How do cancers metastasize?
Metastasis is a complex biological and chemical process, and despite it being highly associated with cancers, they are not the only diseases that involve metastasis. To simplify how it occurs, we have to understand that cancers divide so quickly that the adherence between cells isn’t well established and they can easily break away the “tissue cement” that hold them together and invade nearby blood vessels or lymphatics, then travel to another site. Cancers demand an enormous amount of energy to provide for their rapid growth, so blood vessels develop specifically for them, and since they are rushed, their walls are so weak that they are easily invaded. The site of metastases isn’t chosen randomly, though, and there is a degree of specificity regarding different cancers. Prostatic cancer, for example, metastasizes mostly to the bones of the vertebral column, while cancers of the gastrointestinal tract metastasize to the liver.
The site of metastasis depends on several factors including:
- Line of drainage of lymphatics: cancers tend to spread by lymphatics more than blood, and lymphatics have a line of drainage. Testicular cancers tend to spread to a group of lymph nodes called the para aortic lymph nodes, while breast cancer tends to spread to the axillary lymph nodes located in your armpit.
- Line of drainage by veins: cancers of the gastrointestinal tract are all drained by a venous system through the portal circulation which ends up in the liver, making it the most common site of metastases.
- Molecular specificity: some cancers are inherently more common to spread to specific sites because of receptors found on such cells. This can be seen in cases of distant metastases.
Most blood metastases spread to four organs, more likely the bones, the liber, the lungs and the brain.