The cause of the development of tumours can lie in one or more alterations (mutations) of the genes present in the nucleus of the cells that make up the tumour and which as a whole modify the biological behaviour of the cell.
In rare cases, some hereditary mutations can cause the onset of tumours even in the first years of life. In other cases, some genes may have variants in the population that predispose to the development of tumours that arise if and when the cells have accumulated other pathological mutations.
Unlike rare diseases that are defined on the basis of prevalence (no more than 5 out of 10,000 citizens in the European population), the criterion for defining a tumour as rare is the incidence (number of new cases in a period of time), the threshold is established in 6 cases out of 100,000 in the European population. This criterion, accepted by all at international level, was proposed in 2011 by the European RARE-CARE rare cancer surveillance project.
All together, rare tumours are estimated to represent about 20-25% of all neoplasms and can affect all parts of the body.
