What is Cancer?

*terms will open in new windows for definitions

Cancer is an abnormal proliferation of cells which have uncontrolled growth. They have the ability to grow into adjacent tissue and to spread to distant parts of the body. A cell is the basic building block and the fundamental functioning unit of the body. A sphere about one fourth of an inch in diameter contains about 1,000,000,000 cells (10⁹). Specialized cells are organized into specialized functioning structures called organs. A cell is composed of a nucleus surrounded by the cytoplasm, all contained within a cell membrane. The nucleus contains the genetic material called the chromosomes.

There are 23 paired chromosomes in each human cell nucleus, one pair of which is special in that this pair determines the sex of the individual; XX is female and XY is male. A chromosome is composed of a long double stranded chain of molecules called nucleotides. The specific sequence of these molecules forms a code for the functioning of that chromosome. The chains are also referred to as DNA, deoxyribonucleic acid. It is the nature of cells to divide and increase their number. They do this by a process called mitosis. Some cancers are noted by their high mitotic count which is the number of mitoses counted in a microscopic specimen of the cancer.

Each pair of chromosomes is numbered and can be identified by special microscopic techniques. The genes are located on the chromosomes and are determined by a specific sequence of nucleotides on a segment of the chain. The genes determine all of each individual's characteristics. They also control cell division and the repair of chromosomal defects. A defect on chromosome 13 is thought to be responsible for some cancers that run in families.

Cancer is an abnormal proliferation of cells that divide uncontrollably. Normal cells divide only enough to replace those that are lost or to repair injuries, then stop dividing. Cancer cells have lost this restraint and keep dividing. The mass of cancer cells will eventually become large enough to cause problems and to become detectable. This will produce a lump, mass, or tumor that can be detected. The word oncology means the study of lumps.

The smallest cancer that can be detected by examination, x-ray or scan is a little less than one fourth inch in diameter. A cancer this size contains between 100,000,000 (10⁸) 1,000,000,000 (10⁹) cancer cells. If a cancer starts as a single cell and that cell begins to divide and if the volume of that mass of cancer cells doubles every 45 days, then it would take about 30 doublings for the mass of cancer cells to reach the size of a mass about one fourth inch in diameter. This could take several years. It would only take 10 more doublings to reach a size considered fatal. Forty doublings would be 1,000,000,000,000 (10¹²) cells or about one kilogram(2.2 pounds) of cancer. Of these forty doublings the first thirty occurred prior to the time of the cancer being detectable.

The mass of cancer cells has the ability to invade into adjacent tissue and to spread to distant sites (metastasize). This mass of cancer cells has to have a blood supply so it causes neighboring blood vessels to grow into it. These blood vessels are often faulty and break easily. Bleeding is one of the signs of cancer. They are also poor in carrying blood and oxygen so the mass of cancer cells is often relatively oxygen deficient. Cells deprived of oxygen die and decay, which is why some cancers have an odor. The term for this new blood vessel growth is angiogenesis. There are currently many research studies involved in trying to prevent angiogenesis.

There are several general categories of cancer. Cancers that arise from epithelial surfaces are called carcinomas. If the surface is a glandular type of surface, then they are called adenocarcinomas. Carcinomas and adenocarcinomas are the most common types of cancer. They include lung, breast, prostate, intestinal, skin, ovary, cervix, oral, kidney and bladder. Cancers that arise from supporting structures such as bone, muscle, cartilage, fat and fibrous tissue are called sarcomas. The third major category are the leukemias and lymphomas. These cancers arise from blood cell elements. Brain cancers, nerve cancers, melanomas, and certain testicular and ovarian cancers do not fall into any general category.

Carcinomas arise from the epithelial covering or epithelial lining of structures. Skin is an epithelial lining of the exposed parts of the body. The intestines are lined with a glandular type of epithelial lining. The word epithelium is synonymous with the words lining or covering. This major group of cancers arise from structures that are in contact with the environment. The type of epithelium on the exposed parts of the body is called squamous(squay-mus) epithelium. You can trace a continuous path from the squamous epithelium of the face to the squamous epithelium of the mouth, down the squamous epithelium of the throat and esophagus onto the glandular epithelium of the stomach, then through the small intestine which is also lined with a glandular type of epithelium to the large bowel, also glandular epithelium, and out the anus which is squamous epithelium. All of this is in contact with the external environment. A similar path can be traced from the breast through the nipple into a milk duct and finally into a milk gland.

Cancers can spread to other parts of the body. They can spread by direct extension and invasion into adjacent tissues or along the lining surfaces in the abdomen. Systemic spread throughout the body is by way of the blood stream or by way of the lymphatic vessels. Just as the arteries take blood from the heart to all areas of the body and the veins return it to the heart, there is also a network of lymphatic vessels that drain all areas of the body. The lymphatic vessels converge into regional sites containing nests of lymph nodes. The lymph nodes, or glands, filter the lymph fluid for infectious agents and mount an immune response against them. They are part of the immune system. Cancer can spread by way of the lymph vessels and clog up the lymph nodes and start to grow. Often the success or failure of being able to cure a cancer depends on whether the lymph nodes have cancer in them. When a carcinoma has spread to the regional lymph nodes, those nodes are said to contain metastatic cancer. This is much different than a lymph node cancer in which the cancer originates from the lymph cells of the node. These cancers are called lymphomas.

Cancers are thought to arise as a single cell. The location and type of that cell is the site of the primary cancer. Cancers are named for the primary site of origin, i.e., skin, colon, breast, brain, etc. If a cancer is found in the liver that originated in the colon it is called colon cancer metastatic to the liver, not a liver cancer. Liver cancers are those that originated from a liver cell in the liver. If a liver cancer spreads to the lung it is still liver cancer, not lung cancer. Some cancers have little potential to spread; others are very unpredictable and can spread even before the primary site can be determined. The diagnosis of cancer usually requires that a tissue specimen be removed (biopsy) and sent to a pathologist. The pathologist examines the specimen microscopically to determine if it is cancer and if so what kind of cancer, i.e., primary site and type. He should also indicate the grade. Most cancers are graded as to how much they look like the original tissue. If they look a lot like it then they are called well differentiated. If they do not look much at all like it, then poorly differentiated. Moderately differentiated cancers are of intermediate grade. In general, well differentiated cancers are expected to behave the best, poorly differentiated the worst. Grades are also sometimes designated by Grade I, II, and III.

Following diagnosis, the next step is staging. Stage refers to the extent of the cancer. Each cancer, by organ, has its own staging system. Staging can be designated by one of four stages: Stage I, II, III, or IV. Stage I usually indicates the size of the cancer and that it has not spread beyond the structure of origin; stage II that it has spread into adjacent tissue; stage III that it has spread beyond its own region; stage IV that it involves another organ or a distant site. The official staging system for accredited cancer registries is the TNM system. T indicates the size of the primary tumor. It is usually subdivided into four substages. N indicates the status of the regional lymph nodes. M if distant sites are involved. Stage is assigned by physical examination, x-rays, and sometimes by exploratory surgery. It all depends on what type of cancer is involved.

The important thing about staging is that it determines the appropriate treatment, provides a prognosis, and allows for comparison of treatment results between different treatments and between different institutions. Once a stage is assigned and treatment given the stage is never changed. If a stage I cancer of the cervix is treated and two years later a metastasis is found in the lung it is not now stage IV but remains a stage I, with recurrence to the lung.

Cancers are treated by several methods. Surgery to remove the cancer will be curative only if the cancer is in one place and that place can be removed safely with an adequate margin of normal tissue and if the potential for spread elsewhere is low. Radiation is given to treat an entire area. Chemotherapy is used if the entire body is at risk. Often treatments are combined. Treatment is determined by the type of cancer, the grade, stage, location and the biological behavior expected of that particular cancer. Age and overall physical health also need to be considered. The goal of treatment may not necessarily be for cure but for palliation. If the cancer is essentially incurable but is causing problems that can be remedied, then treatment for relief of symptoms is indicated. In this situation quality of life and not longevity is the goal. Treatment is sometimes given even when all the cancer is thought to have been removed. If there is a significant statistical risk that the cancer will reappear later, then prophylactic treatment is given. This is common in breast cancer where chemotherapy or hormonal therapy is given after surgery to try to prevent a recurrence.

Cancers cause problems because they grow and occupy space. They push on other structures and cause them to malfunction. They erode into other structures and bleed. Cancers have no nerves so they do not hurt. They can cause other structures to hurt because of the destruction or pressure on the normal structures. Cancers require energy to grow and rob the body of the energy it needs for normal function.

Prognosis depends on many factors. For any one individual the probability of being cured is either 0 or 100%. The prognosis for any one individual cannot be predicted, but if there were a large number of people all in the same situation then probabilities of cure, remission, survival and death can be predicted. Some cancers are very predictable, others are not. Some cancers that would be expected to be easily cured are not. Some people live with their incurable untreatable cancers for years. For most cancers, to be cured usually means that the cancer has not reappeared for five years after treatment.

William M. Rich, M.D.
Clinical Professor of Obstetrics and Gynecology
University of California, San Francisco
Fresno-Central San Joaquin Valley
Fresno, Ca.