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

Re: rem vs. rad



Page 9 (32) of ICRP 60 mention:

"Both equivalent dose and effective dose are quantities intended for use in
radiological protection, including the assessment of risks in general terms.
They provide a basis for estimating the probability of stochastic effects
only for absorbed doses well below the thresholds for deterministic effects."

page 15 (57)

The equivalent dose is not always the appropriate quantity for use in
relation to deterministic effects because the values of radiation weighting
factors have been chosen to reflect the relative biological effectiveness
(RBE) of the different types and energies of radiation in producing
stochastic effects. For radiations with a radiation weighting factor larger
than 1, the values of RBE for deterministic effects are smaller than those
for stochastic effects. The use of the equivalent dose to predict
deterministic effects for high LET radiations, e.g. neutrons, will thus lead
to overestimates.

J. J. Rozental <josrozen@netmedia.net.il>
Israel


At 10:53 AM 4/1/98 -0600, you wrote:
>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.
>
>Otto
>		*****************************************************
>		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
>
>