The project Surveillance of Rare Cancers in Europe (RARECARE) collected data on cancers from 89 population-based cancer registries in 21 European countries, making it possible to study the epidemiology of these cancers as a whole in a large and heterogeneous population. Working from this database and the literature, a RARECARE working group produced a new list of cancers and developed a new definition of rare cancers (incidence < 6/100,000/year). This definition and working list are adopted for this rare tumor immunotherapy study.