Rare cancers are generally classified in the group of rare diseases which is defined in the European Union as diseases with a prevalence of fewer than 5 cases out of a population of 10,000. A table published by the European Medicines Agency (EMA).

Hematology is the study of blood in health and disease. It includes problems with the red blood cells, white blood cells, platelets, blood vessels, bone marrow, lymph nodes, spleen, and the proteins involved in bleeding and clotting (hemostasis and thrombosis).