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What Is Cancer?
In the most basic terms, cancer refers to cells that grow out-of-control and invade other tissues. Cells become cancerous due to the accumulation of defects, or mutations, in their DNA. Certain inherited genetic defects (for example, BRCA1 and BRCA2 mutations) and infections can increase the risk of cancer. Environmental factors (for example, air pollution) and poor lifestyle choices -- such as smoking and heavy alcohol use -- can also damage DNA and lead to cancer.
Most of the time, cells are able to detect and repair DNA damage. If a cell is severely damaged and cannot repair itself it undergoes so-called programmed cell death or apoptosis. Cancer occurs when damaged cells grow, divide, and spread abnormally instead of self-destructing as they should.
Tumors: Benign vs. Malignant
A tumor is an abnormal mass of cells. Tumors can either be benign (non-cancerous) or malignant (cancerous). Benign tumors grow locally and do not spread. Malignant tumors have the ability to spread and invade other tissues. This process, which is a key feature of cancer, is known as metastasis.
Metastasis is the process whereby cancer cells break free from a tumor and travel to and invade other tissues in the body. Cancer cells metastasize to other sites via the lymphatic system and the bloodstream. Cancer cells from the original -- or primary -- tumor can travel to other sites such as the lungs, bones, liver, brain, and other areas. These metastatic tumors are "secondary cancers" because they arise from the primary tumor.
Metastatic cancer retains the name of the primary cancer. For example, bladder cancer that metastasizes to the liver is not liver cancer. It is called metastatic bladder cancer. Metastasis is significant because it helps determine the staging and treatment of cancer. Some types of metastatic cancer are curable, but many are not.
Certain genes control the life cycle – the growth, function, division, and death -- of a cell. When these genes are damaged, the balance between normal cell growth and death is lost. Cancer occurs due to DNA damage and out-of-control cell growth. The following is a partial list of factors known to damage DNA and increase the risk of cancer:
- Genetic mutations (for example, BRCA1 and BRCA2)
- Environmental exposure to UV radiation, air pollution
- Bacterial (H. pylori) and viral infections (Epstein-Barr, HPV, hepatitis B and C)
- Lifestyle choices (poor diet, inactivity, obesity, heavy alcohol use, smoking cigarettes and tobacco use, exposure to chemicals and toxins)
- Treatment with chemotherapy, radiation, or immunosuppressive drugs
Cancer Symptoms and Signs
There are more than 100 different types of cancer. Every cancer and every individual is unique. The symptoms and signs of cancer depend on the size and location of the cancer as well as the presence or absence of metastasis. Symptoms and signs such as fever, pain, fatigue, skin changes (redness, sores that won't heal, jaundice, darkening), and unintended weight loss or weight gain are not unique to cancer, but they often occur with cancer. More potential cancer signs and symptoms include the presence of a lump, difficulty swallowing, changes or difficulties with bowel or bladder function, persistent cough, hoarseness, and unexplained bleeding or discharge.
Types of Cancer
Cancer can occur anywhere in the body. Broadly, cancers are classified as either solid (for example, breast, lung, or prostate cancers) or liquid (blood cancers). Cancer is further classified according to the tissue in which it arises.
Carcinomas are cancers that occur in epithelial (lining) tissues in the body. They comprise 80% to 90% of all cancers. Most breast, lung, colon, skin, and prostate cancers are carcinomas. Sarcomas occur in connective tissue like the bones, cartilage, fat, blood vessels, and muscles. Myelomas are cancers that occur in plasma cells in the bone marrow. Leukemias are blood cancers of the bone marrow. Lymphomas are cancers of the immune system cells. Mixed cancers arise from more than one type of tissue.
Cancer is the second leading cause of death in the United States. The most common cancers diagnosed in the U.S. are those of the breast, prostate, lung, colon and rectum, and bladder. Cancers of the lung, colon and rectum, breast, and pancreas are responsible for the most deaths. The prognosis of different cancers is highly variable. Many cancers are curable with early detection and treatment. Cancers that are aggressive or diagnosed at a later stage may be more difficult to treat, and can even be life threatening.
Cancer staging is the process doctors use to classify cancer according to its size, location, and extent of spread. Staging helps doctors determine the prognosis and treatment for cancer. The TNM staging system classifies cancers according to:
- Tumor (T): Primary tumor size and/or extent
- Nodes (N): Spread of cancer to lymph nodes in the regional area of the primary tumor
- Metastasis (M): Spread of cancer to distant sites away from the primary tumor
Some cancers, including those of the brain, spinal cord, bone marrow (lymphoma), blood (leukemia), and female reproductive system, do not receive a TNM classification. Instead, these cancers are classified according to different staging systems.
The TNM classification of a cancer usually correlates to one of the following five stages.
- Stage 0: This refers to cancer that is "in situ," meaning that cancerous cells are confined to their site of origin. This type of cancer has not spread and is not invading other tissues.
- Stage I – Stage III: These higher stages of cancer correspond to larger tumors and/or greater extent of disease. Cancers in these stages may have spread beyond the site of origin to invade regional lymph nodes, tissues, or organs.
- Stage IV: This type of cancer has spread to distant lymph nodes, tissues, or organs in the body far away from the site of origin.
Lymph nodes are small structures that filter lymph -- the clear, watery fluid that contains infection-fighting blood cells -- throughout the body. Lymph nodes are an important part of the immune system. They remove viruses, bacteria, and other waste materials from the lymph. Lymph nodes become inflamed and enlarged when they are fighting an infection or disease process in the body. Groups of lymph nodes are found in the chest, groin, abdomen, neck, and under the arms.
Cancer that originates in the lymph nodes or other area of the lymphatic system is called lymphoma. Cancer that originates elsewhere in the body can spread to lymph nodes. The presence of cancer that has spread to the lymph nodes is significant because it may mean that the cancer is growing quickly and/or is more likely to spread to other sites. The presence of cancer in lymph nodes may affect prognosis and treatment decisions.
Doctors often remove lymph nodes during surgery to remove cancer to determine the extent of spread of the cancer. Removal of lymph nodes can affect the flow of lymph in the body. The removal of many lymph nodes can result in the build-up of lymph fluid, a condition known as lymphedema.
Cancer treatment is highly variable depending on the type and stage of a cancer as well as the overall health of the patient. The most common treatments for cancer are surgery, radiation, and chemotherapy. Other treatments include targeted/biological therapies, hematopoietic stem cell transplants, angiogenesis inhibitors, cryosurgery, and photodynamic therapy.
Every cancer treatment has potential risks, benefits, and side effects. The patient and his or her care team, which may include an internist or other specialist, surgeon, oncologist, radiation oncologist, and others, will help determine the best and most appropriate course of treatment.
Surgery is often performed to remove cancerous tumors. Surgery allows for the determination of the exact size of the tumor as well as the extent of spread and invasion into other nearby structures or lymph nodes – all-important factors in prognosis and treatment. Surgery is often combined with other cancer treatments, such as chemotherapy and/or radiation.
Sometimes, cancer cannot be entirely surgically removed because doing so would damage critical organs or tissues. In this case, "debulking" surgery is performed to remove as much of the tumor as is safely possible. Similarly, "palliative" surgery is performed in the cases of advanced cancer to reduce the effects (for example, pain or discomfort) of a cancerous tumor. Debulking and palliative surgeries are not curative, but they seek to minimize the effects of the cancer.
In other cases, surgery is preventive or prophylactic. A woman who has BRCA1 or BRCA2 gene mutations may elect to have a mastectomy to reduce the risk of developing breast cancer in the future. Reconstructive surgery can be performed to restore the look or function of part of the body after prior cancer surgery. Breast reconstruction after a mastectomy is an example of this kind of surgery.
Radiation is a very common cancer treatment. About 50% of all cancer patients will receive radiation treatment, which may be delivered before, during, or after surgery and/or chemotherapy. Radiation kills cancer cells by damaging their DNA. Unfortunately, radiation also damages the DNA of healthy cells. Radiation can be delivered externally -- where X-rays, gamma rays, or other high-energy particles are delivered to the affected area from outside the body -- or it can be delivered internally. Internal radiation therapy involves the placement of radioactive material inside the body near cancer cells. This is called brachytherapy.
Systemic radiation involves the administration of radioactive medication by mouth or intravenously. The radioactive material travels directly to the cancerous tissue. Radioactive iodine (I-131 for thyroid cancer) and strontium-89 (for bone cancer) are two examples of systemic radiation treatments.
Typically, external radiation is delivered 5 days a week over the course of 5 to 8 weeks. Other treatment regimens are sometimes used.
Chemotherapy, or "chemo," refers to more than 100 different medications used to treat cancer and other conditions. Ideally, chemotherapy cures cancer. If a cure is not possible, the goals of treatment may be to slow the growth of cancer, keep the cancer from spreading, and/or relieve cancer-associated symptoms (such as pain).
Depending on the type of chemotherapy prescribed, the medications may be given by mouth, injection, intravenously (IV), or topically. IV chemo may be delivered via a catheter or port, which is usually implanted in a blood vessel of the chest for the duration of the therapy. Sometimes chemo is delivered regionally, directly to the area that needs treatment. For example, intravesical therapy is used to infuse chemotherapy directly into the bladder for the treatment of bladder cancer.
The chemo regimen a patient receives depends upon the type and stage of the cancer, any prior cancer treatment, and the overall health of the patient. Chemo is usually administered in cycles over the course of days, weeks, or months, with rest periods in between.
Other Cancer Treatments
In addition to surgery, radiation, and chemotherapy, other therapies are used to treat cancer. These include:
- Targeted or biological therapies seek to treat cancer and boost the body's immune system while minimizing damage to normal, healthy cells. Monoclonal antibodies, immunomodulating drugs, vaccines, and cytokines are examples of targeted or biological therapies.
- Hematopoietic stem cell transplants involve the infusion of stem cells into a cancer patient after the bone marrow has been destroyed by high-dose chemo and/or radiation.
- Angiogenesis inhibitors are medications that inhibit the growth of new blood vessels that cancerous tumors need in order to grow.
- Cryosurgery involves the application of extreme cold to kill precancerous and cancerous cells.
- Photodynamic therapy (PDT) involves the application of laser energy of a specific wavelength to tissue that has been treated with a photosensitizing agent, a medication that makes cancerous tissue susceptible to destruction with laser treatment. Photodynamic therapy selectively destroys cancer cells while minimizing the damage to normal, healthy tissues nearby.
Ongoing cancer research continues to identify newer, less toxic, and more effective cancer treatments. Visit the National Cancer Institute (NCI) to see a list of ongoing clinical trails.
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