What is cancer? — for new patients
If you have been recently diagnosed with cancer, you may find that medical terminology can be intimidating. You don’t have to be a doctor, however, to grasp the basic concepts behind what cancer is and how most treatments work. Below, you will find an entry-level explanation of cancer and cell biology designed to help you feel empowered as you navigate your disease.
What is cancer?
Cancer is not actually a single disease: the word “cancer” describes a family of diseases. Each type of cancer is caused by cells reproducing when and where they shouldn’t. Understanding what cancer is requires understanding what cells are, so this explanation will start there.
What are cells?
Cells are the tiny building blocks of the body. Different types of cells have different sizes, shapes, and jobs, but all cells tend to have certain things in common.
Each cell is surrounded by a membrane, which is a fatty “bag” that separates the inside of the cell from the outside world. The cell is full of water mixed with other molecules like salt and sugar; this fluid is called the cytoplasm (“cyto-” means cell). The cell also contains different compartments called organelles, each of which has a specialized job; the nucleus, for example, houses the cell’s DNA. A network of fibers called the cytoskeleton helps give the cell internal structure. You don’t need to remember the names of all of the parts of the cell to understand the rest of this article, but it can be helpful to be able to picture a generic cell in your head.
Each type of tissue in the body is made up of different types of cells. Muscles, for example, are mostly made up of muscle cells, while the brain is mostly made up of neurons and other types of cells that support them. Each type of cell has a different job: muscle cells are able to get shorter (contract) and longer (relax), while neurons specialize in relaying “information” from one part of the body to another.
How do cells reproduce?
New cells are created from old cells through a process called cell division. Generally, this involves a cell growing to roughly twice its size, making a second copy of its DNA, and then dividing in half, producing two new “daughter” cells.
Cells may divide to help the body grow or to replace aging or damaged cells. Certain types of cells are supposed to divide at certain times, while other types of cells may never divide. In general, the more specialized a cell is for performing a specific job — in other words, the more “differentiated” it is — the less likely that cell is to divide. Neurons, for example, do not divide. Stem cells, on the other hand, are “undifferentiated” cells that do undergo cell division. When stem cells divide, some of the “daughter” cells may mature into specialized cells, while other “daughter” cells may remain undifferentiated so they can replace the original stem cells.
How can cell division go wrong?
Cell division is very carefully regulated. Different molecules help determine whether and when a cell will divide. Some molecules act like “green lights”: they tell a cell it should divide. Other molecules act like “red lights”: they prevent a cell from dividing. Cancer is caused by genetic mutations that interfere with the way that cell division is regulated.
Genes are “recipes” that tell the cell how to make different molecules; the recipes are “written” in the cell’s DNA. A mutation in a gene is like an error in a recipe: it can cause the cell to make a molecule that doesn’t work the way it should. Some mutations cause cancer by “turning on” the molecules that act as green lights, while other mutations cause cancer by “turning off” the molecules that act as red lights. Either type of mutation results in increased cell division.
What is a tumor?
When cells divide too much, the extra cells may form a mass of tissue called a tumor. Some tumors do not pose a risk of spreading to other parts of the body; these “benign” tumors are not considered to be cancer. Other tumors do pose a risk of spreading to other parts of the body; these “malignant” tumors are considered to be cancer. Other types of cancer may not involve tumors; in leukemia, for example, excess white blood cells float around in the blood rather than forming a solid, stationary mass.
Different cancers start in different cell types
One of the things that makes different cancers unique is the type of cell in which the cancer-causing mutation takes place. Some cells are more likely to experience these mutations than others. The layer of tissue that covers the outside of the body and the inner and outer surfaces of internal organs is called the epithelium. Most types of cancer (80–90%) originally develop from epithelial cells.
Cancers are generally named after the type of tissue they start in. Cancers of the epithelium are called carcinomas. For example, many skin cancers are carcinomas. There are different types of epithelial cells, so there are different subtypes of carcinoma.
Cancers that start in connective or supportive tissue like cartilage, bone, or muscle are referred to as sarcomas. There are also many different subtypes of sarcoma. For example, osteosarcoma is a cancer of the kind of cell that makes new bone. Sarcomas are relatively rare compared to other types of cancer.
Several types of cancer specifically affect white blood cells, which are part of the immune system. White blood cells are created in the bone marrow, then travel through the body in the blood stream or lymphatic system (a network that helps move fluid around the body).
Leukemia begins in the bone marrow; it produces high numbers of unhealthy blood cells that circulate throughout the body. Leukemia is the most common kind of cancer in children.
Myeloma also begins in the bone marrow. It originates in plasma cells, a type of white blood cell that produce antibodies to help protect the body from infection.
Lymphoma also originates in certain types of white blood cell. Unlike myeloma, which is generally based in the bone marrow, lymphoma tends to affect parts of the lymphatic system, including lymph nodes and the spleen.
As with carcinomas and sarcomas, there are multiple subtypes of lymphoma, myeloma, and leukemia. Some cancers involve a mix of different types or subtypes. For more information on the different types and subtypes of cancer based on tissue type, visit this page from the National Institute of Health’s National Cancer Institute.
Cancer in the same cell type can have different causes
As mentioned above, cancers are caused by mutations in the genes that help control when cells divide. Some mutations may turn on the signals that tell cells to divide, and some mutations may turn off the signals that tell cells not to divide. Cancers in the same cell type may be caused by different mutations in different people.
Knowing the type of genetic mutation that caused the cancer and the type of cell in which the cancer started can help you predict how a cancer will affect your body and how it will respond to treatment. In some cases, we have different treatments for cancers caused by different mutations; with time, this should be increasingly common.
Be your own best advocate
Understanding your cancer can help you be your own best advocate at the doctor’s office. Learn about the specific type of cancer you have been diagnosed with. Ask your doctors about testing for mutations in your tumor’s DNA. Research treatment options that are specific to your kind of cancer and, if possible, your specific mutation. At the very least, this knowledge can help you regain your sense of control.