FGR 13 and Risk Factors

[Charles Willis <CAW@nrc.gov>]

Gene,

It may sound evasive, but risk factors refer to what the authors intend
for them to address.  Some risk factors, such as those of BEIR-V, are
primarily for cancer mortality.  Other risk factors refer to cancer
mortality plus serious genetic defects.  Others, such as ICRP-60, refer
to cancer mortality plus serious genetic defects plus a weighted factor
for cancer morbidity (i.e. being sick from cancer).  There is no standard
in this area.

Also, there is no complete data set for a basis for any of the risk
estimates.  The best we have are the A-bomb survivor data and they
are incomplete because (1) at the time of the last RERF report (1990)
most of those people were still alive, and (2) the data collection process
did not start until, a few years after the exposures so some of the
leukemias may have been missed.  The RERF experts make educated
guesses about what will happen to those still alive and thus produce
risk factors.  Also there are incidence data from the A-bomb survivors
but these data sets are less complete than the mortality data.  As for
human genetic effects, there are no positive data; all the data show is
that the Hiroshima-Nagasaki population was too small for human genetic
effects to be discernible.  Chromosome aberrations can be seen but not
deleterious health effects.  Genetic risk estimates are based largely on
mouse data and the linear, no threshold model (which is a little hard to
square with the observation that doses to female mice have no
discernable genetic effect if the dose are delivered 6 or more weeks
before conception).

Most human mortality data are obtained from death certificates, although
there were autopsy checks on some of the A-bomb survivors. 
Incidence data are obtained from cancer registries (without autopsy
validation, of course).  Also risk estimates can be generated from
mortality data using mortality/incidence ratios

Radiation risk quickly gets to be a big subject.  I hope this bit of it is of
some use.

Charlie Willis
caw@nrc.gov