There are over 100 different types of cancer. It can affect almost any organ in the body. The most common forms of cancer in North America are lung cancer, breast cancer, and prostate cancer. There are 6 major categories of cancers:
- carcinomas are tumours that start in the exterior or interior linings of the internal organs (called epithelial tissue) and on the exterior surface of the body
- leukemias are cancers of the blood-forming tissues
- lymphomas are tumours that originate in the lymphatic system
- melanomas are cancers that start in the cells that form pigment in skin
- sarcomas are tumours that originate in connective tissue, such as muscle, bones, and cartilage
- mixed types are cancers with characteristics from more than one category
As someone's body grows, certain cells divide and multiply to create new tissue, while other cells (like muscle or nerve cells) do not divide and multiply. The body has specific genes called oncogenes that control the ability of cells to divide and grow. Genes called tumour suppression genes help prevent or repair genetic mutations that may lead to cancer. Cancer can occur when either the oncogenes are "turned on" when they aren't supposed to be, or the tumour suppression genes are "turned off" when they're supposed to be on. This results in excess growth in the form of tumours.
Cancer cells go through different stages as they divide and multiply to form a tumour. At first, normal cells divide faster than they should and the total number of cells increases. This is called hyperplasia. At the second stage, called dysplasia, the new cancer cells look misshapen. The cancer cells then form a growing ball of cells, called a primary tumour. The tumour begins to push and squash the cells around it. As the tumour grows bigger, it burrows and invades into surrounding cells – this process is called invasion. When cancerous cells spread into a blood vessel or a lymph node, they can travel in the blood or lymph fluid to other parts of the body where they start to divide once again. This process is called metastasis, which means that the cancer has spread to other areas of the body.
Cancer causes more fear than any other disease. However, many cancers can now be treated and put into remission. This means traces of cancer are no longer found in the body following treatment. For example people with prostate, thyroid, skin, uterine, or breast cancer have at least an 80% chance of being disease-free (without cancer) 5 years after being diagnosed with cancer, assuming the cancer was detected and treated at an early stage.
The exact cause of cancer is not known, but various factors are likely at play. Although genetic factors have been linked to certain types of cancers, less than 5% of cancers are associated with known inherited gene mutations. For example, two genes known to be associated with breast cancer are BRCA1 and BRCA2, but less than 5% of breast cancers are associated with these genes. Most forms of cancer are due to genetic mutations of cells that occur within a person's life as a result of environmental factors such as cigarette smoke or exposure to radiation. Exposure to the following environmental factors can cause cancer:
- tobacco smoking: Smoking causes lung cancer and is also associated with an increased risk for cancers of the mouth (oral cancers), larynx, esophagus, bladder, and cervix.
- chemicals: Exposure to industrial dyes, asbestos, and benzene is linked to cancer.
- ionizing radiation: A connection between ionizing radiation and cancer has been made, but the exact amount of radiation exposure that increases the risk of cancer is not known.
- viruses: Certain viruses, such as the human immunodeficiency virus (HIV, which causes AIDS) are associated with an increased risk of liver cancer, lymphomas, and sarcomas. The human papillomavirus (HPV, which causes venereal warts) is associated with an increased risk of oral, anal, and cervical cancer.
- sunlight: Prolonged exposure (e.g., sun tanning) causes skin damage and may result in skin cancer.
Symptoms and Complications
Cancer can cause many different types of symptoms depending on the type of cancer and what stage it's in. Cancer cells pressing on or invading surrounding cells can cause severe pain. Organs (like the liver or pancreas) that are being invaded by the cancer can't work properly. Some symptoms, called paraneoplastic syndromes, are caused not by the tumour itself but by chemicals or hormones produced by the tumour. The chemicals and hormones can cause an autoimmune reaction where the body produces antibodies against itself. They can also affect the normal functioning of organs or even kill healthy cells.
Some of the complications of cancer can be life-threatening. Cancer can cause fluid to fill the sacs surrounding the heart or lung, making it very hard to breathe. Cancer can also block the veins that return blood from the upper parts of the body to the heart. This causes the veins in the chest and neck to swell. Cancer can also press on the spinal cord or spinal cord nerves, causing pain or the loss of function of the nerve. The longer a nerve has been damaged, the less likely it will recover. Hypercalcemic (high calcium) syndrome occurs either when a cancer produces a hormone that dangerously raises the body's calcium levels or when cancer extensively invades the bones.
Making the Diagnosis
Cancer specialists called oncologists will evaluate symptoms, perform a physical examination, and order blood tests and X-rays. The only way to tell for sure whether cells are cancerous, however, is to take a tissue sample, called a biopsy. The cells are looked at under a microscope. Doctors can tell the type and stage of cancer based on the biopsy sample.
At the time of diagnosis, staging of the cancer is done to help determine the prognosis and type of treatment a patient should receive. Doctors basically classify the cancer according to a staging system called the "TNM system," which describes the size of the tumour and the extent of cancer spread. The choice of treatment will largely depend on the stage of the cancer.
Treatment and Prevention
Certain types of cancers can be prevented through lifestyle modifications – this is known as primary cancer prevention. Tobacco-related cancers (e.g., lung cancer) account for almost one-third of all fatal types of cancer; therefore, quitting smoking is key for preventing cancers of the mouth, throat, esophagus, and lungs. Avoiding and limiting sun exposure, and using adequate protection (SPF lotions and sunscreens) while in the sun, will reduce the risk for developing skin cancer. Diet is another important area for cancer prevention. A high-fat diet is associated with a higher risk for certain cancers (such as breast and prostate), while a high-fibre diet has long been thought to reduce the risk for colon cancer.
Surgery, radiation, chemotherapy, and, for some cancers, hormones or hormone-blocking medications are all used to treat cancer. The goal of cancer treatment is to kill cancerous cells while killing as few healthy cells as possible in the process.
Surgery is used to remove cancer cells that are packed together. Many cancers are treated with surgery. Surgeons will also remove normal cells around the cancerous cells or tumour to determine if the cancer has spread or not. Once the cancer has spread, it's very difficult to remove cancer cells with surgery.
Radiation is used to treat localized cancers. Radiation therapy can take a number of different forms. A beam of radiation can be aimed onto the skin near the site of the cancer. The radiation kills the cancer cells. Unfortunately, it also kills healthy cells. Newer radiation machines are getting better at focusing the radiation only on the cancerous cells, and not the normal cells. Radioactive particles can also be injected into the blood. The particles stick to cancerous cells, but not to normal cells. Sometimes, small radioactive particles are placed right into an organ next to the cancer, giving the cancerous cells a much higher dose of radiation than the normal cells.
Chemotherapy is treatment that uses anticancer medications. It's often used when the cancer has spread throughout several areas of the body. For many cancers, a combination of medications is used because it works better than just one medication. A complete response to chemotherapy is when all detectable cancer has disappeared. However, some cancer cells may still remain in the body and are undetected. As a result, the cancer may grow back after a period of remission. A partial response is when the cancer shrinks by 30% or more. Unfortunately, many cancers become immune to the anti-cancer medications over time. There are certain types of cancers (e.g., breast cancer) that are influenced by hormones; they can be treated with hormones or hormone-blocking medications to slow their growth.
Genetic engineering is being used to control the genes that turn cancer growth on and off and to control the enzymes that allow cancer cells to continually divide and grow. Cancer vaccines, antibodies combined with poisons, and chemicals that turn off blood vessel growth in cancers, are some of the newer developments being investigated in the battle against cancer.
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