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So many different statistics are published about my cancer. Risk factor statistics make me feel guilty that I caused my cancer. Survival statistics terrify me! Can I beat the odds? How do I apply statistics to my own situation?
Christoph Maier, PhD, guest Pro:
To the average person, cancer statistics can be confusing and sometimes even alarming. They are often misinterpreted and misused. They are powerful tools for researchers, policy makers, and insurance companies, all of whom use statistics to make inferences about large, faceless groups of people. However, because statistics are derived from samples and because each person is very different from another in diet, age, general health, fitness level and so on, cancer statistics are only estimates. The study of large groups can make findings hard to apply to an individual person, because large groups average out the differences.
Survival Rates
Survival rates give the percentage of people with a disease who survive, after being diagnosed, over a fixed period of time (usually 5 years). According to the American Cancer Society, the 5-year survival rate for cervical cancer is about 71%. This says that 71% of all women who are diagnosed with cervical cancer will survive at least 5 years, unless they die from another cause. In other words, about 710 out of every 1000 women with cervical cancer will live more than 5 years after diagnosis. What does this mean for an individual person who has just been diagnosed with cervical cancer? Not much! This overall rate lumps everyone together in one group. Many factors influence the survival rate: age, overall health, the stage at which the cancer was diagnosed (since early detection radically improves the survival rate.)
Consider, for example, what happens with early detection. According to the American Cancer Society, the survival rate of invasive cervical cancer is close to 100%, if it is detected at the earliest stage. That of course is far better than 71%. Also keep these facts in mind:
Cancer treatment is improving from year to year. Since statisticians have to use data collected from earlier years to compute the survival rate, the survival rate includes many women who were treated with older, less-effective treatment methods. If you have been recently diagnosed, your survival chances are probably higher. In fact for ovarian cancer, the first year-survival rate steadily improved between 1975 and 2000 by an average of .4% per year. That is a significant 7.7% increase in the first-year survival rate from 1975 to 2000.
The longer you have survived since you were diagnosed, the higher your chances of surviving another year. Let’s modify an analogy from Patricia Kelly, Ph.D., given in her informative book Assess your True Risk of Breast Cancer, to illustrate this point. (The analogy also appears later in this article). Think of a busy street that a woman must cross every year. Let a successful trip across the street represent surviving another year after being diagnosed with cancer. With every successful trip across the street, the woman becomes more adept at safely crossing it. Her chance of dying each year thus decreases.
Using 1994-1998 data from the SEER Cancer Statistics Review, a woman who is diagnosed with invasive ovarian cancer has an 89% chance of surviving the first year. If she survives the first year, she has a 91% chance of surviving the second year. If she survived the second year, she has a 95% chance of surviving the third year.
Cancer statistics indicate that the odds are getting better all the time. But even if the odds are against you, you could be one of the women who beats the odds, so never give up hope!
Risk and Risk Factors
Risk gives the percentage of a specified group of people who will be diagnosed with a disease in a specified period of time. Risk must always be attached to a time frame.
Consider this example:
Approximately .25% of twenty-year-old women (that is, 1 out of every 400 women) will be diagnosed with endometrial cancer between the ages of 20 and 50--a 30-year time frame. So a 20-year-old woman has a .25% risk of being diagnosed with endometrial cancer in the next 30 years.
If the time frame is shortened, the risk decreases. Consider for instance what happens when you use a 20-year time frame instead of a 30-year time frame.
Approximately .06% of 20-year-old women (1 out of every 1667 women) will be diagnosed with endometrial cancer between the ages of 20 and 40. Therefore, a 20-year-old woman has only a .06% risk of being diagnosed with endometrial cancer in the next 20 years.
For a specified time frame, the risk will be lower for younger women than for older women. Age is therefore a risk factor for endometrial cancer.
Remember that the 30-year risk for 20-year-old women is .25%. The 30-year risk for 50-year-old women is 2.0%. The 30-year risk is therefore eight times greater for a 50-year-old woman than for a 20-year-old woman.
Clearly the bad news is that over a specified time period an older woman has a much higher risk. The good news is that as a woman gets older, her lifetime risk decreases.
Patricia Kelly, Ph.D., in her book entitled Assess your True Risk of Breast Cancer, explains it this way, “Think of a busy street that a woman must cross every year of her life. In a single trip (which here would represent a single year’s breast cancer risk) a younger woman, being more agile than an older woman, is less likely to be hit by a car. However, a younger woman also has more trips ahead of her in her lifetime than does an older woman, and so has a higher lifetime risk.”
Patricia Kelly uses breast cancer in her example, but the analogy applies to other cancers as well.
Risk Factors vs. Cause
Smoking is considered by many experts to be a risk factor for cervical cancer. Estimates vary considerably, but many studies indicate that smoking increases a woman’s risk of cervical cancer by at least 1.5 times.
Smoking is a risk factor for cervical cancer. If a woman can quit smoking, should she? Absolutely! Here is why. Consider two groups, each with ten thousand 20-year-old women. The two groups are identical except that in one group all ten thousand women smoke and in the other group all ten thousand women do not smoke. Suppose that in the next 30 years, about .28% of the ten thousand non-smokers (or about 28 non-smokers) will be diagnosed with cervical cancer. If we use a relative risk factor of 1.5, then the number of women among the ten thousand smokers will be approximately 1.5 times higher. That is, 1.5 times 28 non-smokers is approximately 42 smokers. So in the next 30 years, about .42% of the ten thousand smokers (or about 42 smokers) will be diagnosed with cervical cancer. So in the next 20 years, 14 more women in the smoking group will be diagnosed with cervical cancer than in the non-smoking group.
Does this mean that smoking caused the cervical cancer? Absolutely not! Risk factors do not cause cancer. In the group of ten thousand non-smoking women, about 28 women did not smoke but were still diagnosed with cervical cancer. It follows that some of the smokers would have been diagnosed with cervical cancer, even if they had not smoked.
Heredity is sometimes a risk factor for ovarian cancer, so doctors want to know whether there are other women in the family who have had ovarian cancer. But having a sister or mother (or both) with ovarian cancer does not necessarily mean that the family has a strong hereditary predisposition for ovarian cancer. In fact, hereditary ovarian cancer is rare (less than 10% of ovarian cancer cases, according to the National Cancer Institute.) If your mother is among the 1% of women between the ages of 50 and 80 who have ovarian cancer, it is most likely that her largest risk factor is simply her age, and not because ovarian cancer is a hereditary trait in your family.
Christoph Maier, PhD, teaches statistics at the Indiana University of Pennsylvania (IUP), where he is Assistant Professor of Mathematics. Previous to his position at IUP, Dr. Maier worked as a statistician at a pharmaceutical company.
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Risk, Risk Factors, And Survival Rates - What Do These Cancer Statistics Really Mean?