The more widely cancer has spread from the site where it is first diagnosed, the more difficult it becomes to treat. Cancer is labeled stage IV when it is found not only in its original location, but also in distant tissues and organs.
There are several staging systems used by pathologists and cancer specialists to describe the characteristics of a particular tumor. In the most general system:
- Stage 0 means abnormal cells are present, but have not spread to nearby tissue;
- Stage I, II, and III reflect that cancer is present – the higher the number, the larger the tumour and the more it has spread into nearby tissues.
- Stage IV means the cancer has spread, or metastasised, to distant parts of the body.
There are more detailed staging systems that consider the characteristics of the tumour, such as the size and extent of the main tumour, the number of nearby lymph nodes that contain cancer cells, and whether it has metastasised. Stage IV cancer is also defined somewhat differently for each type of cancer.
When cancer is localised, it’s often possible to remove it surgically or eradicate it with radiation treatment. This is sometimes followed by chemotherapy to eliminate cancer cells that have escaped into the bloodstream or the lymph system.
However, when cancer has metastasised, treatments are rarely curative, although there are exceptions. For example, about 80% of men who are treated for metastatic testicular cancer will survive five or more years after diagnosis. A small percentage of patients with stage IV thyroid cancer live beyond 10 years.
Certain types of cancer (such as lung and pancreatic cancer) often do not cause symptoms in their early stages and therefore tend to be diagnosed when they are already advanced and have metastasised to other organs, making them difficult to treat.
Metastatic cancer causes most cancer deaths, but exactly why it is so difficult to treat is not precisely understood. Metastatic tumours often acquire additional genetic changes from those in the primary tumours that spawned them, and these genetic characteristics may cause them to resist standard treatments. Moreover, metastases that have spread to diverse organs may be growing in different micro-environments, causing them to respond differently to treatment. While chemotherapy can often kill small numbers of cancer cells, it is usually less effective in eradicating the larger number of tumour cells present in widespread metastases.
In most cases, treatment of stage IV cancer is aimed at prolonging survival and improving quality of life. These treatments may involve chemotherapy, radiation therapy, biological and hormone therapy, or surgery, or a combination of these modalities. In cases where there are only a few small areas of cancer that has spread, surgeons can remove them in an effort to prolong survival. Radiation therapy is sometimes used to help relieve pain or other symptoms caused by advanced cancer.
New treatments that harness the body’s immune system, such as checkpoint blockers and CAR-T cells, are being tested in late-stage cancers including melanoma and lung cancer. They are not yet labeled as cures, but in some patients they have achieved impressive and long-lasting slowing of advanced cancers, and even eradication of tumors in some patients.