The good news is that many forms of cancer can be avoided, and with early detection, a great number can be cured. In this article, we will look at the many faces of cancer so that you can understand the disease and its treatment, and also so that you learn about steps you can take to limit your exposure!
What we think of as "Cancer" is actually a group of more than one hundred separate diseases. These diseases are all characterized by an abnormal and unregulated growth of cells. This growth destroys surrounding body tissues and may spread to other parts of the body in a process that is known as metastasis. You have probably heard of all of these different types of cancer:
- Skin cancer (squamous cell carcinoma and basal cell carcinoma being the most common)
- Lung cancer
- Brain cancer
- Breast cancer
- Prostate cancer
- Colon cancer
- Ovarian cancer
- Leukemia
- Lymphoma
There are many others as well.
Cancer can develop anywhere in the body, and at any age. Unlike infectious diseases such as AIDS, the flu (influenza), or tuberculosis, cancer is not contagious - cancer is usually caused by genetic damage that happens inside an individual cell. Cells affected by cancer are called malignant cells. Malignant cells are different from normal cells in the body in that they divide (in most cases) much more rapidly than they should. This is important to know because many drugs used to fight cancer (antineoplastic or anticancer drugs) attack malignant cells during the active phase of cell division.
You may know someone who has had cancer, and his or her hair fell out during treatment. That happened because the anticancer drug(s) affected the normal hair follicle cells, which divide rapidly, as well as the rapidly-dividing malignant cells.
When cells divide at an accelerated rate, they often begin to form a mass of tissue called a tumor. The tumor is fed by nutrients that diffuse through neighboring blood vessels and can also grow by forming a substance called tumor angiogenesis (vessel forming) factor. This factor stimulates the growth of an independent blood supply to the tumor. Tumors can cause destruction in three common ways:
- Tumors put pressure on nearby tissues and/or organs.
- Tumors invade tissues and organs directly (direct extension), often damaging or disabling them in the process.
- Tumors make invaded tissues and/or organs susceptible to infection.
Tumors can also release substances that destroy tissues in close proximity to them.
One of the frightening things about cancer is the possibility of metastasis. This is the process where millions of malignant cells are released from the tumor (the primary) into the bloodstream. Fortunately, most of these cells are killed by trauma produced while traveling within the blood vessel walls, or by circulating cells from the immune system, like the Natural Killer (NK) cells and other T lymphocytes.
Other immune cells that battle malignant cells are macrophages, antigen-presenting cells, and substances produced by immune cells called lymphokines. One common lymphokine is called interleukin-2 (IL-2) or interferon. (See How the Immune System Works for details on these different components of the immune system.) In some cases, the circulating malignant cells survive and adhere to the inner muscular lining of the blood vessel walls. Here the process of tumor formation can begin in a different area of the body (the secondary), causing further destruction.
Cancer is caused by a number of factors, some of which we can control, and some we cannot. One of the uncontrollable factors is the presence of gene mutations. One type of gene that plays a role in normal cell growth -- an oncogene -- can be altered to contribute to the uncontrolled growth of a tumor. Oncogenes affect the way cells use energy and multiply. For example, in some cancers, the ras gene (an oncogene) is mutated, and produces a protein that stimulates cells to divide prematurely. Other oncogenes, such as C-myc and C-erb B-2, when amplified, are implicated in small cell lung cancer and breast cancer, respectively.
Mutations in tumor suppressor genes are another common cause of cancer. As you might expect, a tumor supressor gene is supposed to prevent tumors. But when these genes are damaged, they can allow cancer to develop instead of preventing it. One of these genes, p53, normally prevents cells with abnormal DNA from surviving. When p53 is defective, these cells with abnormal DNA survive and can multiply, increasing the probability of developing cancer.
Certain cancers are associated with chromosomal abnormalities. Chromosomes are located within the nucleus of our cells, and contain our genes. When genes are missing, duplicated, or rearranged, a predisposition to develop a tumor is increased. Certain leukemias, sarcomas, lymphomas, and others tumors are associated with chromosomal abnormalities.
There are also viruses associated with cancer. The human papillomavirus (HPV) that causes genital warts is associated with carcinoma of the cervix, and the Epstein-Barr virus that causes infectious mononucleosis, is associated with Burkitt's lymphoma. Diseases or drugs that affect the immune system can also increase the risk for certain cancers. The disease AIDS, for instance, is associated with a high risk of two types of cancer, namely, Kaposi's sarcoma and lymphoma.
Exposure to ionizing radiation can increase the risk of certain cancers. X-rays used to treat disorders such as acne or adenoid enlargement can increase the risk of certain types of leukemias and lymphomas.
Fortunately, there are also factors under our control that can increase the risk of getting cancer, and can therefore be avoided. There are substances called carcinogens (cancer-forming agents) that can increase the risk of getting cancer. Some common carcinogens include:
- Arsenic, asbestos, and nickel, which can cause lung and other cancers
- Benzene, which can cause leukemia
- Formaldehyde, which can cause nasal and nasopharyngeal cancer
and many others...
Carcinogens that are associated with a person's lifestyle include alcohol, which increases the risk of oral, esophageal, and oropharyngeal cancer, and tobacco, which causes lung, head and neck, esophageal, and bladder cancer. Smokeless or chewing tobacco can also increase the risk of oral cancer.
Unprotected exposure to sunlight (ultraviolet radiation) is associated with skin cancer. The main cancers caused by sunlight are basal cell carcinoma, squamous cell carcinoma, and malignant melanoma.
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