Metastatic disease, or more simply "metastasis" (single) or "mestastases" (pleural) occur when a neoplasm (tumor) spreads from one part of the body to another. There are three primary modes of spread:

local invasion or infiltration
blood-borne (hematogenous) dissemination
lymphatic spread

Hematogenous dissemination is the most common and usually the most serious type of tumor spread. Most solid tumors larger than 4-5 milimeters in size grow their own blood vessels. [See Angiogenesis - Click for HyperLink ] The tumor has "angiogenic" factors that stimulate blood vessels. Many of these same tumors also have angioinvasive factors that allow the tumor to penetrate through vessel walls. Therefore, the tumor is richly vascularized - giving it a path to the systemic circulation; and, once the tumor "embolizes" the small tumor fragments are able to erode through the vessel and implant into the surrounding tissue. Thus creating a metastatic deposit.

There is a "cascade" theory for tumor embolization and metastasis. Blood from the gut is "filtered" in the liver, and gastrointestinal metastases usually begin in the liver before they spread to other organs. The lung filters all of the systemic blood as it passes through, and metastatic disease here is common from all tumor sources. Brain and bone metastases usually occur after the lung has been affected - at least in theory. Some solid organs are affected more than others merely because they filter more blood - like the kidney, adrenal gland, and brain.