'Extending' DS86 allows dose assessment
for about 1,200 survivors in Nagasaki who were shielded in factories
and by terrain and for about 10,000 distal survivors who were
in the open
|by Shoichiro Fujita, Department of Statistics
|This article was originally published in RERF
Update 1(2):3, 1989.
|In March 1986,
US and Japanese dosimetry committees approved a new system
of dosimetry to replace the tentative system known as T65D,
which had been used as a basis for risk assessment until that
time. The description of the new system, given the name "DS86,"
together with the scientific basis for it, was published by
RERF in 1987. Computer codes were developed immediately to
implement DS86 on RERF's NEC ACOS-750 computer. The following
describes the versions of this dosimetry system from the inception
of computer implementation.
Version 1: DS86 was installed in early 1986
and covered all survivors with detailed shielding histories
who were at distances less than 2,500 meters from the hypocenter
at the time of the bombings and who were unshielded or shielded
by Japanese-style houses or tenements. It enabled the computation
of free-in-air tissue kerma, tissue kerma adjusted for the
effects of shielding by housing (sometimes loosely called
"shielded kerma"), and absorbed doses, all of these for neutron
and gamma radiation.
|Shoichiro Fujita, center, discusses DS86 with Eizo
Tajima, left, and Masaharu Hoshi at the binational dosimetry
workshop held in Honolulu earlier this year(1989).
|Calculations were possible for males
and females; standing, sitting, or in a prone position, in
either frontal, rear, right or left orientation; for three
age classes (< 3, 3-12, and > 12 years); and for a number
of organs (12 for males and 14 for females): active bone marrow,
bladder, bone, brain, breast, eyes, intestinal tract, liver,
lungs, ovaries, pancreas, stomach, testes, thyroid gland,
and uterus. In addition, DS86 allowed the output of energy-dependent
fluence distributions for each organ dose component, i.e.,
prompt gamma radiation and neutrons, delayed gamma and neutron
radiation, and secondary gammas from prompt and delayed neutrons
created by the shielding and from interactions in the body.
It turned out that the system was able to compute kerma and
dose for 23,422 of the 28,743 survivors (81%) who had been
within 2,500 meters of hypocenter in the ABCC-RERF Master
Sample for which detailed shielding data were available. This
included 18,526 of the 23,420 survivors (79%) in the Life
Span Study (LSS) cohort used in most analyses of radiation
effects among A-bomb survivors.
Because DS86 requires the availability of detailed shielding
information and only covers survivors within 2,500 meters,
it was necessary to "extend" DS86 to enable the assessment
of the doses of survivors, at all distances, for which no
detailed shielding information was available. The method developed
for this purpose became known as the "indirect DS86." It relies
on the use of free-in-air-kerma regression models and the
application of average transmission factors for given classes
of shielding rather than the individual-specific ones used
in the "direct DS86." The indirect DS86 produces only kerma
and dose, as broken down above, but no fluences. Altogether,
71,367 survivors, of whom 57,567 are in the LSS cohort, were
added to the number of direct estimates given earlier.
|Version 2: Early on,
the system was slightly modified to correct software errors
in the original version. It produced only minor changes in
dose estimates and concerned primarily direct DS86 organ doses
for breast, ovary, and uterus in adults and all organ doses
for the age group below three. Average transmission factors
used in the indirect DS86 were also recalculated. Enhancements
to the system permitted calculation of special cases, specifically
factory workers and survivors shielded by terrain (all of
them in Nagasaki), but these enhancements were not implemented
until version 3 was prepared. Therefore, the number of survivors
covered did not change between versions 1 and 2.
|Version 3: Most recently
(in May 1989), kerma and doses for Nagasaki survivors shielded
in factories (361) and by terrain (815) were included (all
direct DS86). New indirect estimates were made for 10,034
distal survivors who were in the open. Kerma and dose estimates
for all Nagasaki survivors were recalculated and slightly
reduced (1%) because of modifying the calculation of slant
distance from the epicenter to each survivor, which reflects
a sea-level correction.
After the latest version, DS86 covers 115,027 survivors for
which T65D estimates were available, 93,741 of whom are in
the LSS cohort. Eighty-eight percent of the proximal survivors
and 97% of the distal survivors now have kerma and dose estimates.
Future efforts will be directed to the inclusion of more special
cases in the proximal group: those who were shielded in concrete
buildings, in air-raid shelters, and in other complex shielding
situations. It appears already, however, that this may produce
dose estimates for at best 3,000 survivors, and will require
considerable effort and time.