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.