Methodological aspects on excess cancer riskafter exposure to ionizing radiation

Sammanfattning: Aims. The aims of this study were to establish a cohort of persons treated with radiotherapy for skin haemangioma in childhood and study the excess risk of cancer after exposure to ionizing radiation. Special attention was paid to ascertainment bias in the excess of parathyroid adenomas and the confounding of reproductive history in the excess of breast cancer among the women in the cohort. A further aim was to analyse and compare excess risk estimates for breast cancer in eight cohorts of women from Japan, North America and Sweden who had been exposed to ionizing radiation of different quality and were living in areas with different background rates of breast cancer.Methods. The concept of epidemiological observational study by cohort analyses was applied. Follow-up was managed through record linkage with Swedish population registers. The analyses of the excess cancer risk were performed by Poisson regression of excess relative risk (ERR) and excess absolute risk (EAR) models accounting for background cancer rates, absorbed dose and other risk factors while allowing for time-dependent variation. For parathyroid adenomas, clinical records were scrutinized to detect differential diagnostic intensity (ascertainment bias), according to earlier exposure to ionizing radiation.Results. An overall excess cancer incidence of 20%, i.e. 228 excess cases, was observed in the haemangioma cohort. For parathyroid adenomas, there was a statistically significant excess incidence of 76%, and an ERR per gray (ERR/Gy) of 1.57, after adjustments made for ascertainment bias. The excess incidence for breast cancer was 25%, 2% of which could be explained by confounding of reproductive history, and a further 12% by the exposure to ionizing radiation (ERR/Gy = 0.35). No simple unified summary could describe the excess breast cancer risks found in the eight studied cohorts of women exposed to different radiation quality.Conclusions. A cohort of radiation-exposed persons was established. There was an excess cancer risk in the cohort, which could in part be explained by a dose-response effect of ionizing radiation. More information on the excess cancer risk and dose-response effect will be gained from the cohort in the future, as the median age at the end of follow-up was 48 years, which implies that 39% of the person-years at risk in the cohort remain to be observed.