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 far has it invaded into surrounding tissues?

2. Have cancer cells spread to nearby (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 number.

What Causes Cancer?

Cancer is often perceived as a disease that strikes for no apparent reason. This is because scientists don't know all the reasons. But many of the causes of cancer have already been identified. Besides heredity, scientific studies point to the existence of three main categories of factors that contribute to the development of cancer: chemicals (e.g., from smoking or diet), radiation, and viruses or bacteria.

Populations-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 vary 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 or environmental factors predominant in one location and not in the other.

Tobacco Use and Cancer

Among the various factors that can cause cancer, the prevalence of tobacco smoking represents the greatest public health hazard. Together cigarette smoke and tobacco contain more than sixty 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 tongue, lips, mouth, throat, esophagus, bladder, and pancreas.

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. The ability of ultraviolet radiation from the sun to cause cancer 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 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 radioisotopes. Because these two types of radiation are stronger than ultraviolet radiation, they can penetrate through clothing and skin and into the body. Therefore, high-strength radiation can cause cancers of internal body tissues. The cancer-causing ability of high-strength radiation has been shown in several instances. 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 divide and grow (proliferate). A prolonged period of time is usually required for these multiple steps. This means there can be a delay of several decades between exposure to a carcinogen and the onset of cancer. For example, a group of young people exposed to carcinogens from smoking cigarettes generally do not develop cancer for twenty to thirty years. This period between exposure and onset of disease is the lag time.


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.