Age-standardisation of relative survival ratios of cancer patients in a comparison between countries, genders and time periods.

A recent method of age-standardisation of relative survival ratios for cancer patients does not require calculation of age-specific relative survival ratios, as ratios of age-specific proportions between the standard population and study group at the beginning of the follow-up are used to substitute the original individual observations. This method, however, leads to direct age-standardisation with weights that are different for each patient group if the general population mortality patterns for the groups are different. This is the case in international comparisons, and in comparisons between genders and time periods. The magnitude of the bias caused by the differences in general population mortality is investigated for comparisons involving European countries and the USA. Patients in each country are assumed to have exactly the same age-specific relative survival ratios as those diagnosed in Finland in 1985-2004. An application of a properly functioning age-standardisation method should then give exactly equal age-standardised relative survival ratios for each country. However, the recent method shows substantial differences between countries, with highest relative survival for populations, where the general population mortality in the oldest ages is the highest. This source of error can thus be a serious limitation for the use of the method, and other methods that are available should then be employed.