until the tritium concentration in the urine drops below one nCi/ml or as determined by the RSO. Dose will be calculated if tritium intake would result in greater than 50 mrem. Anyone with such levels is informed and an entry is made in his or her permanent dose record.