Cancer

Cancer is a group of more than 100 different diseases which characterized by uncontrolled cellular growth, local tissue invasion and distant metastasis (Chabner, 2006).

It has been difficult to develop an accurate definition for cancer. The reputed British oncologist Willis has defined cancer as "an abnormal mass of tissue, the growth of which exceeds and is uncoordinated with that of the normal tissues and persists in the same excessive manner after cessation of the stimuli which evoked the change" (Cotran et al., 1999).

The mechanisms by which cancer occurs are incompletely understood. The cancer is thought to develop from cells with changed the typical mechanisms for manage of proliferation and growth. Recent proofs strengthen the notion of carcinogenesis as a genetically regulated multistage process (Mediana et al., 2008).

The first step in this process is initiation which started by exposure of cells to carcinogenic substances which lead to genetic damage that, if not fixed, results in irreversible mutations. The mutated cells start growth till formation of a colony. The second stage is a promotion, in which carcinogens or other factors modify the environment in a way assists growth of mutated cell population over normal cells. The main difference between the these two stages is that the later one is reversible, so it may be the target of future chemoprevention strategies (Mediana et al., 2008).

The next stage is transformation of mutated cells to cancerous cells; 5-20 years may require for the transition of benign carcinogenic phase to the fully developed malignant stage where the cancer can be detected clinically. The last stage called progression where further genetically changes take place leading to increase the proliferation and metastasis (Weinberg, 1996, Compagni and Christofori, 2000).

1.2. Cancer epidemiology:

Cancer is a major public health problem in many parts of the world. Over ten million new cases of cancer, with over six million deaths were estimated in the year 2000 (Parkin, 2001). The estimated numbers of incidence and mortality in 2002 were markedly increased as 10.9 million new cases, 6.7 million deaths with cancer (Parkin et al., 2005). Even developed countries suffering from cancer, in USA it was expected 2677860 new cases of cancer will be diagnosed in 2009. In that same year , more than 562,340 deaths will occur due to cancer, this number represent 25 % of all deaths and makes cancer as a second leading cause of death after heart diseases (Jemal et al.