Principles Underlying Dose Limits

Radiation protection deals with the protection of individuals, their offspring, and the general population against potentially harmful effects of radiation. There are two types of cellular damage that may occur following exposure to ionizing radiation: deterministic and stochastic damage [15].

Deterministic effects occur above a certain threshold. The probability of damage resulting in cell death or loss of reproductive potential will increase with the radiation dose. The extent of damage will also increase with the dose.

If cells are ''modified'' instead of killed, the outcome is different. In the clones of cells resulting from the reproduction of the modified cells, a malignant tumor may develop. This may occur even after a prolonged delay. The probability of tumor development following radiation increases with the increment of the radiation dose. However, the severity of the tumor is not related to this dose. This effect is called stochastic. Stochastic damage can be transmitted to later generations when it occurs in germ cells. The radiation dose used for determination of the stochastic risk is calculated in the following way. The ''absorbed dose'' (D) is the basic dosimetric unity. It is expressed in energy per unit mass

(joules per kilogram, J/kg) and is called the Gray (Gy), where 1 J/kg is equivalent to 1 Gy. The probability of biological damage depends not only on the amount of Gy but also on the type of radiation and the organ or tissue that is irradiated.

The influence of the type of radiation on biological damage is expressed as a ''radiation weighting factor,'' WR. For example, gamma rays (WR = 1) cause less damage than alpha particles (WR = 20) when the absorbed dose is the same. When this weighting factor is taken into account, the result is the ''equivalent dose'' (H), which is expressed in Sievert (Sv). The equivalent dose in an organ or tissue (HT) is the sum (X) of the weighted doses of all radiation types:

where DTR is the absorbed dose in the organ Tdue to radiation R. The radionuclide technetium-99m (99Tc), as used in the tracers for sentinel node labeling, emits only gamma rays and has a weighting factor of 1.

The ''effective dose'' is the dosimetric unit that takes into account the sensitivity to radiation damage of the different organs and tissues of the body. For example, the gonads are more susceptible to stochastic radiation damage than the skin. If the body is irradiated uniformly, the contribution of each tissue Tto the total detriment resulting from the exposure is represented by a ''tissue weighting factor,'' WT. The effective dose (E) is the sum (X) of all weighted equivalent doses to all tissues and organs:

where HT is the equivalent dose in a tissue or organ. The effective dose is, like the equivalent dose, expressed in Sv. The effective dose limit for the general population is accepted to be 1 mSv/year [15,16].

The average annual effective radiation dose from natural sources amounts to 2.4 mSv [17]. This is supplemented by an average of 0.4 mSv/year that is caused by man-made sources like radiographs and nuclear medicine procedures. The natural annual radiation dose differs according to geographical location and altitude. The ICRP recommends that any exposure to amounts over the natural background radiation levels should be as low as reasonably achievable (ALARA), but always below individual dose limits. Separate dose limits have been established for radiological personnel (e.g., those working in radiology and nuclear medicine departments) [16]. Employees in surgery and pathology departments also are to be considered as members of the general public with according dose

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