Spatial clustering and space-time clusters of leukemia among children in Germany, 1987-2007

Leukemia is the most frequent malignancy in children under the age of 15 years. The question of whether childhood leukemia has a tendency for clustering or forms clusters has been studied for several decades. The environmental risk factor discussed most often is infection, which might result in spatial clustering and space-time clusters. The German Childhood Cancer Registry provided data on 11,946 children with leukemia diagnosed during 1987-2007, as classified in the International Classification for Childhood Cancer (third edition), aggregated by municipality. We used the Potthoff-Whittinghill model to test for a general trend for clustering and the spatial scan statistic to search for localized clusters. No evidence of global clustering was found, neither for the whole study population nor in sub-groups by age, period or population density, or for different types of leukemia. A similar result was found for localized clusters. The analysis shows no evidence of a tendency to clustering, however, aggregation of data at the municipality level might have diluted small localized clusters. The results of this study do not provide support for the hypothesis of an infectious or a spatial environmental etiology of childhood leukemia.