More than a million European children undergo CT scans annually. While ionizing radiation at moderate to high doses (>100 mGy to >1 Gy) is established as a risk factor for haematological malignancies, the risk associated with lower doses (<100 mGy), typical in CT scans for children and adolescents, remains unknown.

Now, a pan-European study suggests that, for every 10,000 children scanned at 8 mGy per scan, one to two could develop radiation-related haematological malignancies in the next 12 years (1.4 cases, 95% CI, 1-2). The study confirms a higher cancer risk even at low radiation doses, emphasising the importance of justifying each paediatric CT scan and optimizing delivered doses. Of note, CT scans can be replaced by MRI or ultrasound.

A Comprehensive European Cohort

Previous studies assessing the risk for haematological malignancies linked to ionizing radiation from CT scans in young patients have shown an increased risk for leukaemia with repeated scans. However, they faced limitations, such as statistical power or biases related to confounding factors. 

The EPI-CT study, coordinated by the International Agency for Research on Cancer, aimed to address this gap by evaluating the risk for cancer after exposure to low doses of ionizing radiation from CT scans during childhood or adolescence.

The study presented a risk assessment of haematological malignancies resulting from CT scans in a multinational European cohort of 948,174 individuals scanned before the age of 22.

Ionizing radiation doses to the bone marrow were estimated based on the body region, patient characteristics, scan year, and machine parameters. 

The final analysis included 876,771 individuals who underwent 1,331,896 scans (mean 1.52 scans per patient) and a follow-up of at least 2 years after the first scan. 

Results

In total, 790 haematological malignancies were diagnosed, including 578 lymphoid malignancies and 203 myeloid malignancies and acute leukaemias, with a 7.8-year average follow-up. At diagnosis, 51% of patients were younger than 20 years old, and 88.5% were under 30. The cumulative dose showed a significant association with haematological malignancy risk, with a relative risk of 1.96 (95% CI, 1.10-3.12) per 100 mGy.

This corresponds to a 16% increased risk (95% CI, 8%-24%) per scan (with an average dose per scan of 8 mGy). Higher risks were observed for doses >10 mGy, reaching a relative risk of 2.66 (95% CI, 1.92-3.70) for doses >50 mGy compared with doses <5 mGy.

The risk increased with older age at the time of radiation exposure, especially for lymphoid malignancies. The risk in the 5- to 9-year-old and >10-year-old groups was 2 and 3-4 times higher respectively, than in the <5-year-old group. The risk decreased over time, with the highest risk observed between 2 and 5 years after exposure and the lowest after 10 years.

Despite some missing or approximate data, this study, involving nearly a million individuals, has higher statistical power than previous studies. It demonstrates an association between cumulative bone marrow dose and the risk of developing haematological malignancies, both lymphoid and myeloid, even at low doses (10-15 mGy).