Epidemiological research indicates that children have higher relative risks of cancer per unit dose of radiation than adults. The thyroid gland, breast tissue, and reproductive glands are structures that have an increased sensitivity to radiation in growing children. In addition to the increased organ sensitivity, small children also receive a greater radiation dose than larger children or adults from the same CT scanner settings. Several investigators have attempted to derive estimates of the number of excess cancer cases possibly attributable to CT scans and other radiological diagnostic procedures, using radiation risk coefficients derived from the atomic bomb survivor studies (Brenner 2001; Brenner 2007). However, the epidemiological information regarding radiation health effects of early childhood exposures to low doses of ionizing radiation remains limited, particularly by the lack of information on early childhood cancers in the years immediately after the atomic bombings in Japan.

In order to quantify directly the cancer induction rate per unit dose delivered to the organ, a group of patients (cohort) exposed to ionizing radiation from CT will be assembled from major paediatric hospitals. Data collection commences by enumeration of a roster of patients who underwent CT scans in a particular hospital. The data collection period can be split into two, the first - before the introduction of the Picture Archiving Computerized System (PACS), the second - after the introduction of PACS. Both data periods will give rise to similar information on included patients and information available for prospective follow-up, though the level of detail available for dose reconstruction and assessment of confounders will vary.

For every member of the cohort it will be assessed if he/she developed a cancer after the CT examination. The number of observed cancers in the cohort will be counted. From cancer registries we will obtain information on how many cancers are expected per year at certain ages. Consequently, the observed number will be compared to the expected one, revealing the proportion possibly caused or prevented by the CT examination. Using the individual radiation dose estimates we will estimate the probability of leukaemia and cancer risk per unit of absorbed dose to the bone marrow or to the organ within the scanned area.

Cancer, fortunately, is a rare disease in young patients. To detect even a very small effect of exposure to CT radiation with substantial statistical certainty, a very large multinational cohort is necessary. Because 5 to10 years are needed for development of a possible cancer following the exposure to CT radiation, the patients included into the cohort will be those who already received a CT scan in the past. In some countries, the enrolment will continue for 3-4 years after the start of the project, which will allow for more statistical power to assess possible risk of various site-specific cancers in the future. The international cooperation also allows to analyze impact of other factors (such as socio-economic status) or other diseases than leukaemia and cancer on the relationship between the dose from CT scan and the outcome of interest.

Description of national cohorts

    Cohort and exposure information  Outcomes
Country Age range Start cohort accrual Source of cohort information Cohort size2 Childhood cancer incidence Adult cancer incidence
Belgium 0-15     2002  PACS       30 000 YesYes
Denmark 0-18     2000  PACS       30 000 Yes Yes
France 0- 51     2000  RIS/PACS       90 000 Yes Possible
Germany 0-15     1985  RIS/PACS     140 000 Yes No
Netherlands 0-18     1998  PACS       40 000 Yes Yes
Norway 0-20     2005  RIS/PACS       20 000 YesYes
Spain 0-20     2005  RIS/PACS/other     200 000 YesSince 2010
Sweden 0-18     1984  RIS/PACS/other       95 000 Yes Yes
UK 0-21     1985  RIS/PACS/other     400 000 Yes Yes
Total 0-21 1984-2002   1 045 000

1 0-10 years, starting from 2007
2 Estimated size, corrected for multiple examinations per patient and oncologic indications, as applicable