Understanding Cancer.

What is Cancer?

Cancer is a renegade system of growth that originates within a patient's biosystem, more commonly known as the human body. There are many different types of cancers, but all share one hallmark characteristic: unchecked growth that progresses toward limitless expansion.

It is difficult to imagine anyone who has not heard of this illness. Most people have been affected because either they or their loved ones or friends are cancer survivors.

Because cancer is so prevalent, people have many questions about its biology, detection, diagnosis, possible causes, and strategies for prevention.

Different Kinds of Cancer

Cancer can originate almost anywhere in the body.

Carcinomas, the most common types of cancer, arise from the cells that cover external and internal body surfaces. Lung, breast, and colon are the most frequent cancers of this type in the United States.

Sarcomas are cancers arising from cells found in the supporting tissues of the body such as bone, cartilage, fat, connective tissue, and muscle.

Lymphomas are cancers that arise in the lymph nodes and tissues of the body's immune system.

Leukemias are cancers of the immature blood cells that grow in the bone marrow and tend to accumulate in large numbers in the bloodstream.

Naming Cancers

Scientists use a variety of technical names to distinguish the many different types of carcinomas, sarcomas, lymphomas, and leukemias. In general, these names are created by using different Latin prefixes that stand for the location where the cancer began its unchecked growth. For example, the prefix "osteo" means bone, so a cancer arising in bone is called an osteosarcoma. Similarly, the prefix "adeno" means gland, so a cancer of gland cells is called adenocarcinoma--for example, a breast adenocarcinoma.

Loss of Normal Growth Control

Cancer arises from a loss of normal growth control. In normal tissues, the rates of new cell growth and old cell death are kept in balance. In cancer, this balance is disrupted. This disruption can result from uncontrolled cell growth or loss of a cell's ability to undergo cell suicide by a process called"apoptosis." Apoptosis, or "cell suicide," is the mechanism by which old or damaged cells normally self-destruct.

Example of Normal Growth

To illustrate what is meant by normal growth control, consider the skin. The thin outermost layer of normal skin, called the epidermis, is roughly a dozen cells thick. Cells in the bottom row of this layer, called the basal layer, divide just fast enough to replenish cells that are continually being shed from the surface of the skin. Each time one of these basal cells divides, it produces two cells. One remains in the basal layer and retains the capacity to divide. The other migrates out of the basal layer and loses the capacity to divide. The number of dividing cells in the basal layer, therefore, stays the same.

The Beginning of Cancerous Growth

During the development of skin cancer, the normal balance between cell division and cell loss is disrupted. The basal cells now divide faster than is needed to replenish the cells being shed from the surface of the skin. Each time one of these basal cells divides, the two newly formed cells will often retain the capacity to divide, thereby leading to an increase in the total number of dividing cells.

Tumors (Neoplasms)

This gradual increase in the number of dividing cells creates a growing mass of tissue called a "tumor" or "neoplasm." If the rate of cell division is relatively rapid, and no "suicide" signals are in place to trigger cell death, the tumor will grow quickly in size; if the cells divide more slowly, tumor growth will be slower. But regardless of the growth rate, tumors ultimately increase in size because new cells are being produced in greater numbers than needed. As more and more of these dividing cells accumulate, the normal organization of the tissue gradually becomes disrupted.

Invasion and Metastasis

Cancers are capable of spreading throughout the body by two mechanisms: invasion and metastasis. Invasion refers to the direct migration and penetration by cancer cells into neighboring tissues. Metastasis refers to the ability of cancer cells to penetrate into lymphatic and blood vessels, circulate through the bloodstream, and then invade normal tissues elsewhere in the body.

Malignant versus Benign Tumors

Depending on whether or not they can spread by invasion and metastasis, tumors are classified as being either benign or malignant. Benign tumors are tumors that cannot spread by invasion or metastasis; hence, they only grow locally. Malignant tumors are tumors that are capable of spreading by invasion and metastasis. By definition, the term "cancer" applies only to malignant tumors.

Why Cancer is Potentially Dangerous

A malignant tumor, a "cancer," is a more serious health problem than a benign tumor because cancer cells can spread to distant parts of the body. For example, a melanoma (a cancer of pigmented cells) arising in the skin can have cells that enter the bloodstream and spread to distant organs such as the liver or brain. Cancer cells in the liver would be called metastatic melanoma, not liver cancer. Metastases share the name of the original ("primary") tumor. Melanoma cells growing in the brain or liver can disrupt the functions of these vital organs and so are potentially life threatening

Cancer Detection and Diagnosis

Detecting cancer early can affect the outcome of the disease for some cancers. When cancer is found, a doctor will determine what type it is and how fast it is growing. He or she will also determine whether cancer cells have invaded nearby healthy tissue or spread (metastasized) to other parts of the body. In some cases, finding cancer early may decrease a person's risk of dying from the cancer. For this reason, improving our methods for early detection is currently a high priority for cancer researchers.

Early Cancer May Not Have Any Symptoms

Some people visit the doctor only when they feel pain or when they notice changes like a lump in the breast or unusual bleeding or discharge. But don't wait until then to be checked because early cancer may not have any symptoms. That is why screening for some cancers is important, particularly as you get older. Screening methods are designed to check for cancer in people with no symptoms.

Cervical Cancer Screening

A screening technique called the Pap test (or Pap smear) allows early detection of cancer of the cervix, the narrow portion of the uterus that extends down into the upper part of the vagina. In this procedure, a doctor uses a small brush or wooden scraper to remove a sample of cells from the cervix and upper vagina. The cells are placed on a slide and sent to a laboratory, where a microscope is used to check for abnormalities. Since the 1930s, early detection using the Pap test has helped lower the death rate from cervical cancer more than 75 percent.

Should abnormalities be found, an additional test may be necessary. There are now 13 high-risk types of human papillomaviruses (HPV) recognized as the major causes of cervical cancer. The U.S. Food and Drug Administration has approved an HPV test that can identify their presence in a tissue sample. This test can detect the viruses even before there are any conclusive visible changes to the cervical cells.

Breast Cancer Screening

Breast cancer can sometimes be detected in its early stages using a mammogram, an X-ray of the breast. Mammography is most beneficial for women as they age and undergo menopause. Mammography is a screening tool that can detect the possible presence of an abnormal tissue mass. By itself, it is not accurate enough to provide definitive proof of either the presence or the absence of breast cancer. If a mammogram indicates the presence of an abnormality, further tests must be done to determine whether breast cancer actually is present.

Prostate and Ovarian Cancer Screening

The U.S. Food and Drug Administration has approved the PSA test along with a digital rectal exam to help detect prostate cancer in men age 50 and older . Doctors often use the PSA test and DRE as prostate cancer screening tests; together, these tests can help doctors detect prostate cancer in men who have no symptoms of the disease. Most men with an elevated PSA test, though, turn out not to have cancer; only 25 to 30 percent of men who have a biopsy due to elevated PSA levels actually have prostate cancer, so researchers are working hard to find new clues. Experts are trying to develop better blood tests that might alert people to malignancies while the cancers are still in their early stages. For example, several new blood tests for ovarian or prostate cancer are under development.

Colon Cancer Screening

A procedure called a fecal occult blood test (FOBT) detects invisible amounts of blood in the feces, a possible sign of several disorders, including colon cancer. The test is painless and can be done at home or in the doctor's office along with a rectal exam. With an application stick, a dab of a stool specimen is smeared on a chemically treated card, which is tested in a laboratory for evidence of blood. If blood is confirmed in the stool, more elaborate tests may be performed to find the source of the bleeding.

Some other options include sigmoidoscopy and colonoscopy. The former exam uses a lighted instrument called a sigmoidoscope to find precancerous or cancerous growths in the rectum and lower colon. The latter exam uses a lighted instrument called a colonoscope to find precancerous or cancerous growths throughout the colon, including the upper part.

Biopsy

To diagnose the presence of cancer, a doctor must look at a sample of the affected tissue under the microscope. Hence, when preliminary symptoms, Pap test, mammogram, PSA test, FOBT, or colonoscopy indicate the possible existence of cancer, a doctor must then perform a biopsy, which is the surgical removal of a small piece of tissue for microscopic examination. (For leukemias, a small blood sample serves the same purpose.) This microscopic examination will tell the doctor whether a tumor is actually present and, if so, whether it is malignant (i.e., cancer) or benign. In addition, microarrays may be used to determine which genes are turned on or off in the sample, or proteomic profiles may be collected for an analysis of protein activity. This information will help doctors to make a more accurate diagnosis and may even help to inform treatment planning.

Microscopic Appearance of Cancer Cells

Cancer tissue has a distinctive appearance under the microscope. Among the traits the doctor looks for are a large number of irregularly shaped dividing cells, variation in nuclear size and shape, variation in cell size and shape, loss of specialized cell features, loss of normal tissue organization, and a poorly defined tumor boundary.

Hyperplasia

Instead of finding a benign or malignant tumor, microscopic examination of a biopsy specimen will sometimes detect a condition called "hyperplasia." Hyperplasia refers to tissue growth based on an excessive rate of cell division, leading to a larger than usual number of cells. Nonetheless, cell structure and the orderly arrangement of cells within the tissue remain normal, and the process of hyperplasia is potentially reversible. Hyperplasia can be a normal tissue response to an irritating stimulus. An example of hyperplasia is a callus that may form on your hand when you first learn to swing a tennis racket or a golf club.

Dysplasia

In addition to hyperplasia, microscopic examination of a biopsy specimen can detect another type of noncancerous condition called "dysplasia." Dysplasia is an abnormal type of excessive cell proliferation characterized by loss of normal tissue arrangement and cell structure. Often such cells revert back to normal behavior, but occasionally they gradually become malignant. Because of their potential for becoming malignant, areas of dysplasia should be closely monitored by a health professional. Sometimes they need treatment.

Carcinoma in Situ

The most severe cases of dysplasia are sometimes referred to as "carcinoma in situ." In Latin, the term "in situ" means "in place," so carcinoma in situ refers to an uncontrolled growth of cells that remains in the original location. However, carcinoma in situ may develop into an invasive, metastatic malignancy and, therefore, is usually removed surgically, if possible.

Tumor Grading

Microscopic examination also provides information regarding the likely behavior of a tumor and its responsiveness to treatment. Cancers with highly abnormal cell appearance and large numbers of dividing cells tend to grow more quickly, spread to other organs more frequently, and be less responsive to therapy than cancers whose cells have a more normal appearance. Based on these differences in microscopic appearance, doctors assign a numerical "grade" to most cancers. In this grading system, a low number grade (grade I or II) refers to cancers with fewer cell abnormalities than those with higher numbers (grade III, IV)

Tumor Staging

After cancer has been diagnosed, doctors ask the following three questions to determine how far the disease has progressed:

1. How large is the tumor, and how deeply has it invaded surrounding tissues?
2. Have cancer cells spread to regional lymph nodes?
3. Has the cancer spread (metastasized) to other regions of the body?

Based on the answers to these questions, the cancer is assigned a "stage." A patient's chances for survival are better when cancer is detected at a lower stage.

What Causes Cancer?

Cancer is often perceived as a disease that strikes for no apparent reason. While scientists don't yet know all the reasons, many of the causes of cancer have already been identified. Besides intrinsic factors such as heredity, diet, and hormones, scientific studies point to key extrinsic factors that contribute to the cancer's development: chemicals (e.g., smoking), radiation, and viruses or bacteria.

Population-Based Studies

One way of identifying the various causes of cancer is by studying populations and behaviors. This approach compares cancer rates among various groups of people exposed to different factors or exhibiting different behaviors. A striking finding to emerge from population studies is that cancers arise with different frequencies in different areas of the world. For example, stomach cancer is especially frequent in Japan, colon cancer is prominent in the United States, and skin cancer is common in Australia. What is the reason for the high rates of specific kinds of cancer in certain countries?

Heredity? Behaviors? Other Factors?

In theory, differences in heredity or environmental risk factors might be responsible for the different cancer rates observed in different countries. Studies on people who have moved from one country to another suggest that exposure to risk factors for cancer varies by geographic location. For example, in Japan, the rate of colon cancer is lower, and the rate of stomach cancer is higher, than in the United States. But this difference has been found to gradually disappear in Japanese families that have moved to the United States. This suggests that the risk of developing the two kinds of cancer is not determined primarily by heredity. The change in risk for cancer for Japanese families could involve cultural, behavioral, or environmental factors predominant in one location and not in the other.

Tobacco Use and Cancer

Among the various factors that can cause cancer, tobacco smoking is the greatest public health hazard. Cigarette smoke contains more than two dozen different chemicals capable of causing cancer. Cigarette smoking is the main cause of lung cancer and contributes to many other kinds of cancer as well, including cancer of the mouth, larynx, esophagus, stomach, pancreas, kidney, and bladder. Current estimates suggest that smoking cigarettes is responsible for at least one out of every three cancer deaths, making it the largest single cause of death from cancer. Other forms of tobacco use also can cause cancer. For example, cigars, pipe smoke, and smokeless tobacco can cause cancers of the mouth.

Low-Strength Radiation

Some atoms give off radiation, which is energy that travels through space. Prolonged or repeated exposure to certain types of radiation can cause cancer. Cancer caused by the sun's ultraviolet radiation is most common in people who spend long hours in strong sunlight. Ultraviolet radiation from sunlight is a low-strength type of radiation. Effective ways to protect against ultraviolet radiation and to prevent skin cancer are to avoid going into strong, direct sunlight and to wear protective clothing. Sunscreen lotions reduce the risk of some forms of skin cancers.

High-Strength Radiation

Increased rates of cancer also have been detected in people exposed to high-strength forms of radiation such as X-rays or radiation emitted from unstable atoms called radioisotopes. Because these two types of radiation are stronger than ultraviolet radiation, they can penetrate through clothing and skin into the body. Therefore, high-strength radiation can cause cancers of internal body tissues. Examples include cancer caused by nuclear fallout from atomic explosions and cancers caused by excessive exposure to radioactive chemicals.

Lag Time

Chemicals and radiation that are capable of triggering the development of cancer are called "carcinogens." Carcinogens act through a multistep process that initiates a series of genetic alterations ("mutations") and stimulates cells to proliferate. A prolonged period of time is usually required for these multiple steps. There can be a delay of several decades between exposure to a carcinogen and the onset of cancer. For example, young people exposed to carcinogens from smoking cigarettes generally do not develop cancer for 20 to 30 years. This period between exposure and onset of disease is the lag time.

Viruses

In addition to chemicals and radiation, a few viruses also can trigger the development of cancer. In general, viruses are small infectious agents that cannot reproduce on their own, but instead enter into living cells and cause the infected cell to produce more copies of the virus. Like cells, viruses store their genetic instructions in large molecules called nucleic acids. In the case of cancer viruses, some of the viral genetic information carried in these nucleic acids is inserted into the chromosomes of the infected cell, and this causes the cell to become malignant.