Effect of follow-up period on minimal significant dose level and radiation risk estimate in the cohort of Chernobyl liquidators

The article presents results of estimating the lowest dose level in the cohort of Russian Chernobyl liquidators at which excess relative radiation risk is a significant value. The liquidators were selected in accordance with developed criteria, all of enrolled people had officially certified individual doses of whole-body gamma-radiation. The average age of the liquidators at the time of entry into the exclusion (accident) zone was 33.5 years. The average dose of external whole-body gamma-radiation exposure, that liquidators received for the period of their work within the accident zone, was 132.7 mGy. For the follow-up period lasted from 1992 over 2016, 6530 primary solid cancer cases were diagnosed. The 24-year follow-up period was divided into three shorter follow-up periods: 1992-2009, 1992-2013 and 1992-2016 years. For each follow-up period a minimal dose level (pmin was 0.25 Gy for 1992-2009; 0.175 Gy for 1992-2013 and 1992-2016 years. Estimated ERR per Gy tended to be higher with longer follow-up: ERR/1Gy=0.56 (95% CI: 0.09, 1.09) for the follow-up period 1992-2009, observation period 1992-2013, ERR/1Gy=0.96 (95% CI: 0.13, 1.90 for 1992-2013 and ERR/1Gy=0.95 (95% CI: 0.19, 1.80) for 1992-2016. Thus, it is revealed that the minimum dose level decreases with an increase of the period of observation of the cohort.