9. Radiation effects on embryo and fetus.

The Commission has reached the following conclusions on the in-utero risks of tissue injury and malformation at doses below about 100 mGy of low-LET radiation. In respect of the induction of malformations, the new data strengthen the view that there are gestational age-dependent patterns of in-utero radiosensitivity with maximum sensitivity being expressed during the period of major organogenesis. On the basis of animal data it is judged that there is a true dose threshold of around 100 mGy for the induction of malformations; therefore, for practical purposes, the Commission judges that risks of malformation after in-utero exposure to doses well below 100 mGy are not expected. The Commission recognises that there are particular uncertainties on the risk of radiation-induced solid cancers following in-utero exposure. The Commission considers that it is prudent to assume that life-time cancer risk following in-utero exposure will be similar to that following irradiation in early childhood, i.e., at most, about three times that of the population as a whole.

10. An absorbed dose.

Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation per unit mass. It is equal to the energy deposited per unit mass of medium, which may be measured as joules per kilogram and represented by the equivalent SI unit, gray (Gy), or the antiquated units, rad. The absorbed dose depends not only on the incident radiation but also on the absorbing material: a soft X-ray beam may deposit four times more dose in bone than in air, or none at all in a vacuum.

11. An effective dose.

The effective dose in radiation protection and radiology is a measure of the stochastic (i.e., probabilistic) effect on a whole organism due to ionizing radiation delivered non-uniformly to part(s) of its body. The effective dose is a reasonable way to assess the health effects of beta and alpha radiation on the skin and eyes, or the effects radiotherapy applied selectively to part of the body. The effective dose is not intended as a measure of acute or threshold effects of radiation exposure such as erythema, radiation sickness or death.

12. The system of radiological protection of humans.

Everybody is exposed to ionising radiation from natural and man-made sources. Protection can be achieved by taking action at the source, or at points in the exposure pathways, and occasionally by modifying the location or characteristics of the exposed individuals. The assumed proportional relationship between an increment of dose and an increment of risk of stochastic e?ects makes it possible to deal separately with di?erent parts of this network of events and situations leading to exposure, and to select those parts that are of relevance in a given situation. To make these selections, however, it is necessary to de?ne, for each part of the network, the objectives, the organisations (and individuals) responsible for protection, the lines of responsibility, and the feasibility of obtaining the necessary information.