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| Cancer Screening |
What tests can reduce deaths from malignant disease?
Alice Fisher
In our November/December 2007 issue, we discussed how to lower your cancer risk through good lifestyle habits such as limiting alcohol, eating a healthy diet and being physically active. Certain screening tests can help, too, by picking up precancerous abnormalities or the signs of early cancers still at a treatable stage. Other screening tests don’t have the science behind them to warrant widespread use. In allocating large sums of public-health dollars to population-based screening programs, authorities must be sure that reductions in disease burden and deaths merit the expenditure. Here are the Canadian Cancer Society’s current recommendations on screening for six cancers.
Effective Tests
Breast Cancer
Breast cancer deaths could be reduced by as much as one-third if women ages 50 to 69 were screened every two years with mammography. In this procedure, X-ray images of the compressed breasts are taken from the top and one side of each breast to generate a mammogram. This shows often subtle changes or abnormalities in breast tissue that may signal the presence of malignancy.
Current recommendations
Ages 40 to 49 Breast examination by a trained health-care professional at least every two years. Discuss your risk of breast cancer and the benefits and risks of
mammography with your doctor.
Ages 50 to 69 Breast examination by a trained health-care professional at least every two years. Mammogram every two years.
Age 70 and older Talk to your doctor about how often you should be screened for breast cancer.
Cervical cancer
Since the Pap smear was introduced about 50 years ago, the incidence and death rates for cervical cancer have
declined by about 50% and 60%, respectively. In the
future, the Pap smear, which takes cells from the neck
of the uterus and examines them for abnormalities,
may be combined with an even more sensitive test that detects the presence of genetic material from the human papillomavirus (HPV), the organism that can cause
cervical cancer. Comparing the two tests in more than 10,000 women, a team led by Dr. Marie-Hélène Mayrand, an assistant professor of obstetrics and gynecology at the University of Montreal, found that looking for viral DNA caught almost 95% of cancers or precancerous lesions. The Pap smear detected 55%.
Current recommendations
Women who are sexually active should have a Pap
smear and pelvic examination every one to three years, depending on provincial screening guidelines. Women who are no longer sexually active should discuss with their doctors how often to undergo screening. Vaccines now approved for use in Canada to prevent HPV
infection complement but do not replace screening.
Colorectal Cancer
Fecal occult blood testing (FOBT) can detect blood in the stool from precancerous polyps and early cancers. FOBT could reduce colorectal cancer mortality rates by 17% if 70% of Canadians ages 50 to 74 were tested every two years. Only three provinces — Ontario, Manitoba and Alberta — have announced organized screening programs, says Dr. Elizabeth McGregor, a research scientist with the Alberta Cancer Board in Calgary. There is no evidence from randomized trials that two more costly screening tests, colonoscopy and sigmoidoscopy, can reduce colorectal cancer deaths. These tests image the colon via a tube fitted with a tiny video camera.
Current recommendations
Men and women age 50 and older and at average risk should have FOBT at least every two years. A positive test could lead to a follow-up with sigmoidoscopy, colonoscopy or a double-contrast barium enema with an X-ray of the large intestine. Those at elevated risk (a parent or sibling who’s had colorectal cancer, a personal history of colorectal cancer, inflammatory bowel disease, inherited conditions or benign colorectal polyps) should discuss monitoring and surveillance with their doctors.
Biological Markers The burgeoning field of molecular markers — biological signs such as gene proteins, enzymes and antigens that suggest a greater susceptibility to cancer or exposure to carcinogens — could revolutionize cancer screening and prevention. “When you’re diagnosed with cancer, it hits you in the face. Wham, I’ve got cancer!” says Dr. Richard Gallagher, leader at the Cancer Control Research Program of the BC Cancer Agency in Vancouver. “But, in fact, the precursors have probably been with you for seven to 10 years.” We need to know how factors such as diet, exercise, sun, infections and other environmental exposures spur some precursors to progress to cancer, he adds. Already, two genetic markers for breast cancer — BRC1 and BRC2 — are used to test women at high risk. Researchers are on the verge of identifying more cancer genes and biomarkers. If you can identify these markers, you might somehow prevent full-blown malignant disease. Down the road, says Gallagher, “we might be able to test for these things and say, ‘Your blood markers indicate that you have a higher risk of eventually developing such and such a cancer, and here’s something that can help modify those markers.’” The hope is that cancer may become totally preventable. |
Controversial Tests
Prostate cancer
There is insufficient evidence to support across-the-board screening for prostate cancer with the prostate-specific antigen (PSA) test. The PSA test measures blood levels of this substance, which is produced by cells in the prostate gland. Levels can rise in the presence of cancer but also in the presence of benign enlargement
of the prostate. The test does not differentiate between aggressive cancers that require immediate treatment
and slower-growing tumours that do not.
“Prostate cancer has a slow clinical course for many men, and treatment can have a lot of morbidity, including impotence or incontinence,” says McGregor. “We have to make sure that a screening test is going to pick up the prostate cancers that really need to be treated.” Dr. Richard Gallagher, leader at the BC Cancer Agency’s Cancer Control Research Program in Vancouver, agrees. “The PSA test, even in combination with other markers, is not a foolproof indication of the presence of cancer or how aggressive a tumour is. Most men diagnosed with prostate cancer are not going to die of it.”
Other experts disagree, noting that in jurisdictions where PSA screening is common — such as Canada, the U.S., France and Austria — deaths from prostate cancer have declined, whereas they are rising in Sweden, Greece and Spain, where screening is not recommended.
Recommendations
All men age 50 and older should discuss with their doctors whether they need testing for prostate cancer with a digital rectal exam and a PSA test. Younger men who may be at a higher risk because of a family history (two first-degree relatives affected) or African ancestry should talk to their doctors about getting tested sooner.
Lung cancer
Researchers are looking into the feasibility and cost- effectiveness of using high-powered CT scans to detect early lung cancers. To date, though, there is insufficient evidence to warrant organized lung cancer screening because of the scans’ high false-positive rates. That means that the test often suggests that a malignancy is present when, in fact, it is not. Furthermore, there is no evidence that screening for this cancer saves lives.
No current recommendations.
Ovarian cancer
Screening with biomarkers — for example, the antigen CA-125 — has not reduced deaths from this malignancy. This protein is made by some ovarian cancer cells but also by other cancer cells and non-cancerous cells.
No current recommendations.
As for the future, the Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial is a massive study evaluating nearly 155,000 men and women ages 55 to 74 in the U.S. Its goal is to see to what extent screening can reduce deaths from these diseases. Together, these four cancers account for some 42% of all diagnosed cases of malignant disease. The study wrapped up last year, and preliminary results may be available soon. Perhaps data emerging from this trial will lend support for screening for prostate, lung and ovarian cancers.
No screening test is 100% accurate, but the good ones can reduce the number of cancer deaths and improve quality of life for those with malignancies by allowing for earlier and gentler treatment.
