Question

In the system of protection devised by the International Commission on Radiological Protection ICRP, absorbed dose to organs and tissues are converted to equivalent dose and effective dose. Explain how and why this is done.

Answer 1

The absorbed dose is the radiation energy imparted per unit mass of an irradiated body. It is measured in joule per kilogram, a unit which is also called the gray (Gy). Multiplying the absorbed dose by appropriate weighting factors depending on the type of radiation, creates the equivalent dose in the relevant organ or tissue. By weighting the equivalent dose in each organ in proportion to the probability and severity of the harm done by radiation, and adding the weighted contributions from each organ to a total body dose, a third dose, the effective dose is obtained. The effective dose is defined by the Commission as the sum of the equivalent doses in the principal tissues and organs in the body, each weighted by a tissue weighting factor, wT. This weighting factor takes account of the probability of fatal cancer, the probability of nonfatal cancer, weighted for severity, and the average length of life lost due to an induced cancer. A contribution for severe hereditary disorders is also included. Detriment, as used by the Commission, can be thought of as the probability of causing a level of total harm judged to be equivalent to one death that causes, on average, a loss of lifetime of 15 years. The committed effective dose is the sum of the committed equivalent doses each weighted by the appropriate tissue weighting factor. In radiation protection it is usually the effective dose that is determined for comparison with dose limits or for assessments of risks. Both the equivalent dose and the effective dose are also measured in joule per kilogram, but in these cases the unit is called the sievert (Sv). For x rays and gamma rays the absorbed and equivalent doses in gray and sievert are numerically equal.

This leads to the basic system of protection which has three components.

(1) the justification of a practice, which implies doing more good than harm,

(2) the optimisation of protection, which implies maximising the margin of good over harm,

(3) the use of dose limits, which implies an adequate standard of protection even for the most highly exposed individuals.

A simple proportional relationship also has some important practical implications. It allows

(a) doses within an organ or tissue of the body to be averaged over that organ or tissue,

(b) doses received at different times to be added,

(c) doses received from one source to be considered independently of the doses received from other sources.