Quality Factors

Abstract

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.

The divergence of science and simplicity in radiation protection

“…The situation became even more complicated when x-and gamma radiation were found to have different RBE's depending upon energy 3 (Hettinger et al 1965, Amols et al 1986, ICRU-40 1986, Brenner et al 1999, Heyes and Mill 2004, Heyes et al 2006, Hunter and Muirhead 2009, Nikjoo and Lindborg 2010. The higher-energy gamma radiation was found to be less effective than the x-rays (Q(L) ∼ 0.6 for 60 Co) or, conversely, the x-rays were found to be more effective (Q(L) ∼ 2) than the gamma radiation, depending on which radiation was taken to be the reference radiation (Kerr 1988, Straume 1995. This phenomenon has been attributed to the predominance of the photoelectric effect at low energies (Brenner et al 1999).…”

Inconsistencies and open questions regarding low-dose health effects of ionizing radiation.

“…Life Span Study mortality, 1950Study mortality, -1985 from all cancers except leukemia. Cumulative mortality per 104survivors for the lowest six colon dose subcohorts (DS86) 0, 1-5, 6-9, 10-19, 20-49, 50-99 cGy (triangles) and for the two combined 0-5 (mean dose 0.7) and [6][7][8][9][10][11][12][13][14][15][16][17][18][19] (mean dose 10.9) cGy subcohorts (squares) versus mean colon dose (cGy). Standard error bars are shown.…”

“…Standard error bars are shown. The increase in mortalities between the 0-5 and [6][7][8][9][10][11][12][13][14][15][16][17][18][19] cGy subcohorts is statistically significant (p < 0.01). The solid line is an error-weighted linear fit to the five data points below 40 cGy mean dose (see Table 2).…”

Inconsistencies and Open Questions regarding Low-Dose Health Effects of Ionizing Radiation

“…While that is not the conclusion of this Committee, which did not consider this question in detail, it could be argued that since the risk estimates that are presented in this report are derived chiefly (or exclusively) from the Japanese experience they should be doubled as they may be applied to medical, industrial, or other X-ray exposures. (2: 218) The physical basis for such a possible effect is the roughly fourfold higher ionization density in tissue by medical X-rays than that by high-energy 7-rays (6).…”