11.1: Understanding Cancer

Defining Cancer

Cancer is a group of more than 100 diseases, all of which involve abnormal cell growth with the potential to invade or spread to other parts of the body. Cancer can start almost anywhere in the human body when just one of the trillions of cells changes. Normally, human cells grow and multiply in a controlled process as the body needs them. When cells grow old or become damaged, they are supposed to die, and new cells take their place. Sometimes, abnormal or damaged cells grow and multiply when they should not. These cells typically form tumors, which are lumps of tissue.

In general terms, cancer occurs when the cell cycle is deregulated due to DNA damage. The number of potential underlying causes of this DNA damage is great, so there are many different risk factors for cancer. Any cells that become cancerous divide more quickly than normal cells. They may form a mass of abnormal cells called a tumor. The rapidly dividing cells take up nutrients and space, damaging the surrounding normal cells. If cancer cells spread to other parts of the body, they can invade and damage tissues and organs. They may eventually lead to death.

By far, the most common of the 100-plus types of human cancer is basal cell carcinoma, a type of skin cancer. Basal cell carcinoma makes up 40 percent of all new cancers each year in the United States. Other common types of cancer include lung, colorectal, prostate (in males), and breast (in females) cancers. These cancers are not as common as skin cancer, but they cause the majority of cancer deaths.

What is Cancer?

A 3D animation describing cancer.

Normal Cells versus Cancer Cells

Cells group together to form the tissues and organs of our bodies, and we usually have just the right number of each type of cell. This is because cells produce signals that control how often and how much they divide. For example, when the body is growing, such as from childhood to adulthood, it signals for more cells as its tissues expand. Normal healthy cells also like each other; they know which other cells to join, and they stay together without crowding one another.  Cancer cells do not like each other and do not stick together as normal cells do.

Normal cells:

• control their growth, reproducing when and where it’s needed.
• stick together and remain in their intended location in the body.
• die when they become damaged or too old to maintain homeostasis.
• become specialized in specific functions.

Cancer cells:

• grow in the absence of signals telling them to grow (normal cells only grow when they receive such signals).
• ignore signals that normally tell cells to stop dividing or to die.
• invade into nearby areas and spread to other areas of the body.
• tell blood vessels to grow toward tumors.
• hide from or trick the immune system. The immune system normally eliminates damaged or abnormal cells. Cancer cells can trick the immune system into helping cancer cells stay alive and grow.
• accumulate multiple changes in their chromosomes, such as duplications and deletions of chromosome parts. Some cancer cells have double the normal number of chromosomes.
• rely on different kinds of nutrients than normal cells. In addition, some cancer cells generate energy from nutrients in a way different from most normal cells. This lets cancer cells grow more quickly.

How do cancer cells behave differently from healthy ones?

Dig into the science of how cancer cells grow, and why their rapid cell division is the disease’s strength— but also its weakness.

Other Tissue Changes

Not every change in the body’s tissues is cancer, but some may develop into cancer if they are not treated. Examples of tissue changes that are not cancer but may be monitored include:

• Hyperplasia occurs when cells within a tissue multiply faster than normal, building up extra cells. The cells and the tissue's organization are still normal. Hyperplasia can be caused by several factors, including chronic irritation.
• Dysplasia is more advanced than hyperplasia but also includes an excess of cells. In dysplasia, the cells appear abnormal, as does the tissue's organization. In general, cancer risk increases with the abnormality of cells and tissues. Some types of dysplasia may need to be monitored or treated, but others do not. An example of dysplasia is an abnormal mole that forms on the skin. A dysplastic nevus can turn into melanoma, although most do not.
• Carcinoma in situ is even more advanced than dysplasia. It is sometimes called stage 0 cancer, precancer, or a low malignant potential (LMP) tumor. They have not invaded nearby tissue and may never do so. But because some carcinomas in situ can progress to cancer, they are usually treated.

Adapted from the National Cancer Institute

How Cancer Occurs

Cancer occurs when a normal cell is transformed into a cancer cell. This occurs when mutations alter the genes that regulate cell growth and differentiation. The genes involved include proto-oncogenes and tumor-suppressor genes.

Proto-oncogenes are genes that normally promote the growth and division of normal cells.

Tumor-suppressor genes are genes that normally inhibit the division and survival of abnormal cells.

Genetic alterations may result in the formation of cancer-causing oncogenes, overexpression of normal proto-oncogenes, or underexpression or inactivation of tumor-suppressor genes.

As shown in Figure \(\PageIndex{1}\), changes in multiple genes are typically required to transform a normal cell into a cancer cell. The entire process is similar to a chain reaction. Initial DNA errors are compounded by additional errors, and each error progressively allows the cell to escape more controls on cell growth and cell division.

Figure \(\PageIndex{1}\): Transformation of a normal cell into a cancer cell is a multi-stage process involving accumulated genetic damage to proto-oncogenes and tumor-suppressor genes. Oncogene activation, Amoriesunshine CC0

What is Cancer? Oncogenes

Cancer is caused by changes to DNA in genes. If a gene involved in normal cell growth is changed so that cell growth doesn’t stop, that gene is called an oncogene. Uncontrolled cell growth leads to cancer.

What is Cancer? Tumor Suppressor Genes

Cancer is caused by changes to DNA in genes. In healthy cells, tumor suppressor genes slow or stop cell growth. In cancer cells, mutations in tumor suppressor genes can inactivate them, leading to uncontrolled cell growth.

Benign versus Malignant Tumors

When cells grow and divide, they may form tumors, which are lumps of tissue. These lumps might be benign or malignant. A benign tumor is not cancerous; a malignant tumor is cancerous. However, not all cancers form tumors; leukemia, a cancer of the blood cells, does not. Instead, the cancer cells accumulate in the blood and sometimes in the bone marrow.

The main difference between a cancerous (malignant) tumor and a non-cancerous (benign) tumor is the ability of a cancerous tumor to spread to other parts of the body, called metastasis. Most benign tumors do not spread into, or invade, nearby tissues and are usually left in the body unless they are causing pain or are life-threatening, such as those in the brain. Cancerous (malignant) tumors spread into, or invade, nearby tissues and can travel to distant places in the body to form new tumors.

Benign tumors:

• usually grow slowly
• do not generally spread to other parts of the body
• usually have a covering made up of normal cells

Malignant tumors:

• usually grow faster than benign tumors
• spreads into surrounding tissues and causes damage
• may spread to other parts of the body (metastasis) in the bloodstream or through the lymph system to form secondary tumors.