Cancers are capable of spreading throughout the body by two mechanisms: invasion and metastasis. Invasion refers to the direct migration and penetration by cancer cells into neighboring tissues. Metastasis refers to the ability of cancer cells to penetrate into lymphatic and blood vessels, circulate through the bloodstream, and then invade normal tissues elsewhere in the body. Invasion and metastases kill hosts through two processes: local invasion and distant organ injury. Local invasion can compromise the function of involved tissues by local compression, local destruction, or prevention of normal organ functioning. The most significant turning point in the disease, however, is the establishment of metastasis. At this stage, the patient can no longer be cured by local therapy alone. The patient with metastatic disease most commonly succumbs to injury caused by cancer dissemination or less often to complications associated with cytotoxic therapies.Our understanding of the processes of invasion and metastases has improved, but our ability to detect occult metastatic disease or metastatic potential prior to development of occult disease still lags. Some patients with cancer have benefited from improved screening techniques wherein they are being diagnosed earlier, such as in breast cancer. Despite this, approximately 30% of patients still will have clinically detectable metastases at the time of initial diagnosis, and a further 30 to 40% of patients will harbor occult metastases. source 1:Invasion and Metastases - Holland-Frei Cancer Medicine - NCBI Bookshelf source 2:Comprehensive Cancer Information - National Cancer Institute