I just finished typing this up for a post on a House forum (summarized from my biology 40A notes from last quarter), and thought it might be of interest to the layman. This is a brief explanation of what cancer is for people who have no idea.
Fundamentally, cancer is uncontrolled mitosis of cells, i.e. the mitosis (replication) phase of cell activity is turned on and doesn’t turn off. This uncontrolled mitosis is the common element in all types of cancer. The type of cells involved and the manner of disease progression varies widely, and there are literally hundreds of different kinds of cancer. Specific cancers have certain progressive patterns, but these are not well understood.
Genes control the cell cycle. A mutation occurs in a gene that regulates cell cycle, and the mitosis phase gets turned on permanently. This is a physical alteration of the DNA at the chemical level, and when the cell replicates, the new cells also have this mutation, and thus continue the uncontrolled mitosis.
Sources of these gene mutations are either inherited (a “proto-onco” gene, predisposed to breaking), environmental (chemicals, radiation, viruses), or a combination of the two. UV radiation, for example, breaks the hydrogen bonds between thymine amino acids, and so long term UV exposure can cause cancer.
BENIGN VS. MALIGNANT
This uncontrolled mitosis is what the tumor is — a big ball of cells that keep reproducing (tumor grows). If the tumor is confined to the type of tissue in which it originates, it is a “benign” tumor.
If the cell mitosis spreads to a different kind of surrounding tissue, that is a “malignant” tumor. It is possible for cancerous cells to break away from the original tumor and spread to other parts of the body via the blood or lymph vessels.
If a new tumor beings somewhere else in the body, it is a “metastatic tumor” and the cancer is said to have “metastasized”. The original tumor is the “primary tumor”. Lung cancer with a metastasis in the brain is still lung cancer, not brain cancer.
Cancer that originates with epithelial tissue (coverings, linings of body cavities, ducts) is called a “carcinoma”. If it originates with the cells in red bone marrow, it is “leukemia”. Leukemia is not tumorous.
There are 4 treatment options for cancer:
1) Surgery – aka “debulking”, in which the tumor or some of the tumor is physically removed. This is not always possible, especially in vital organs. But debulking can be useful in relieving pressure on neighboring body structures.
2) Chemotherapy – systemic delivery of drugs that kill cells actively engaged in mitosis. This includes good cells as well as cancerous ones, although drugs can sometimes be targeted to certain kinds of cells. Cancer cells are always in mitosis, so this kills a lot of them. It is the destruction of healthy cells that leads to the side effects like hair loss, nausea, fatigue, etc.
3) Radiation – a high dose of radiation targeted and delivered directly to the cancerous tissue (tumor). A radioactive substances can also be implanted inside the tumor (it diffuses, so the location and amount of radioactive material used are carefully chosen). Precision is highly desirable with this kind of therapy.
4) Antiangiogenesis therapy – “angiogenesis” means “growth of new blood vessels”. Some kinds of tumors grow their own blood supply by inducing angiogenesis. Drugs are introduced that inhibit angiogenesis, depriving the tumor of nutrients. This treatment does not cure cancer, but can significantly slow its progress. Not all types of cancer induce angiogenesis, so this treatment isn’t always useful.