Equivalent dose (symbol HT) is a dose quantity calculated for individual organs (index T – tissue). The equivalent dose is based on the absorbed dose to an organ, adjusted to account for the effectiveness of the type of radiation. An equivalent dose is given the symbol HT. The SI unit of HT is the sievert (Sv) or but rem (roentgen equivalent man) is still commonly used (1 Sv = 100 rem). Unit of sievert was named after the Swedish scientist Rolf Sievert, who did a lot of the early work on dosimetry in radiation therapy.
As was written, for radiation protection purposes, the absorbed dose is averaged over an organ or tissue, T. This absorbed dose average is weighted for the radiation quality in terms of the radiation weighting factor, wR, for the type and energy of radiation incident on the body. The radiation weighting factor is a dimensionless factor used to determine the equivalent dose from the absorbed dose averaged over a tissue or organ based on the type of radiation absorbed. The resulting weighted dose was designated as the organ- or tissue equivalent dose:
Quality Factor
The quality factor of a radiation type is defined as the ratio of the biological damage produced by the absorption of 1 Gy of that radiation to the biological damage produced by 1 Gy of X-rays or gamma rays.
The Q of a certain type of radiation is related to the density of the ion tracks it leaves behind in tissue. The quality factors for the various types of radiation are listed in the table.
These quality factors are restricted to the dose range of interest to radiation protection, i.e., to the general magnitude of the dose limits. In special circumstances where one deals with higher doses that can cause deterministic effects, the relevant RBE values are applied to obtain a weighted dose.
Radiation Weighting Factor
In radiation protection, the radiation weighting factor is a dimensionless factor used to determine the equivalent dose from the absorbed dose averaged over a tissue or organ based on the type of radiation absorbed. In the past, a similar factor known as the quality factor was used for this purpose. The radiation weighting factor estimates the effectiveness per unit dose of the given radiation relative to a low-LET standard.
Before 1990, dose-equivalent quantities were defined in terms of a quality factor, Q(L), that was applied to the absorbed dose at a point to take into account the differences in the effects of different types of radiation. In its 1990 recommendations, the ICRP introduced a modified concept. For radiological protection purposes, the absorbed dose is averaged over an organ or tissue, T. This absorbed dose average is weighted for the radiation quality in terms of the radiation weighting factor, wR, for the type and energy of radiation incident on the body.
The reason for replacing the quality factor, i.e., the Q–L relationship, with wR values in the definition of the organ-equivalent doses and the effective dose was that the Commission believed:
‘that the detail and precision inherent in using a formal Q–L relationship to modify absorbed dose to reflect the higher probability of detriment resulting from exposure to radiation components with high LET is not justified because of the uncertainties in the radiological information.’
Noteworthy, these two factors, the radiation weighting factor and the quality factor, are restricted to the dose range of interest to radiation protection, i.e., to the general magnitude of the dose limits. In special circumstances where one deals with higher doses that can cause deterministic effects, the relevant RBE values are applied to obtain a weighted dose.
Special Reference: ICRP, 2003. Relative Biological Effectiveness (RBE), Quality Factor (Q), and Radiation Weighting Factor (wR). ICRP Publication 92. Ann. ICRP 33 (4).
