These analyses were based on the NRRW-3 cohort (with minor corrections) with follow-up extended by 10 years and include 34,819 informative deaths among 3.68 million person-years a large increase over the 23,326 deaths and 2.43 million person-years seen in the previous 10 year lagged mortality analysis.

General patterns of mortality and cancer incidence

The estimates of the excess risk of death and incidence from all malignant neoplasms excluding leukaemia (and NMSC) remained statistically significantly raised while the span of associated the 90% confidence bounds were narrower. In both instances the strength of support for these not being chance findings (p = 0.017 and p = 0.005, respectively) increased compared to NRRW-3. The additional exclusion of lung and pleural cancer had little impact on the results and an analysis of chronic obstructive pulmonary disease (COPD) did not show any positive association of risk with dose (Table S3) although it did show some evidence of a negative association. These results taken together with the lung cancer estimates suggest that smoking is unlikely to be a positive potential confounder when examining external radiation effects. If anything the low estimates for COPD and lung cancer may point towards some negative confounding effect of smoking on radiation estimates. However, non-cancer diseases also associated with smoking (although less strongly so) have shown positive associations with radiation exposure in prior analysis of this cohort so the potential confounding effects of smoking may be not be open to any simple interpretation.

Restricting analyses to data regarding doses below 400, 200 and 100 mSv indicated that the raised risk estimates per unit dose at low doses were not reliant solely on the data in the high dose categories. The attenuation of risk per unit dose at higher cumulative doses may in part be the result of a selection process with workers who remain employed for long periods and thus accumulate higher doses tend to be healthier than those who leave employment (the ‘healthy worker survivor effect’ (HWSE)).15 Although the HWSE may play a role in the pattern of results observed an adjustment for duration of employment did not materially affect the risk estimates and it may be that selection into internal radiation work (the majority of high dose workers were also internally monitored) results in a selection effect beyond the normal HWSE.

The impact of exposure to internal emitters on the estimates of risk from external exposure was assessed by incorporating an additional baseline stratum into the model but this did not materially affect the overall risks. However, excluding internally monitored workers from the analysis resulted in a doubling of the point estimate of mortality risk and an increase of 60% of the incidence risk for all malignant neoplasms excluding leukaemia. In both instances the width of the 90% confidence interval was also doubled. Overall 24% of the workers analysed were monitored for potential internal exposures but this group, who tended to receive higher external doses, account for 67% of the overall cancer deaths amongst workers who received doses in excess of 100 mSv. Below 100 mSv the results for the monitored and non-monitored groups were nearly identical with an ERR/Sv of 1.2. Thus attenuation in risk is happening for both groups when including higher doses but the effect is stronger for monitored workers perhaps suggesting the HWSE or a selection effect into continued employment or internal work. All NRRW analyses to date have used a fixed internal monitoring factor that assumed monitoring starts from the beginning of radiation work. This is not ideal as some workers will not have been monitored for internal exposure from first employment so their early person-years will be misclassified but the additional information to construct a time varying factor is currently not available for the cohort. However a time varying factor was used for the recent BNFL cohort analysis16 and this showed the same pattern of risk but further investigation of this issue is needed.

Comparison with other studies

There was good agreement of the risks for both mortality and incidence of solid cancers and solid cancers excluding lung and pleural cancer from this study and risks derived for this study by fitting a linear model to a comparable subset of the LSS14 data17,18 (Table 2). The largest difference was for solid cancer mortality where the updated NRRW result was 17% smaller than the comparable LSS14 based estimate. Comparing this study with the International Workers Study (INWORKS),19 which incorporated the 85% of NRRW-3 cohort who had nuclear industry employers, the INWORKS risk estimate for all solid cancer was 37% higher. Excluding lung cancer reduced this difference only slightly. Given the span of the confidence limits these results were still in broad agreement.

Table 2 Comparison of estimates of linear ERR per Sv (and 90% CI) with other studies Full size table

The risk estimates for cancer mortality and incidence from the Mayak worker cohort20,21 are lower than the other risk estimates in Table 2. The discordance between the Mayak risks and those of the LSS and other occupational studies has been noted previously.

Individual cancer sites

The significantly raised risks for mortality and incidence of rectal cancer found here were seen in NRRW-3 but those for bladder cancer were not. In the LSS mortality analysis17 the overall risk for rectal cancer was not significantly raised nor was it for males but for females the risk was raised (ERR/Gy = 0.66; 95%CI 0.06; 1.5; p = 0.03, 228 deaths). Unfortunately, in this study the female-specific analysis of rectal cancer was uninformative as there were only 14 deaths of which eight were in the lowest dose category. For bladder cancer mortality the overall and sex-specific LSS mortality risks were all significantly raised with that for females having almost twice the risk of males. Here only nine bladder cancers were seen among females and the main mortality result which is almost a male-specific result was close to the LSS14 result for females. This difference may be explained by variation in underlying rates of disease between males and females and countries rather than a real difference in radiation risks.

For ovarian cancer, lymphatic and haematopoietic cancer, pleural cancer, non-Hodgkin’s lymphoma and multiple myeloma significantly raised risks were seen in this study for incidence but not mortality. Only for the latter two diseases were there comparable raised risks seen in the NRRW-3 analysis. The ovarian cancer result derived here was based on 61 cases but only two occurred in workers with doses of 50 mSv or more so is not very reliable. Incidence risks derived from the LSS18,22 for all these cancer groupings (except pleural cancer), provided either weak or no evidence of raised risk. The site-specific analyses restricted to workers who were not monitored for internal exposure revealed that only for rectal cancer incidence was there a statistically significant excess risk (ERR/Sv = 1.70; 90%CI 0.16, 3.94; p = 0.031; 769 cases) which was higher than the overall rectal cancer result.

Limitations of the study

The biggest source of uncertainty likely relates to the dosimetry. In particular early dosimeters (film badges) had high threshold detection limits and the assessment of neutron exposure particularly in the first 20–30 years of follow-up was poor. Attempts to take account of neutron exposure information in the recent INWORKS19 project resulted in a variation of the solid cancer risk estimate of a factor of 2 from the primary result. A further drawback was the lack of quantitative internal dose estimates, the use of an indicator in the model stratification revealed issues that need further consideration. Although this study is a predominately a low dose and dose rate study earlier workers who accrued larger exposures (>100 mSv) over a number years do have a significant leverage on overall risks estimates but significant risks were still observed when restricting the analysis to low doses (<100 mSv) allbeit with greater uncertainty. Finally, potentially confounding lifestyle factors such as smoking, hypertension and BMI were not available.

Future work

In further work the shape of dose–response will be examined in detail as will the temporal variation in radiation risk (mortality and cancer incidence) with age and time since exposure. A analysis on the potential association between non-cancer disease mortality risk and radiation exposure is already underway.