Birth order and risk of nasopharyngeal carcinoma in multiplex families from Taiwan.

A small proportion of individuals infected with Epstein-Barr virus (EBV) develop nasopharyngeal carcinoma (NPC). Timing of initial exposure could alter immunological responses to primary EBV infection and explain variation in cancer risk later in life. We measured early life family structure as a proxy for the timing of primary EBV infection to examine whether earlier age at infection alters NPC risk. We utilized data from 480 NPC cases and 1,291 unaffected siblings from Taiwanese NPC multiplex families (>/= 2 family members with NPC, N = 2,921). Information on birth order within the family was derived from questionnaires. We utilized logistic regression models to examine the association between birth order and NPC, accounting for correlations between relatives. Within these high-risk families, older siblings had an elevated risk of NPC. Compared with being a first-born child, the risk (95% CIs) of NPC associated with a birth order of two, three, four and five or more was 1.00 (0.71, 1.40), 0.88 (0.62, 1.24), 0.74 (0.53, 1.05) and 0.60 (0.43, 0.82), respectively (P for trend = 0.002). We observed no associations between NPC risk and the number of younger siblings or cumulative infant-years exposure. These associations were not modified by underlying genetic predisposition or family size. We observed that early life family structure was important for NPC risk in NPC multiplex families, with older siblings having a greater risk of disease. Future studies focusing on more direct measures of the immune response to EBV in early childhood could elucidate the underlying mechanisms.