The cause of tumor development may lie in one or more alterations (mutations) of the genes present in the nucleus of the cells that make up the tumor, and which as a whole change the biological behavior of the cell. In rare cases, some hereditary mutations can provoke the onset of tumors even in the first years of life. In other cases, some genes may have variants in the population that predispose to the development of tumors that arise if and when the cells have accumulated other pathological mutations.
Unlike rare diseases that are defined according to their prevalence (not more than 5 out of 10,000 citizens in the European population), the criterion for defining a rare tumor is the incidence (number of new cases in a time interval), the threshold of which is 6 out of 100,000 cases in the European population. This criterion, accepted by all internationally, was proposed in 2011 by the European project on rare RARE-CARE tumors. Together, it is estimated that rare tumors represent around 20-25% of all malignancies and that they can affect all areas of the body.
