EDITOR,—M A Richards and J C Parrott raise important issues about tertiary cancer services.1 Sadly, their paper does not contribute to our understanding of benchmarking for tertiary cancer centres because the calculation of population denominators is seriously flawed. The standard method of calculating catchment populations for specific services uses the proportions of current activity provided by different providers to allocate a district's resident population to those providers.2 The key principle is that the catchment populations of all providers add up to the total populations of the …