Sex difference in the incidence of nasopharyngeal carcinoma in Hong Kong 1983-2008: Suggestion of a potential protective role of oestrogen

Background: Nasopharyngeal carcinoma (NPC) shows a male predominance in incidence while the underlying reasons have rarely been explored. Methods: We analysed incident cases of NPC recorded in Hong Kong Cancer Registry during the period 1983-2008. All cases were divided into 5-year age groups. Age group specific incidence rates of NPC by sex and male to female ratios in incidence rate by age group were calculated. A curve fitting approach was taken to quantitatively describe the age-specific incidence rates of NPC using non-linear regressions. Results: During the period 1983-2008, a total of 27,579 new cases of NPC were identified (20,060 males and 7519 females) in Hong Kong. The overall male to female ratio of the annual age-standardised incidence rates of NPC ranged 2.2-3.1. The male to female ratio of NPC incidence increased with age until peaking at ages 55-59 years and showed a decline thereafter. An additional minor increase at ages 15-19 years was also observed. Modelling of the age-specific incidence curves suggested divergent slopes for men and women and a delay in increasing incidence with age among females, by around 5-10 years before menopause ages. Conclusion: The age-dependent pattern of the sex difference in the incidence of NPC could not be completely explained by known risk factors for NPC. The contributions of intrinsic exposures, such as sex hormones, merit consideration and further investigations. (C) 2012 Elsevier Ltd. All rights reserved.