Value of prostate cancer screening
By John Messmer, M.D.
Penn State Family and Community Medicine
Penn State Milton S. Hershey Medical Center
Penn State College of Medicine
It's the second most frequently diagnosed cancer in men and the second leading cause of cancer death among men after lung cancer. So, one might think it makes sense to screen for it. Screening tests are well established for other cancers: colonoscopy for colon cancer, mammography for breast cancer, Pap smears for cervical cancer. Why not for prostate cancer?
When a test is done because of a symptom or physical finding, it is a diagnostic test, not a screening test. Screening is a way of finding a problem when no symptoms or other abnormalities exist. A screening test for prostate cancer should be fairly sensitive, that is, it should find all the cancers it is looking for. It should also be fairly specific, that is, when it is positive, it is because of the disease it is screening for, not because of some other problem.
Screening tests are only good for certain groups. If a disease only occurs in women, it makes no sense to screen men for it. Similarly, if detecting and treating the problem will only add ten years of life, it may be inappropriate for people with a life expectancy of less than ten years. If the disease diagnosed by screening is mild and the treatment results in a high rate of unpleasant side effects, perhaps screening should be avoided.
More men over age 50 are screened for prostate cancer than for colon cancer according to a 2003 article in the Journal of the American Medical Association. That's interesting, because colon cancer screening is more likely to save lives than prostate cancer screening. Of course, screening for prostate cancer is easier - a blood test and rectal examination vs. laxative preparation, sedation and special equipment for colonoscopy. Nonetheless, men of all ages over 50 are more likely to have had prostate screening within the preceding year than colonoscopy within five years.
Proponents of prostate cancer screening might point out there is an increase in the diagnosis of prostate cancers due to screening and increased survival with early diagnosis. But the counter argument is that this represents biases based on lead time and length. That is, increased amounts of screenings will find tumors earlier, but these will include a number of non-aggressive tumors that would never have caused a problem for the men with them. Screening found them, but they were never a threat to begin with or the men would have died of some other problem before the prostate cancer affected them.
If early diagnosis results in cure, should we really care whether or not the cancer would have lead to death? Isn't it better just to have the cancer out? Maybe not, since this is one disease for which the treatment might worse than the disease in some cases. Some of the medications used can increase the risk of heart disease, a more common disease in men than prostate cancer. The treatments can result in impotence and urine leakage after surgical treatment and pain in the bladder or rectum from radiation. If screening is supposed to help, it should result in increased quality of life.
Prostate cancer screening commonly means a blood test for prostate specific antigen (PSA) and might include a digital rectal examination (DRE) of the prostate. PSA is a chemical made only by the prostate which serves to liquefy the semen after ejaculation. A small amount is found in the blood normally, but elevated levels are found in prostate cancer and in a number of non-malignant prostate conditions, most usually non-malignant prostatic hypertrophy or enlargement that comes with age. DRE can find larger tumors, but tends to miss tiny ones. However, it is useful when the PSA is only slightly elevated. If DRE shows the prostate to be very large, the PSA may be high because of the increased size of the prostate.
Unfortunately, low levels of PSA do not guarantee the absence of prostate cancer and high levels do not mean cancer is definitely present. Much research is ongoing to determine if the rate of increase in PSA is diagnostic or if the amount of PSA compared to the size of the prostate is more useful. Some PSA is bound to blood proteins and some is not. The latter is called "free" or "unbound" PSA. Lower levels of "free" PSA may be more indicative of cancer, but there is currently insufficient data to say that with certainty.
Since the scientific data is not able to show that prostate screening is always beneficial, recommendations on screening are different from different groups. The US Public Health Service Task Force makes no recommendation on DRE and says there is insufficient evidence to recommend PSA testing. The American Cancer Society and American Urologic Association recommend DRE annually for men over 40 and PSA annually for men over 50 after being informed of the risks vs. the benefits. The Canadian Task Force on Prevention Health Care says there is poor evidence to do DRE at all and it recommends against doing PSA on anyone.
So, what's a guy to do? There is no single answer applicable to all men. Higher risk individuals include African-American and Hispanics, obese men, and men with a family history of prostate cancer under age 60, so those men might benefit more from screening starting at age 40 or so. Other men can be informed of the risks and benefits of screening and make individual decisions. Since prostate cancer in men under age 70 tends to be more aggressive, PSA may be useful in this age group as long as their life expectancy is more than ten years. After age 70 it is even less clear, since prostate cancer is less likely to be a cause of death. Otherwise healthy men with longer life expectancies may be helped by screening, but the anxiety of having to deal with a borderline PSA may create unnecessary unhappiness.
It's a good idea to have a personal clinician to help with the decision to screen or not. Everyone's circumstances are different so having someone to help sort the ifs, ands, and buts can make it easier. For men without a personal clinician, a number of hospitals participate in prostate cancer screenings during September which is prostate cancer awareness month. Regardless where screening is performed, men must ask what the results mean to them as individuals and whether it is even appropriate to screen them at all.