Mechanisms of metastasis.

The mechanism by which a malignant tumor cell establishes a metastatic foci is a complex process requiring the successful completion of a series of steps. If any step cannot be completed, the entire process fails. The tumor cell must separate from the primary tumor, gain access to an efferent channel (lymphatic or vascular), survive transport to a distant capillary bed, exit the vessel at the distant site, and obtain a new blood supply within the distant organ, if successful macrometastases are to develop. As the mysteries of this process are being unraveled, numerous heretofore beliefs have been disproven and new beliefs established. The primary malignant tumor is a heterogenous mixture of malignant cells, each subcell type with unique properties. Both lymphatic and vascular channels offer an efferent pathway for tumor cells, whether they be carcinomatous or sarcomatous. The location of the clinical metastasis is not a haphazard process, but rather a combination of selection by the tumor cell and mechanical trapping of the tumor cell fibrin-platelet clot. Finally, investigations are beginning to demonstrate that the immune process is exceedingly complex and its role in defense against malignancy is far from understood. As more is learned, new treatment methods will become available to combat the metastatic process. If metastasis were to be controlled, cancer would be conquered.