The ancient Greeks used the word metastasis
to mean "removal from one place to another".
Nowadays metastasis is used to describe the movement or spreading of cancer cells from one organ or tissue to another. Cancer cells can break away from a primary tumor, penetrate into lymphatic and blood vessels, circulate through the bloodstream, and grow in a distant focus (metastasize) in normal tissues elsewhere in the body.
Systemic metastasis is the primary cause of mortality for most cancer types and represents a major therapeutic challenge in oncology. As a matter of fact approximately 90% of all cancer deaths arise from the metastatic spread of primary tumours. When cancer is detected at an early stage, before it has spread, it can often be treated successfully by surgery or local irradiation, and the patient will be cured. However, when cancer is detected after it is known to have metastasized, treatments are much less successful. Furthermore, for many patients in whom there is no evidence of metastasis at the time of their initial diagnosis, metastases will be detected at a later time, years or even decades after apparently successful primary treatment. These metastases can show an organ-specific pattern of spread, for example, breast cancer often metastasizes to bone.
Surprisingly though, in spite of the clinical importance of metastasis, much remains to be learned about the biology of the metastatic process. In part, knowledge is limited because metastasis is a 'hidden' process, which occurs inside the body and so is inherently difficult to observe.Many molecular factors have been identified as contributing to the formation of detectable metastases, and additional factors will rapidly be identified from studies that use genomic and proteomic approaches. However, the identification of molecules and genes that are associated with a metastatic end point does not, in itself, provide information about how these molecules contribute to the metastatic process. Therefore studies using in vivo imaging techniques and quantitative approaches that follow the fate of cancer cells in the body can shed light on this 'occult' process.