metastasis

Metastasis is the process by which cancer cells break away from where they started and set up new colonies in distant parts of the body. Picture dandelion seeds carried off on the wind from one plant, each able to take root somewhere far away — except here the seeds are tumor cells traveling through the bloodstream or lymph channels to organs they were never meant to grow in.

For lung cancer this is the crucial event that turns a potentially curable local problem into widespread disease. Lung cancer commonly spreads to the brain, bones, liver, adrenal glands, and the other lung. The presence of even one such distant deposit is what defines stage IV disease and usually means that local treatments alone — surgery or radiation to the original tumor — can no longer cure it.

Confusingly to newcomers, a cancer is always named for where it began: lung cancer that has spread to the brain is still lung cancer in the brain, made of lung cells, and is treated as lung cancer rather than as a brain tumor. This is why finding the original (primary) tumor and its tissue type matters even when the metastasis is what first causes symptoms.

An important nuance: not all metastatic disease is treated identically. Modern systemic therapies can control widespread cancer for a long time, and in selected cases with only one or a few metastases, focused treatment of those spots is sometimes pursued.