Abstract
In this chapter, we will understand that malignant primary tumors become dangerous when angiogenesis allows their massive growth and in particular when some of the cancer cells spread to other organs and form metastases. Cancer cells that have obtained the hallmark “activating invasion and metastasis” use the EMT process, in order to leave the malignant primary tumor. They circulate in the blood stream until they colonize in distant organs, which often follows a specific tropism. The metastatic cascade filters for very potent few cancer cells that grow from micrometastases to macrometastases, but the clinical manifestation of metastases often takes years. Metastatic tumors take not only space and resources of their host tissues but they impact also other organs, such as skeletal muscle, via secreted cytokines and other factors. This leads in the majority of metastatic cancer patients to the muscle wasting syndrome cachexia, which finally causes multiorgan failure and death.