Space-time Clustering of Childhood Lymphatic Leukaemias and Non-Hodgkin's Lymphomas in Sweden

Background: The discussion concerning clusters of childhood leukaemia has mainly been focused on their relation to the time and place of diagnosis. Recently some studies have indicated clustering not only at diagnosis, but also around time and place of birth. Space-time clustering at time of birth could be of special interest if the aetiological agent is of infectious origin and the induction of leukaemia either occurs pre- or perinatally or an infection at that time favours a poor subsequent immune response to the agent.

Methods: To identify possible space-time clustering we have used the close-pair method of Knox. One-thousand-twenty recorded cases (0-14 years) of childhood acute lymphatic leukaemia and 293 cases (0-14 years) of malignant non-Hodgkin's lymphoma from Sweden between 1973-1996 were analysed. The records include date of birth and of diagnosis as well as addresses at birth and at diagnosis.

Results: A significant excess of case-pairs (25 observed, 14.9 expected, p = 0.01) was observed close in date and place of birth in the 4-14 year age group with acute lymphatic leukaemia (ALL). However there was no statistically significant clustering found around time of diagnosis. When the cases of leukaemia and the non-Hodgkin's lymphomas were combined no statistically significant clustering was obtained neither at birth nor at diagnosis.

Conclusions: This study strengthens the evidence of space-time clustering around the birth date in children whom later developed ALL. This observation is in support of the hypothesis that pre- or perinatal infections can induce a process leading to ALL.