ABSTRACT
The human body consists of billions of different cells and thousands of cell types organised in many tissues. These cells complete their specific function according to a program using an information flow proposed by Crick (1970) in the central dogma of molecular biology (see Appendices). Cancer cells arise from these normal cells. Cancers are classified into four large groups according to the normal cell from which they originate. The majority of cancers (80%) arise from ephitelial normal cells and forms the first group called carcinomas∗ (e.g. breast, ovary, cervix, prostate, lung, pancreas, colon, etc.). All the other cancers arise from nonepithelial normal cells. The second group contains sarcomas∗ which derive from connective or supportive tissue (e.g. bone, cartilage, fat, muscle, blood vessels) and soft tissues. The third group arises from hematopoetic tissues (i.e. blood-forming cells) and includes lymphomas∗ and leukaemias∗. The last group consists of tumours arising from the central and peripheral nervous system and includes glioblastomas∗, neuroblastomas∗, schwannomas∗ and medulloblastomas∗. All cancers form solid tumours except in the case of leukaemias which generate circulating tumoral cells in the blood. Cancer is a heterogeneous disease in terms of morphological, clinical and molecular characteristics (see Section 1.4).
