Answer to Question #7868 Submitted to "Ask the Experts"

Category: Radiation Basics — Radiation Quantities and Units

The following question was answered by an expert in the appropriate field:

Q
Keeping in mind the definition provided on the Health Physics Society website about "effective dose" (dose representation the total body could receive), I would like to go farther to learn the pros and cons the "effective dose" term itself has, as a radiation safety measurement, for instance in a daily working environment dealing with radiation protection (i.e., a medical physics department at a hospital), as I intend to work in this area in the near future.

A

As discussed on the HPS Ask the Experts website (e.g., see question 1252), the effective dose is a quantity calculated by multiplying the equivalent dose received by every significantly irradiated tissue in the body by a respective tissue weighting factor (this factor reflects the risk of radiation-induced cancer to that tissue) and summing together the individual tissue results to obtain the effective dose. Such a dose, in theory, carries with it the same risk of cancer as would an equal equivalent dose delivered uniformly to the whole body. (Note that equivalent dose is the product of the absorbed dose and a radiation weighting factor, which accounts for differences in biological impact of different radiation types.) 

From a practical standpoint, in dealing with occupational doses from external radiation sources, there is no practical way to measure the effective dose. Rather we use a radiation dosimeter, such as a TLD or film, and measure dose at some fixed depth. In general, dosimeters are designed to measure dose at a depth equivalent to 1 cm in soft tissue and to use this number as a surrogate for effective dose. Monte Carlo calculations have shown that, for most photon energies of concern, the 1 cm dose is a conservative estimate of effective dose when the dosimeter is worn so as to face the source. When the body is irradiated from various directions and the worker is wearing a dosimeter at a single location on his body, the 1 cm dose may not be a very realistic measure of effective dose—e.g., if a worker wears his dosimeter on his/her chest and his/her body is irradiated from the back side, the interpreted effective dose may be too low. When multidirectional external radiation fields are anticipated, use of multiple dosimeters is often prescribed so that doses are not seriously underestimated.

The concept of effective dose is used, perhaps more explicitly, in the interpretation of dose from radionuclides that are taken into the body. Then mathematical models that describe the expected distribution within and removal from the body of such material are used to calculate the committed effective dose (calculation is projected over 50 years). In the medical physics arena, internal uptakes of radionuclides by workers is not a common occurrence, although some potential for small uptakes is present, perhaps most notably for staff who are involved in the administration of therapeutic radioiodine to patients and for staff involved with the in-hospital care and handling of such patients. Naturally, the patients themselves who receive diagnostic administrations of radionuclides will receive some doses from such administrations, and work has been done to estimate expected effective doses to typical individuals from various procedures, and this information is generally available to staff who may be requested to provide such information to patients.

Hope this helps. Good luck in your career in the medical physics area.

George Chabot, PhD, CHP
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