PNNL-13501 - Pacific Northwest National Laboratory

Figure 2. Argonne human radium data for bone cancer, rolling 50-person average, showing observed fractions of bone sarcomas
plotted on both a. logarithmic and b. linear dose scales. Dose threshold is about 8 Gy as inferred from nonlinear least-squares
fitting.
significantly exposed to an alpha-emitting heavy metal
(Stannard 1988) and have been studied internationally.
Research was conducted by others to verify the Advisory
Committee statement using the latest available data from
Danish (Andersson and Storm 1992; Andersson et al.
1994; Advisory Committee on Radiological Protection
1996), German (van Kaick et al. 1984; van Kaick et al.
1999), Japanese (Kido et al. 1999; Mori et al. 1999a; Mori
et al. 1999b), Portuguese (dos Santos-Silva et al. 1999),
and Swedish (Martling et al. 1999) studies. We conclude
that the apparent threshold may be an artifact of the
assumed 15-year latency period for Thorotrast-initiated
liver cancer, and that the significant, possibly causal, dose
accumulates over a latency period (even for minimal
Thorotrast administrations). However, even at the lowest
dosages of Thorotrast administered, the radiation dose to
liver was great, admitting the possibility of a threshold.
Another trend we noticed in the Thorotrast data was that
the latency period between the time of injection and death
by liver cancer increased with decreasing amount
injected. In the Danish studies (Andersson et al. 1994),
the Kaplan-Meier curves are shifted to the right about
6 years between the high and medium dosage groups, and
are shifted again about 6 years between the medium and
low dosage groups. This trend implies that the dose rate
is inversely related to the latency. Since the latency for
liver cancer from Thorotrast is already significant (48
years to reach 50% cumulative frequency in this study), it
is very possible that a dose rate exists at which the latency
is so long that there is no risk of dying of liver cancer.
There was inadequate data, at present, for us to estimate
this “effective threshold” for Thorotrast dose rate.
280 FY 2000 Laboratory Directed Research and Development Annual Report
Human Plutonium Exposure Related to Bone and Liver
Cancer
Two papers (Gilbert et al. 2000, Koshurnikova et al.
2000) shed light directly on the human bone and liver
cancer associated with very high exposures to plutonium.
These authors analyzed dosimetry and medical records for
11,000 workers who began working at Mayak in 1948 to
1958, among whom were 23 cases that had bone cancer
and 60 cases that had liver cancer at death. Of the 5,500
who were monitored for plutonium in urine (beginning in
1970), 10 had bone cancer and 36 had liver cancer.
Stratifying by age and sex and accounting for external
dose on a linear basis, the authors computed relative risk
for the two upper ranges of body burdens compared with
the lowest range (Figure 3). For both bone cancer and
liver cancer the relative risk for cancer in the intermediate
body burden range (1.48 to 7.4 kBq) was not statistically
different from the lowest group. It was only in the highest
group (of 251 individuals) that an increased relative risk
can be seen. The authors did not give skeletal dose values
for all cases, but sufficient information was available to
infer that individuals in the middle group had skeletal
doses on the order of 1 to 7 Gy. Thus, these data suggest
a threshold for osteosarcoma above 7 Gy of alpha dose to
the skeleton.
Impact on Cleanup Standards
If there are thresholds for bone and liver cancer in humans
from relatively high doses of alpha irradiation, as our
investigation shows, then DOE’s cleanup standards and
occupational dose limits for plutonium are too restrictive