Equivalent dose and effective dose (sievert)
A way of converting a physical radiation dose into a single number for comparing radiation risk across different types of radiation and different organs. It is used for radiation-protection purposes, not for prescribing cancer treatment doses.
Overview
Equivalent dose (H) adjusts the absorbed dose in an organ or tissue (measured in gray) by a radiation weighting factor (wR) that reflects how biologically damaging a given type of radiation is, compared with photons or beta particles of the same absorbed dose: H_T = sum over R of wR times D_T,R. Current ICRP values are wR = 1 for photons, electrons and beta particles, wR = 2 for protons, and wR = 20 for alpha particles, reflecting alpha particles' much higher biological potency per unit of physical dose. Equivalent dose is expressed in sievert (Sv), the same unit (J/kg) as gray but with a different name to signal that a biological weighting has been applied.
Effective dose (E) goes one step further, multiplying the equivalent dose to each organ by a tissue weighting factor (wT, which sums to 1 across the whole body) that represents that organ's relative contribution to overall cancer and heritable-disease risk from uniform whole-body irradiation, then summing across organs. Effective dose, also in sievert, is the standard quantity used in radiation protection to compare the stochastic (cancer-risk) burden of different exposures, for example a diagnostic PET/CT scan versus natural background radiation.
These are protection quantities developed by the ICRP for population-level risk management and regulatory dose limits; they are not used to prescribe or evaluate radiopharmaceutical therapy, where the organ-specific absorbed dose in gray (via the MIRD formalism), sometimes adjusted for relative biological effectiveness rather than the fixed regulatory wR, is what guides treatment planning.
Showing the technology this term belongs to: In vivo dosimetry and patient-specific quality assurance.
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