once again that BEIR VII cancer incidence estimates were based on LSS data that included 11 more years of follow-up than the data analyzed by the UNSCEAR committee.

UNCERTAINTIES IN LIFETIME RISK ESTIMATES

As noted early in this chapter, quantitative estimates of cancer risk are subject to several sources of uncertainty, which come about because of limitations in epidemiologic data and because the populations and exposures for which risk estimates are needed nearly always differ from those for which epidemiologic data are available. Several organizations have conducted detailed uncertainty assessments, which are described in Annex 12A. The NCRP (1997) evaluated uncertainties in the lifetime risk of total cancer mortality, and EPA (1999) provides extensive discussion of sources of uncertainty and gives example quantitative evaluations for lung cancer and leukemia. The NIH (2003) conducted a comprehensive evaluation of uncertainty in the excess relative risk used to calculate the assigned share, and it would be possible to extend this to lifetime risk estimates.

Quantitative Evaluation of Uncertainty

The lifetime risk estimates shown in Tables 12-5, 12-6, and 12-7 are accompanied by subjective confidence intervals that quantify the most important uncertainty sources: (1) sampling variability in risk model parameter estimates from the LSS data, (2) the uncertainty about transport of risk from a Japanese (LSS) to a U.S. population, and (3) the uncertainty in the appropriate value of a DDREF for adjusting low-dose risks based on linear-in-dose risk models estimated from the LSS data. This section gives more details on the allocation of uncertainty by source and discusses sources of uncertainty that were not included in the committee’s quantitative assessment.

As an example, Table 12-10 displays the estimated lifetime attributable risks of cancer incidence for various sites shown in Table 12-5A, corresponding to a population of persons of mixed ages exposed to 0.1 Gy. The confidence intervals in Table 12-5A were constructed from the standard error of the estimated logarithm of LAR. This standard error is conveyed in Table 12-10 as the coefficient of variation, which is the standard error of LAR as a percentage of the

TABLE 12-10 Estimated Lifetime Attributable Risks of Solid Cancer Incidencea for a Population of Mixed Ages Exposed to 0.1 Gy (Corresponding to Table 12-5A)

Site

LAR (per 105)

CV (%)

Variance of log (LAR)

Variance (Percentage) Due to

Estimation

Transport

DDREF

Males

Stomach

34

176

1.41

0.10 (7)

1.22 (86)

0.09 (6)

Colon

154

46

0.19

0.15 (40)

0.08 (13)

0.09 (47)

Liver

27

127

0.96

0.14 (15)

0.73 (76)

0.09 (9)

Lung

138

55

0.26

0.16 (60)

0.01 (5)

0.09 (34)

Bladder

98

69

0.39

0.28 (72)

0.02 (5)

0.09 (23)

Other solid

285

46

0.19

0.09 (45)

0.02 (8)

0.09 (46)

All solid

669

36

0.12

0.02 (18)

0.01 (8)

0.09 (74)

Leukemia

101

61

0.32

0.23 (72)

0.09 (28)

Females

Stomach

43

161

1.28

0.05 (4)

1.14 (89)

0.09 (7)

Colon

96

57

0.28

0.15 (54)

0.04 (14)

0.09 (32)

Liver

12

184

1.48

0.31 (21)

1.08 (73)

0.09 (6)

Lung

304

51

0.23

0.04 (16)

0.10 (44)

0.09 (39)

Breast

462

36

0.12

0.03 (25)

0.00 (0)

0.09 (75)

Ovary

60

85

0.54

0.42 (79)

0.02 (5)

0.09 (17)

Bladder

94

63

0.34

0.19 (58)

0.05 (15)

0.09 (27)

Other solid

288

45

0.19

0.06 (32)

0.04 (20)

0.09 (48)

All solid

1048

34

0.11

0.01 (11)

0.006 (6)

0.09 (83)

Leukemia

72

71

0.41

0.24 (58)

0.17 (42)

NOTE: Number of excess cases per 100,000 exposed.

aAlso shown are the coefficients of variation (estimated standard deviation as a percentage of the estimated LAR value) and the variance of log (LAR) due to each of the three sources considered: sampling variability in the parameter estimates, uncertainty in the transport model (ERR or EAR), and presumed uncertainty in the DDREF. The approach for obtaining a single LAR and its uncertainty is detailed in Annex 12C.



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