Exposure to Radiation Earlier in Life Increases Leukemia Risk

Exposure to ionizing radiation at cumulative doses <100 mSV in childhood or adolescence significantly increases a person’s risk for acute myeloid leukemia and acute lymphoblastic leukemia, according to the results of a recent study published in The Lancet Haematology (2018 Jul 16. Epub ahead of print).

Previous studies have demonstrated the increased risk for leukemia associated with childhood exposure to moderate or high doses of radiation, but evidence of increased leukemia risk from low doses (≤100 mSv) of radiation is lacking.

Thus, Mark P. Little, DPhil, Radiation Epidemiology Branch, National Cancer Institute, Bethesda, MD, and colleagues, conducted a pooled analysis to estimate the risk for leukemia associated with low-dose radiation exposure in childhood or adolescence.

“Because radiation exposure in childhood has been shown to confer the highest proportionally increased risks relative to background risk of leukaemia, and because the relative risk of leukaemia is higher in children exposed to radiation than that of any other malignancy, individuals with childhood exposures to radiation afford the best opportunity for studying the risks of cancer associated with low-dose radiation,” they said.

Using individual patient data gleaned from 9 international cohort studies of patients who were first exposed to radiation before age 21 years, Dr Little and colleagues evaluated leukemia and myeloid malignancy outcomes per International Classification of Diseases and International Classification of Diseases for Oncology criteria. Eligible cohorts had ≥5cases of the relevant hematopoietic neoplasms, included patients who had not received treatment for malignant disease, and had projected individual active bone marrow (ABM) doses. The analysis was restricted to patients who had mean cumulative ABM doses <100 mSv, and who had been enrolled in the historical cohort studies between June 4, 1915, and December 31, 2004. 

From a total of 262,573 eligible individuals, Dr Little and colleagues identified 154 myeloid malignancies—including 79 acute myeloid leukemias, 8 myelodysplastic syndromes, and 36 chronic myeloid leukemias—and 40 acute lymphoblastic leukemias; overall, 221 leukemias (excluding chronic lymphocytic leukemia) were identified. The mean follow-up time was 19.63 years, and the mean cumulative ABM dose was 19.6 mSv.

At doses of 100 mSv, the fitted relative risks were 3.09 (95% confidence interval [CI], 1.41-5.92; P trend= .008) for acute myeloid leukemia and myelodysplastic syndromes combined, 2.56 (95% CI, 1.09-5.06; P trend= .033) for acute myeloid leukemia, and 5.66 (1.35-19.71; P trend = .023) for acute lymphoblastic leukemia. No clear dose-response was reported for chronic myeloid leukemia, which had a relative risk of 0.36 (0.00-2.36; P trend = .394) with 100 mSv.

In addition, Dr Little and colleagues noted that the significant dose-response persisted even at doses <50 mSv for acute myeloid leukemia and myelodysplastic syndromes combined and for acute lymphoblastic leukemia. Excess absolute risks ranged between 0.1 and 0.4 cases or deaths per 10,000 person-years at doses of 100 mSv.

“The risks of acute myeloid leukaemia and acute lymphoblastic leukaemia were significantly increased after cumulative doses of ionising radiation of less than 100 mSv in childhood or adolescence, with an excess risk also apparent for cumulative radiation doses of less than 50 mSv for some endpoints....These findings suggest that there is risk of leukaemia associated with low-level exposure to radiation. Since most exposures to workers and the public are from low doses, the present study, among others, suggests that the current system of radiological protection is prudent and not overly protective,” they concluded.—Janelle Bradley