Quality factors

The quality factor, Q, is a dimensionless modifier used in converting absorbed dose, expressed in gray (or rad), to dose equivalent, expressed in sievert (or rem). The dose equivalent is used in radiation protection to account for the biological effectiveness of different kinds of radiation. The quality factor is related to both linear energy transfer (LET) and relative biological effectiveness (RBE). The RBE obtained from biological experiments depends in a complex way on the observed biological effect, the specific test organism and the experimental conditions. Judgment is involved, therefore, in the choice of Q. Questions regarding the adequacy of current Q values for neutrons were first raised in a 1980 statement by the National Council on Radiation Protection and Measurements (NCRP) and later in a 1985 statement by the International Commission on Radiological Protection (ICRP). In 1980, the NCRP alerted the technical community to the possibility of a future increase between a factor of 3 to 10 in the Q for neutrons, and in 1985, the ICRP suggested an increase by a factor of 2 in Q for fast neutrons. Both these advisory groups are now recommending essentially the same guidance with regard to Q for neutrons: an increase by a factor of 2. The Q for neutrons is based on a large, albeit unfocused, body of experimental data. In spite of the lack of focus, the data supporting a change in the neutron quality factor are substantial. However, the proposed doubling of Q for neutrons is clouded by other issues regarding its application. These issues are discussed, together with the current database for the neutron quality factor. Improvements are needed to provide better guidance with regard to both Q for neutrons and its application in radiation protection.