Doctoral dissertation

Even low doses of ionizing radiation can increase the risk of childhood leukemia

A doctoral thesis carried out at Tampere University evaluated the effect of natural background radiation, indoor radon and computerized tomography on the risk of childhood leukemia. Complete residential histories increased the accuracy of the estimates of background radiation exposure. Using high-quality Finnish register-based data, a slightly elevated risk of childhood leukemia from low-dose ionizing radiation was found especially for the younger (2–7 years) children.

All articles in the thesis were based on a case-control design with all 1,093 cases of childhood leukemia diagnosed in 1990–2011 identified from the Finnish Cancer Registry. Three times as many controls were chosen from the Population Register Center matched by sex and age.

The first article assessed leukemia risk from natural background radiation. Exposure assessment was based on high-quality measurements by STUK – Radiation and Nuclear Safety Authority and complete residential histories from the Population Register Centre. The results suggest a marginal increase in childhood leukemia risk particularly for children aged 2–7 years and a specific subtype of leukemia with high hyperdiploidy.

The second article estimated the utility of complete residential histories in evaluating the role of natural background radiation. The results indicate that the dwelling at the time of birth or diagnosis can provide a reasonable estimate of exposure, even though full residential histories are preferable. A strong correlation between dose rates in successive dwellings was also found, due to short distance between them.

The third article focused on the effect of computerized tomography, and the information on CT scans in the ten largest hospitals in Finland was obtained. The doses to the red bone marrow were estimated with a software developed for the purpose. The results suggest a slightly elevated risk of leukemia due to radiation from computerized tomography.

The fourth article deals with residential radon and childhood leukemia. A model for predicting indoor radon concentration was constructed in collaboration with the Radiation and Nuclear Safety Authority. The model performed reasonably well when compared with similar models from other studies. When it was used to predict radon levels in the case-control study, no evidence was found for increased  childhood leukemia risk related to higher indoor radon concentrations.

Nikkilä concludes that: “In general, the results provide support to the notion that even low doses of ionizing radiation can increase the risk of childhood leukemia.” However, the effect is small and these sources of exposure are expected to cause only few cases of childhood leukemia in Finland annually. As childhood leukemia is a rare disease and the risk from low doses of ionizing radiation is small, large international collaborative analyses offer more precise risk estimates. To advance the field, the research group will keep participating in international collaboration also in the future.

Nikkilä was born and raised in Jyväskylä, graduated as a Licentiate of Medicine at Tampere University and currently works as a general practitioner in Hämeenlinna. He plans to specialise in pediatrics.

Licentiate of Medicine Atte Nikkilä will defend his doctoral thesis in epidemiology titled “Low doses of ionizing radiation and childhood leukemia” at the Faculty of Social Sciences of Tampere University on Friday 27 of September in the lecture hall F211A+B of the Arvo building, address: Arvo Ylpön katu 34. The opponent, Professor Mark Pearce, is from the University of Newcastle, England. The custos is Professor Anssi Auvinen, from the Faculty of Social Sciences.

The thesis is available online at