Jeff Schein
More than one million individuals were told last year that they have cancer. Despite progress, about half this number will die of cancer this year at a rate slightly more than one per minute. Ranking second only to heart disease, cancer is the next most common cause of death in the United States.
The subject of cancer often comes up in newspaper and magazine articles. Even at dinner parties, it is not uncommon to hear "everything" causes cancer. While much knowledge about cancer remains to be discovered, scientists have uncovered many useful findings. Fortunately, everything does not cause cancer. We have evidence that certain agents in the environment are highly unlikely to be related to cancer while others are clearly linked to the disease. What`s more, many cancer-causing agents cause certain cancers, but not others. Asbestos, for example, has been linked to lung and gastrointestinal cancers, but not to brain, kidney or prostate cancer. In another example, survival rates for lung and pancreatic cancer are dim, while a relatively common form of skin cancer known as basal cell carcinoma is nearly 100 percent curable.
This is important because not all cancers are alike. In fact, scientists consider cancer a group of diseases. What causes one type of cancer may have no bearing at all on another type of cancer. Similarly, one medication may bring an excellent response in one type of cancer and no effect in another.
Available statistics record cancer rates for the general U.S. population. Certain subgroups may have a much higher rate of particular types of cancer. For example, blacks are twice as likely as whites to develop multiple myeloma, a rare form of cancer in which a type of plasma cell which produce antibodies (a key element of our immune system) becomes malignant. Another subgroup are certain workers; if cancer is found in people who work with certain chemicals such as lead or solvents, it is more likely to be a cancer of the kidney or the liver.
The problem with studies
While scientific studies are the foundation upon which medical decisions are made, one important caveat called confounders must always be remembered. The meaning of confound is to confuse or to perplex. Likewise, confounders confuse scientific findings. For example, in a study on the relationship between dietary fat and breast cancer, exercise may be a confounder. Women who eat less fat may be more health-conscious and, therefore, be more likely to exercise. As a result, the study may be designed in a way to ensure that women who eat low-fat diets exercise at a similar level as their counterparts who eat fat-rich meals. Many studies published in reputable medical journals control for confounders, but many do not.
A 1984 study investigating the link between activity level and colon cancer suggested that people with sedentary occupations had a 60 percent increased risk of developing colon cancer compared with people who were engaged in more physically demanding jobs.
When reading studies of this sort one must ask: How do the two groups differ? In this case, how do persons with sedentary occupations differ from their more active counterparts? Perhaps the inactive persons are engaging in more stressful occupations. Perhaps active people eat a diet rich in fiber or low in fat.
All of these differences must be taken into account before fair and appropriate comparisons can be made. In the exercise-colon cancer example, additional studies would be needed to show that exercise has a protective effect independent from that of diet. Scientists make comparisons fair by adjusting for various factors, such as age, socioeconomic class, sex, and many other items.
Age is important, for example, because if one were to compare the number of new cases (called the incidence) of cancer in Florida with those found in California (you may even conclude that Florida has been hit by a cancer epidemic). The problem with this comparison is that it has not been age-adjusted. Without doing so, we are largely seeing the effects of advanced age, not of cancer. After age-adjustment, the two incidences of cancer are very similar.
Epidemiology is a branch of medicine that deals with determinants or factors related to diseases. Cancer epidemiologists try to determine what factors are associated with particular types of cancer. For example, while alcohol intake has been linked to oral cancer, a cancer epidemiologist would ask and design an experiment to answer does the person who drinks alcohol have other lifestyle habits (such as cigarette smoking) that are associated with the development of oral cancer.
Chemicals and cancer
Many patients with cancer ask whether their condition could be related to a particular exposure. One person may have worked as a gasoline attendant, another in a chemistry lab. Some people ask, "Could my job have had something to do with my developing cancer?" Exposure to certain chemicals could have something to do with certain cancers. (Occupational exposure to certain cleaning fluids, for example [such as trichloroethane], has been linked to gastrointestinal cancers but not to lung, skin, or brain cancer.) Furthermore, linking a specific chemical to the development of cancer is complex; people who work with chemicals typically are exposed to several chemicals. So identifying the contribution of each individual chemical is no easy task.
One thing we do know with certainty is that cancer takes several years to develop. As a result, someone who develops cancer one or two years after starting a new job with an entirely new exposure, such as working with chemicals, is highly unlikely to have cancer from the exposure. It`s much more likely that some combination of factors (perhaps including an exposure to a certain chemical decades ago) has resulted in or contributed to this person developing cancer.
Putting risk in perspective
The latest figures from the National Cancer Institute are frightening: One in eight women will develop breast cancer. Not only is this a higher number, it is an increase over the one in nine women cited only two years ago.
It is important to note, though, that the one in eight risk is a lifetime risk. That is, it represents the risk of a woman developing breast cancer anytime between birth and death (living to age 90). The risk of breast cancer in any given year is substantially less. Likewise, the risk over a decade is also less than this often-cited figure. For example, a 40-year-old woman`s risk of developing breast cancer by age 50 is one out of 63.
Remember these are average risks. Women with other risk factors such as a family history of breast cancer, early onset of menstruation, and no children have a higher risk.
Although not fool proof, mammography is a good early detection tool. In many cases, mammography can detect a cancerous growth up to two years before a woman or a doctor can feel a small lump. In women over age 50, mammography is especially useful (younger women have denser breast than older women, making tumor detection more difficult). Surprisingly, a survey by the Jacobs Institute of Women`s Better Health (founded by the American College of Obstetricians and Gynecologists) found that at least one-quarter of women over age 50 have never had a mammogram.
Fortunately, the Mammogram Quality Standards Act went into effect October 1, 1994. This federal act regulates the quality of mammograms. All mammography centers will be certified and have staffs that meet certain training requirements. Centers that fail their requirements will not be able to receive reimbursement from Medicare or Medicaid.
In the meantime, to identify a high quality mammography facility, find out whether:
- The facility is accredited by the American College of Radiology.
- The equipment is calibrated each year by a radiation physicist.
- The radiation dose delivers no more than 0.4 rads per view.
- The radiologist who reads the mammography is certified by the American Board of Radiology.
Two types of prostate growth
The most common cancer in men is prostate cancer, which caused about 35,000 deaths in the United States in 1993.
Prostate cancer is totally different from another condition called benign prostatic hyperplasia, which is basically an enlarged prostate. Although this condition involves prostate growth, it is noncancerous growth. A trained physician (typically a urinologist) can often distinguish between benign prostatic hyperplasia and prostate cancer by performing a few tests. One test is simply feeling the prostate through the rectum. The prostate feels soft in benign prostatic hyperplasia; prostate cancer, however, is typically felt as a small hard lump.
Much research on the relationship between fat and cancer has focused on breast and colon cancer. We`ll discuss both of these.
The premise that a high fat diet can lead to breast cancer stems largely from two pieces of information: animal studies and human population studies. Rats fed a fat-rich diet developed more breast tumors than rats fed a low-fat diet. Clinicians are quick to point out, however, that applying the results of animals to humans is problematic. For instance, many animals when exposed to certain environmental agents (such as high-fat meals) develop certain outcomes that humans do not (and vice versa) simply due to differences in species. Differences in breast tissues is another limitation. Breast tumors in rats typically do not have the tendency to spread to other parts of the body; in humans, however, the phenomenon is all-too commonly observed.
Population studies compare cancer rates in different populations with disparate diets. Scientists have observed that breast cancer rates are lowest in the countries where dietary fat intake is lowest. For example, breast cancer is highest in countries such as the United States, Canada, and Sweden - all of which have high fat consumption. On the other hand, the countries with low-fat diets - Japan, China, and Hong Kong - all have much lower rates of breast cancer. Further evidence comes from migration studies that track cancer rates in people who immigrate from one country to another. The risk of breast cancer rises in women who have moved from countries who have low-fat diets to countries with higher-fat diets.
Population studies are often plagued by confounders. Many other determinants of disease or other factors associated with cancer are often missed in population studies. That`s not to say that these studies are not useful. Each study (if well designed), be it animal, population, migration, or clinical, adds to the weight of evidence implicating a particular environmental agent that may contribute to the development of cancer.
In regards to colon cancer, a study in the New England Journal of Medicine followed the dietary patterns of nearly 90,000 female nurses for six years; 150 of them developed colon cancer. The researchers found that the women who ate red meat daily were 2.5 percent more likely to develop cancer of the colon compared with the women who ate red meat infrequently.
Critics of the study noted that the frequency of daily eating red meat in the study is somewhat extreme, as is the infrequency of just once a month. In-between levels of intake would be more realistic. Another criticism is that women who eat high levels of red meat probably eat much less fiber (which may protect against colon cancer).
Once again migration studies have been used to support an association between high-fat diets and colon cancer. For example, as Japanese citizens who eat low-fat diets move to western countries (where the diets are typically high in fat), colon cancer rates have risen sharply.
While all the evidence is not yet in concerning fat and cancer, cutting back on fat seems to be good advice. Less fat means fewer calories, and the typical American can gain better health by reducing his or her calorie intake. Animal studies have consistently shown that reduced calorie intake even when fat intake is unchanged leads to fewer tumors. In humans, the evidence to eating less is also thought provoking: the American Cancer Society found that women who are 40 percent over ideal body weight are at greater risk of dying from cancer (up to a 55 percent increased risk) than their normal-weight counterparts. Obese men had up to 33 percent greater risk of dying from cancer compared with more slender men.
Cigarette smoke`s established risks
The U.S. Public Health Service has stated that cigarette smoking is responsible for one-third of all cancers. Smoking has been strongly associated with cancer of the lungs, larynx, mouth, and esophagus. A weaker association has been found between kidney, bladder, and pancreatic cancer.
Cigarette smoke is a complex mixture of more than 4,000 chemicals. Many of these chemicals have been shown to start a new cancer or promote an ongoing cancer in laboratory animals.
The effects of cigarette smoking extends far beyond cancer. Smoking can cause or exacerbate various respiratory disorders such as asthma, bronchitis, or emphysema. A 1989 report of the Surgeon General stated that 80 to 85 percent of deaths from chronic obstructive pulmonary disease - a general term for breathing disorders (such as chronic bronchitis and emphysema) - are attributed to cigarette smoking. Ulcers also occur more often in smokers than in non-smokers. Smoking can also alter the way that certain medications are handled in the body, either making them less effective or increasing the likelihood of side effects.
When interpreting numbers, it`s important to break down subpopulations. For example, the incidence of lung cancer overall has increased over the last few years.
But careful study of the numbers, however, shows that this increase is largely the reflection of increased lung cancer in women (26 percent) in the last three years. This increase in women both black and white is most likely due to an increase in cigarette smoking in this group.
Lung cancer in white men, on the other hand, has decreased since 1985; unfortunately, the same cannot be said of black men who still show an increase each year.
The age connection
One of the greatest risks for developing cancer is one often overlooked - simply growing old. More than half of all cancers occur in persons over the age of 65 (who make up only 14 percent of the population).
Several changes occur in the body as we age. Scientists speculate that several of these are related to cancer. Much evidence has been brought to suggest that two changes that occur as we age increase our vulnerability toward cancer. The first is the immune system, which as we grow older, is less effective at monitoring our body for foreign invaders and getting rid of them. The second change involves DNA, the genetic material responsible for inherited traits found in every cell. With each breath, new DNA is continuously being produced or copied from a master template. It is a natural process for a certain very small percentage of this new DNA to be erroneous, or what scientists call mutations. As we age, however, the ability to correct these mistakes decreases.
To illustrate this idea, think of a photocopy machine. You need to make 100 copies of a one-page document. Out of the 100 copies you made, one of them is blurry. The blurry copy is the DNA mistake. If you carefully sift through each and every copy as your body normally does when making copies of DNA you`ll find the mistake. However, as we grow older, our body`s ability to find the poor copy is diminished.
What to do?
Prevention has long been promulgated, and for good reason. If you work with chemicals, look into protective clothing and gear. If you smell noxious fumes, discuss this with the safety engineer, industrial hygienist and physician at your job.
This advice also goes for household projects. If you are using chemicals whether to refinish a table top or to polyurethane a floor make sure you have good ventilation (windows open, ceiling fan on).
Concerning diet, there does seem to be some connection between a high-fat diet and an increased risk of cancer. Whether this finding is related to people who are on fat-rich diets, eat less fiber, or are more sedentary is as yet unknown. Nevertheless, try to eat less fat and more fruits and vegetables. Try to increase your fiber intake and try to exercise more.
All of these measures are practical, common-sense advice based on what we currently know about cancer.
Jeff Schein is a medical writer based in Highland Park, New Jersey.
Cancer Screening: Is It Worth It?
Screening is a means of detecting cancer at an early stage. An effective screening test must:
- Test for a disease that has a high prevalence in the population.
- Be able to do follow-up tests to confirm a positive (meaning you have the disease) screening test.
- Be sensitive (too many false positives mean the test is erroneously classifying too many people as sick who are really well).
- Be specific (too many false negatives mean the test is erroneously classifying too many people as well who are really sick).
To illustrate, ovarian cancer is the fifth leading cause of cancer deaths in women, killing at least 13,000 women a year. Several screening tests have been developed. A common one known as CA-125 measures a protein in the bloodstream that is often produced by ovarian tumors. The test is not highly sensitive, so many women who actually do not have ovarian cancer are classified as having the disease. As a result, there is an increased chance of women receiving unnecessary surgery
Even more potentially dangerous is the high rate of false negatives: About 20 percent of women with ovarian cancer will not have elevated levels of the protein. As a result, these women (who have the cancer) will be missed by screening, classified as healthy. They probably will not be treated until the cancer has developed to a more advanced stage. (Fortunately, doctors can combine the CA-125 test with other tests, such as ultrasound, which aid in diagnosis.)
On the other hand, breast cancer screening is relatively sensitive and specific for all women over age 50. It screens for a disease with a high prevalence (about one out of every eight women will develop breast cancer in a lifetime).
These are better numbers compared with our example of ovarian cancer, which only one out of 70 women will develop. (However, the lifetime risk of developing ovarian cancer for women with a strong family history may approach one in two.)
Physicians can explain how sensitive and specific a test is. Next, find out whether a follow-up test can be performed to confirm the findings. Maintain good communication with the physician. Try to compile a family tree of health for your physician (and for your own knowledge). This involves identifying relatives who have been diagnosed with any diseases, including cancer. Since genetics play a major role in the development of cancer (as well as other diseases), communicating this information can be very important.
