It is the nature of cells to divide and increase their number in a process called mitosis. Normal cells divide to replace those lost, or to repair injuries only, then stop dividing.
Cancer is an abnormal, continuous multiplying of cells. The cells divide uncontrollably and may grow into adjacent tissue or spread to distant parts of the body. The mass of cancer cells eventually become large enough to produce lumps, masses, or tumors that can be detected. However, not all tumors are cancer. Tumors can be benign or malignant:
Are not cancer
Can usually be removed
Do not come back in most cases
Do not spread to other parts of the body, and the cells do not invade other tissues
Are rarely a threat to life
Can invade and damage nearby tissues and organs
Metastasize. Cancer cells break away from a malignant tumor and enter the bloodstream or lymphatic system to form secondary tumors in other parts of the body.
There are several general categories of cancer, with carcinomas and, more specifically, adenocarcinomas being the most common:
Carcinomas. Cancers that occur in epithelial surfaces; the cells that form the outer surface of the body to line or cover the body's cavities, tubes, and passageways.
Adenocarcinomas. Cancers that form on a glandular surface, such as the lung, breast, prostate, ovary, or kidney. Adenocarcinomas are considered a subtype (specific type) of carcinomas.
Sarcomas. Cancers that occur in supporting structures, such as bone, muscle, cartilage, fat, or fibrous tissue.
Leukemias. Cancers that occur in blood cell elements, such as bone marrow.
Lymphomas. Cancers that are found in cells of the immune system.
Brain cancers, nerve cancers, melanomas, and certain testicular and ovarian cancers do not fall into a general category.
Cancers begin in a single cell, and that cell is the site of the primary cancer. The cancer is named for the primary site of origin, such as skin, colon, or breast. For example:
When cancer that started in the colon is found in the liver, it is called colon cancer that has metastasized to the liver, not liver cancer.
Lung cancers are those that started from a lung cell. If it spreads to the brain, it is still lung cancer, not brain cancer.
When cancer spreads to the regional lymph nodes, those nodes are said to contain metastatic cancer. (Cancers that originate in the lymph cells of a node are called lymphomas.)
Cancer can spread from where it started to other parts of the body.
Spreading may occur by direct extension or invasion into nearby tissues.
Systemic spread throughout the body may occur by way of the cancer cell getting into and travelling through the:
Blood system. Arteries and veins take blood to and from all areas of the body.
Lymphatic system. A network of lymphatic vessels in all areas of the body that drain and filter infectious agents.
When cancer spreads to another part of the body, it is called metastatic cancer.