Why cancer cells that travel keep the identity of where they started
When breast cancer spreads to the lungs, the new growth is made of breast cells, not lung cells, and doctors call it metastatic breast cancer. That journey from one organ to another, known as metastasis, separates malignant tumours from benign ones and accounts for over 90 percent of cancer deaths.
The escape begins when malignant cells break off the original tumour and attack the protein mesh, called the extracellular matrix, that walls it off from neighbouring tissue. Having broken through, some slip into lymph vessels or blood vessels and ride the circulation, sometimes singly and sometimes in clusters that may keep their group structure and seed new growths more successfully. Lymph channels are the usual first route for carcinomas, while sarcomas tend to spread through the blood. Veins, with thinner walls than arteries, are invaded more often, so spread tends to follow venous flow: colorectal cancer, for instance, usually reaches the liver through the portal vein.
Destinations are not random. In 1889 Stephen Paget proposed his seed and soil idea, that certain tumour cells thrive only in certain organs. Today this preference is called organotropism. Prostate cancer tends to settle in bone, colon cancer in the liver, and breast cancer in bones and lungs, steered partly by signalling molecules such as chemokines. The lungs, liver, brain and bones are the most frequent sites overall.
Researchers still argue about the exact mechanism. Leading ideas include cells switching between a tightly bound and a mobile state, a special population of cancer stem cells, and fusion between cancer cells and immune cells called macrophages. Physical forces matter at every stage, since squeezing through vessel walls and surviving shear in the bloodstream is hard; cancer cells also switch opportunistically between crawling styles. Tumours recruit new blood vessels, a process called angiogenesis, and chemical tags on DNA packaging appear to help scattered cells survive in distant organs.
The body fights back with roughly a dozen known metastasis suppressor proteins. Still, spread places a cancer at the highest stage in standard grouping, which is why researchers look for ways to block the steps cells need to move.
Source: Metastasis
