Summary A statistical model was developed based on Poisson regression of incidence of childhood leukaemia and non-Hodgkin's lymphoma (NHL) in relation to population mixing among all 119 539 children born to mothers living in Cumbria, north-west England, (excluding Seascale). This model was used to predict the number of cases in Seascale (the village adjacent to the Sellafield nuclear installation) children, born and diagnosed before 1993. After allowing for age, the incidence of acute lymphoblastic leukaemia (ALL) and NHL was significantly higher among children born in areas with the highest levels of population mixing, relative risk (RR) = 11.7 (95% confidence interval (CI) 3.2-43) and was highest among children of incomers. The model predicted up to 3.0 (95% CI 1.3-6.0) cases of ALL/NHL in children born in Seascale compared to six observed and 2.0 (95% CI 1.0-3.4) cases in children resident, but not born, in Seascale compared to two observed. Population mixing is a significant risk factor for ALL/NHL, especially in young children, accounting for over 50% of cases in Cumbria and most cases in Seascale.Keywords: retrospective cohort study; childhood leukaemia; non-Hodgkin's lymphoma; population mixing; Seascale; epidemiology
144British Journal of Cancer (1999) 81(1), 144-151 © 1999 Cancer Research Campaign Article no. bjoc.1999 Received 16 November 1998 Revised 7 January 1999 Accepted 27 January 1999 National Statistics (ONS) and entered onto a computer database . The motherÕs address was grid-referenced and assigned to one of the 171 electoral wards used in the 1991 census. The fathersÕ and mothersÕ counties of birth (or regions of Scotland) were coded (Mason, 1986). Children were grouped into families using algorithms based on parentsÕ names and measures of population mixing and social class in a ward were based on characteristics of parents.
Case ascertainmentCancer registrations for the cohort, recorded throughout the UK, were obtained from ONS, from six regional and national cancer registries and from scrutiny of all relevant death registrations, also obtained from ONS Prior to analysis, it was decided to consider (i) all cases of leukaemia and NHL (leukaemia/NHL); (ii) the following subgroups: common ALL (cALL), other ALL, other leukaemias, NHL; and (iii) solid tumours.
Follow-upThere is evidence that many leukaemias diagnosed under the age of 1 year are different at a molecular level from other childhood leukaemias, possibly reflecting a different aetiology (Greaves, 1996(Greaves, , 1997 and so these were excluded. Each child was therefore followed up from age 1 year until he or she reached 15 years, died or emigrated, or until the end of 1992, whichever was the earliest. Hence the person-years at risk were calculated (Table 1).
Explanatory variablesAt the individual level, a measure of population mixing was derived from the place of birth of the childÕs parents: each child was categorized as having either both, one or neither parent born outside Cumbria. Additional individual characteristics consi...