An Introduction to Cancer
For many families in southern Louisiana, cancer has become a common condition. Cancer strikes about one in every three Americans, so all of us have loved ones who have suffered from this disease. I very often receive calls from concerned friends and relatives who have been diagnosed with cancer or who have had a close friend or loved one diagnosed. The most common questions are "What is cancer?" and "How did I get it?"
WHAT IS CANCER?
In addition to performing their assigned jobs, cells are also good citizens. Cells respect the space of the other cells around them and support the healthiness of those cells. Occasionally, cells begin to grow in an uncontrolled fashion, causing many problems for the body. Cancer is a disease of uncontrolled cell growth (or proliferation). Cancer cells are no longer good citizens. For instance, a liver cell that becomes cancerous no longer does its job of detoxifying the body. In addition, cancer cells do not respect their neighboring cells and will crowd them out of existence. Cancer cells push normal cells out of the way and use up all of the nutrients in the body to fuel their own uncontrolled growth. A group of cancer cells in the body is often called a tumor (a swelling), but not all tumors are cancerous. This concept is illustrated in Figure 1. Tumors are classified as benign or malignant. Benign tumors typically do not threaten the life of the patient, although this is not always the case. Benign tumors that develop in the brain can cause serious health effects or even death because the brain is contained within the skull. As the benign tumor expands, the normal brain has no place to go to get out of the way and can be damaged by the expanding benign tumor. Malignant tumors (or cancer) are always serious and will often lead to death if not treated promptly. Malignant tumors often spread (or metastasize) to other organs.
Ultimately, it is this lack of respect for the body by cancer cells that dooms a person to death. To return to our example of liver cancer, the cancerous liver cells do not perform the job of the liver and destroy the normal liver cells around them by their lack of respect for those cells. In the end, the person is left without a functional liver and dies of liver failure.
Cancer is many different diseases. Cancer is not one disease,but literally hundreds of different diseases. This is of great practical importance to both physicians and patients, because different cancers are treated differently and have different outcomes for the patient. For instance, breast cancer is different from brain cancer: If caught early, breast cancer is a very treatable disease, and patients can look forward to a cure and a normal life expectancy. On the other hand, brain cancer is an extremely aggressive disease, and the outcome is usually poor, regardless of how early the cancer is detected. Furthermore, brain cancer itself is not one disease but many diseases, each with different potential outcomes and treatments. In fact, if not caught early, breast cancer very often spreads (metastasizes) to the brain, and the breast tumor grows in the brain. Ultimately, for a typically non-aggressive disease like breast cancer, it is the spread of the tumor to a vital organ like the brain that causes illness and death.
HOW DID I GET
Cancer forms when
genes within a normal cell are damaged
and mutated. Mutations in
DNA can occur for many reasons.
Cigarette smoke contains chemicals
that will damage DNA. Solar radiation
from the sun contains ultraviolet
(or UV) rays that will damage DNA.
In most instances, the DNA damage
will not lead to cancer or other
diseases, but in some cases the
damaged DNA does lead to cancer.
There are about 25,000 genes in
each human cell, but, in most cases,
a mutation within a gene will not
lead to the development of cancer.
It is only when mutations occur
in certain key genes that cancer
develops. These key genes can be
grouped into three classes:
Multiple genes are defective in cancers. Cancer does not occur from a single gene mutation in a single gene. Instead, the development of cancer involves multiple mutations within several key genes, including mutations in proto-oncogenes, tumor suppressor genes, and DNA repair genes (Figure 2). The process of accumulating mutations in several genes like this normally takes many years, and this is why cancer is more frequently seen in older individuals.
How key genes become defective. The answer to the question "How did I get cancer?" is very often difficult to address. For those who smoke cigarettes and develop lung cancer, the answer is clear: The chemicals in the cigarette smoke caused mutations in key genes in the cells of your lungs and led to the formation of lung cancer. If you had never smoked, or if you had quit smoking at an early age, you probably would not have developed lung cancer. However, for a young woman who exercises and eats correctly but who develops breast cancer at age 41, the question is much more difficult to answer in an acceptable fashion. We know that mutations occurred in key genes in the cells of her breast, but identifying the source of those mutations is often impossible. The newly diagnosed woman is likely to ask, "What did I do to cause this? What could I have done differently to avoid my cancer?" The answers to these questions remain largely unknown. In some instances, we know that having a mother or sister who developed breast cancer at a young age puts a woman at an elevated risk for also developing breast cancer at an early age. In this case, breast cancer is not only genetic, but there is also a hereditary component to the disease. For breast cancer, the best we can say at this time is this: If you are at elevated risk for breast cancer, talk to your doctor about setting up a screening program with routine examinations.
DOES THE ENVIRONMENT
CONTRIBUTE TO MY CHANCES OF DEVELOPING
Environmental risks for developing cancer. We predict that as many as 80% of cancers may be attributable to environmental risk factors. The major environmental risk factors are listed below:
Among these risk factors, the use of tobacco (cigarette, cigar, pipe, or smokeless tobacco), unhealthy diet (rich in fatty foods), and physical inactivity (leading to obesity) are more likely to increase a person's cancer risk than the very low levels of pollutants in food, drinking water, and air. However, the risk from these pollutants increases with larger concentrations and longer duration of exposure. For instance, significant increases in cancer risk are associated with workers that have been exposed to high concentrations of ionizing radiation, certain chemicals, metals, and other substances (see Table I, below). Fortunately, industrial pollution has not had a major effect on cancer incidence. These pollutants are present in the environment at very low levels, and it does not seem likely that they are a major contributor to total cancer incidence.
The major risks are easily controllable. Luckily, the environmental risk factors that most significantly contribute to cancer risk are those that we as individuals can do something about. The use of tobacco causes about 30% of all cancer deaths. It is by far the major identified cause of human cancer. When a person stops smoking, it substantially reduces the risk of cancer. After about 20 years of not smoking, the risk of lung cancer for an ex-smoker is about the same to that for a non-smoker and about ten times less than if smoking had continued. Diet is also a major risk factor for cancer that can be controlled. Obesity increases the risk of breast cancer (in post-menopausal women), colon cancer, endometrial cancer (uterine corpus cancer), prostate cancer, uterine cervical cancer, ovarian cancer, and cancer of the gallbladder. The relative risk of breast cancer in post-menopausal women is 50% higher for the obese. The relative risk for colon cancer in men is 40% higher for the obese. The relative risks of gallbladder and endometrial cancer are five times higher for obese individuals compared with individuals with a healthy weight. Other studies have also shown a possible association between obesity and cancers of the kidney, pancreas, rectum, esophagus, and liver.
Use of cellular phones is NOT an environmental risk for cancer. Finally, types of questions that are often asked today are: "Do cell phones cause cancer?" and "Can I get cancer from living underneath power lines?" Cell phones use radio waves, a very low-energy form of radiation, to transmit signals between caller and receiver. In addition, they generate very low-energy "power frequency" radiation, just as household appliances and electrical wires. Low-energy radiation like radio waves, power frequency radiation, radar, and microwaves, have not been proven to cause cancer. Some early studies suggested that these forms of radiation were associated with cancer, but most of the new, more extensive research in this area does not suggest an association. The answer to the question is "No": Cellular phones do not cause brain cancer and living under power lines does not cause leukemia or any other kind of cancer.
low-energy forms of radiation do
not cause cancer, exposure to high-energy
radiation, like X-rays and ultraviolet
radiation, is a cancer risk. For
this reason, a person's exposure
to ultraviolet rays from the sun
or from tanning booths (despite
how safe tanning booths claim to
be) should be limited to short exposures.
The X-rays a person receives in
the hospital or clinic when X-rays
are taken is not only a
short exposure but is also a
safe level of energy. For this reason,
there is virtually no risk of developing
cancer from medical X-rays.
HOW TO LEARN
Jay D. Hunt, III, Ph.D., is Associate Professor of Biochemistry and Molecular Biology, Stanley S. Scott Cancer Center, LSU Health Sciences Center at New Orleans and Adjunct Associate Professor of Biochemistry, Tulane University Health Sciences Center in New Orleans. His research involves finding the specific gene defects that lead to cancers.