[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: rem vs. rad

April 1, 1998
Davis, CA

The idea that equivalent dose only applies to so-called stochastic effects
of radiation is wrong. Equivalent dose is defined on the basis of radiation
quality that depends on specific ionization intensity. You must consider
the equivalent dose to estimate non-stochastic effects such as formation of
cataracts of the eye or cell killing!

Ionizing radiation of various types displays different intensity and
pattern of ionization reactions in body tissues. In particular, the
specific ionization intensity per unit of path length has a marked effect
on the potential biological response, whether stochastic or nonstochastic.
For this reason the absorbed dose that is calculated from one type of
radiation cannot be arithmetically added to the absorbed dose from other
types of radiation. For example, the dose associated with recoil nuclei
during alpha emissions is characterized by high intensity of ionization but
very short range in tissues. The alpha particles' longer ranges in tissue
(usually about 30 to 70 Ám)
yield less intense ionization tracks than the recoil nuclei, but
considerably more intense than the tracks associated with energetic
electrons, beta particles, or gamma photons. These inherent differences in
specific ionization intensity should be expected to yield differences in
relative biological effectiveness (RBE) among the different types of
ionizing radiation
depending upon the circumstances. Even without a clear understanding of
these differences in RBE, the dosimetrist is obligated to clearly segregate
doses of different radiation quality. 

The radiation quality factor, Q, has been defined to represent in a general
quantitative fashion the relative biological effectiveness (RBE) that might
be expected for a given radiation type as a function of LET. From this a
biologically equivalent dose can be defined as H=DQN where H is the
equivalent dose in sieverts (Sv), N is a dose distribution or modifying
factor (usually assumed to be exactly 1 or dropped) and the quality factor,
Q, has the effective dimension of Sv/Gy. Clearly, the biologically
equivalent dose is a non-physical abstraction that should be used with
caution in radiation dosimetry.

A functional relationship of radiation quality factor, Q, and unrestricted
LET, L, was given by the ICRP (1991). For L 10 keV/Ám, Q(L)=1. For 10<L<100
keV/Ám, Q(L)=0.32L 2.2. For L 100 keV/Ám, Q(L)=300/ L . Since L may vary
over a given radiation path, so will Q. It may be useful to define a mean
quality factor averaged over paths or whole organs (ICRU, 1993). The newer
reports of the International Commission on Radiological Protection use the
term radiation weighting factor, wR, in place of the quality factor to
describe nominal values of Q.

The equivalent dose is useful in radiation safety work and in setting
standards. It cannot be used to predict risk of any given effect because
the Q or wR is not equal to the RBE.

		Prof. Otto G. Raabe, Ph.D., CHP
                [President, Health Physics Society, 1997-1998]
		Institute of Toxicology & Environmental Health (ITEH)
		     (Street address: Old Davis Road)
		University of California, Davis, CA 95616
		Phone: 530-752-7754  FAX: 530-758-6140 [NEW AREA CODE]
		E-mail ograabe@ucdavis.edu