Brain and other nervous system tumours.

Interpretation of time trends in incidence rates for cancers of brain and other and unspecified parts of nervous system is complicated by variation in registration efficiency over time and place, statistical instability of rates, changes in the ICD classification of the tumours, variation in registration practice for the benign and unspecified tumours over time and changes in methods of diagnosis. The reader will undoubtedly have the impression that in this chapter there are too many differences in the content of the data over time and place to have confidence in any of the conclusions drawn. To derive a well documented and sustainable assessment on secular trends will require an international collaborative study collecting incidence data for the malignant, benign, uncertain and unspecified tumours of the brain and other parts of the nervous system by histology and by method of diagnosis. By providing a separate rubric for the meninges, the 10th Revision of the ICD (WHO, 1993) should--combined with the morphology rubrics of the second edition of the International Classification of Diseases-Oncology (Percy et al 1990)--make such an endeavour somewhat easier. The data presented above nevertheless suggest that, as for several other cancer sites, on the whole there has been a slow overall increase in the incidence of brain and other and unspecified nervous system neoplasms on the order of 1-2% per year over the past 30 years. Increases appear to have been greater in older people. The extent to which these increases are due to the unquestioned improvements in methods and precision of diagnosis is not clear. Birth cohort data suggest that in recent years, this influence may have been greatest for those birth cohorts born prior to 1900, and this effect is unlikely to disappear until cohorts born about 1910-1920 have passed on. There is currently little evidence to support the contention that the recent increase in older people cannot be largely explained by improvements in diagnostic methods, wider access to medical care and more intensive investigation. Further work is needed.