# Equivalent dose

The equivalent dose (HT) is a measure of the radiation dose to tissue where an attempt has been made to allow for the different relative biological effect of different types of ionizing radiation. Equivalent dose is therefore a less fundamental quantity than radiation absorbed dose, but is more biologically significant. Equivalent dose has units of sieverts.

The equivalent dose should not be mistaken for dose equivalent. Dose equivalent (H) presents the absorbed dose at a specific location in tissue weighted by a distribution of quality factors (Q). These are influenced by the LET (linear energy transfer) distribution of the radiation at that site.

Equivalent dose (E) is calculated by multiplying the absorbed dose (D) with the evaluation factor. Formerly, The International Radiological Protection Commission recommended the use as the evaluation factor of the quality factor Q (that estimates the relative biological effectiveness, RBE) only. The value of Q is 1 for x-rays, gamma rays and beta particles, but higher for protons, neutrons, alpha particles etc.

In 1990, the organisation issued new recommendations. The radiation weighting factor (wR) should be used as the evaluation factor. It is calculated by multiplying the quality factor Q and the modified factor N. The modified factor has value of 1 when used for external sources of irradiation, but is defined arbitrarily by a competent authority when used for internal ones. The equivalent dose is thus calculated by the equation:

[itex]H_{T,R} = wR * \ D_{T,R}[itex]

## References

• ICRP. ICRP Publication 60: 1990 Recommendations of the International Commission on Radiological Protection. Elsevier Science Pub Co (April 1, 1991). ISBN 0080411444.
• ICRP. ICRP Publication 92: Relative Biological Effectiveness (RBE), Quality Factor (Q), and Radiation Weighting Factor (wR). Elsevier Science Ltd, Oxford, UK. November 2003. ISBN 0080443117.

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