Effective dose is a fundamental concept in radiological protection, defined by the International Commission on Radiological Protection (ICRP). It serves as a measure to quantify the stochastic health risk (e.g., cancer and hereditary effects) from exposure to ionising radiation, taking into account the type of radiation and the differing sensitivities of various organs and tissues in the human body. The unit of effective dose is the sievert (Sv).
Effective dose allows for the comparison of different exposure scenarios, such as external radiation fields, internal contamination, or medical exposures, by normalising the risk across these varied situations. It is a risk-weighted sum of the equivalent doses to all organs and tissues of the body.
The effective dose () is calculated as the sum of the equivalent doses () in all specified tissues and organs () of the body, each multiplied by a corresponding tissue weighting factor (). The mathematical representation is:
Where:
Tissue weighting factors are crucial for the calculation of effective dose. They are dimensionless factors that account for the varying radiosensitivity of different organs and tissues to the induction of stochastic effects. For example, some tissues like bone marrow and the colon are more sensitive to radiation-induced cancer than others, such as the skin or bone surface. The ICRP periodically reviews and updates these values based on evolving scientific understanding of radiation effects. The most recent set of values are provided in ICRP Publication 103, which include factors for organs such as bone marrow, colon, lung, stomach, breast, gonads, bladder, oesophagus, liver, thyroid, and others, as well as a remainder category for less sensitive tissues.
Effective dose is widely used in national and international radiological protection programmes and regulations. Its primary applications include:
It is important to understand that effective dose is a calculational quantity designed for radiation protection purposes and should not be interpreted as a direct measure of individual harm. While it provides an estimate of the overall potential stochastic risk to a population from an exposure, it is not suitable for predicting deterministic effects or for assessing the precise individual risk to a single person. Its application is primarily for prospective assessments and regulatory compliance, rather than for retrospective clinical dose assessment of individual patients.