Example of Prostate Cancer Screening

Incidence and Mortality

The American Cancer Society estimated that, in 2004, 230,110 men would be diagnosed with prostate cancer; 29,900 men would die of this disease.20 The age-adjusted prostate cancer incidence in nine Surveillance, Epidemiology, and End Results (SEER) registries between 1996 and 2000 was about 173 per 100,000 men.21 The mortality during that period was about 33 per 100,000 men. The probability at birth of being diagnosed with prostate cancer by age 80 is about 14%; the probability at birth of dying of this disease by age 80 is about 1.26%.22 The difference between prostate cancer incidence and mortality is one of largest for any cancer; this difference increased greatly after PSA screening became widespread. This is a strong indication that at least some prostate cancers now detected by screening would never become clinically important.

The incidence of prostate cancer increased dramatically after the beginning of PSA screening in the late 1980s and then stabilized in the later 1990s. Mortality from prostate cancer decreased after about 1992, a total reduction of about 20% by 2000. Screening is one of several possible interpretations of this reduction in mortality.57

Screening Tests

The two most common screening tests for prostate cancer are prostate-specific antigen (PSA) and digital rectal exam (DRE). No well-conducted RCT of prostate cancer screening has been completed; two large studies are under way.

Because of the uncertainty about which prostate cancers are clinically important, the sensitivity and specificity of screening is difficult to determine. DRE detects fewer cancers than PSA. Various approaches have been suggested for increasing the sensitivity and specificity of screening, but whether these approaches improve detection of clinically important cancers and reduce detection of unimportant cancers is unknown.58

Rationale for Screening

Because of the absence of clear evidence that screening reduces mortality from prostate cancer, the rationale for screening is not established. However, many men are still being screened.59 Many believe that the ecologic evidence (showing a reduction in mortality after the start of PSA screening) justifies screening; others find that screening has a strong intuitive appeal.

A single well-conducted RCT compared radical prostatectomy and watchful waiting in men with clinically detected prostate cancer.60 After 8 years, fewer men in the prostatectomy group had died of prostate cancer [13.6% versus 7.1%; absolute difference, 6.6% (2.1%—11.1%)]. The groups did not differ in all-cause mortality. As the cancers in this study were more advanced than those usually detected by screening PSA, this study does not provide adequate evidence about the effectiveness of screening.

Harms of Screening

Two major sources of the harms of prostate cancer screening are false positives and overdiagnosis and overtreatment. False-positive tests are common. On the initial screening round, from 5% to 27% of men (depending on age) have a PSA greater than 4.0 (the traditional cut-point); about 30% of these men will have prostate cancer diagnosed by biopsy.57 A problem for men with a negative biopsy is that biopsies often miss some prostate cancers; thus, even a negative biopsy does not assure a man that he does not have cancer,61 and this uncertainty could increase anxiety.

The frequency of overdiagnosis and overtreatment of prostate cancer caused by screening is uncertain. Surveillance data show a large increase in the number of new cases of prostate cancer, with only a small absolute reduction in mortality, after the introduction of PSA screening in the late 1980s.

If, as seems likely, most of the new cases detected would never have been fatal, then more than half of screening-detected prostate cancers do not require major treatment.

Although a small percentage of prostate cancers have his-tologic characteristics that reliably predict either a very small or a very large malignant potential, most prostate cancers have intermediate histology, leaving us uncertain about the likely prognosis. Because of the inability to determine prognosis from clinical and histologic data, most men under age 70 years receive aggressive treatment: either radical prostatectomy or radiation therapy. These treatments have important adverse effects, including impotence and incontinence, for some 50% of men being treated.57 Thus, if there are a substantial number of men who do not need treatment but receive it, many of them will be harmed unnecessarily. The exact magnitude of this problem is uncertain, but it may be quite large.

Balance of Benefits and Harms

For screening for prostate cancer, the benefits are not clear whereas the harms are very clear. Thus, the net balance between the two is currently impossible to determine. Given this information, some men will choose to have screening while others will choose not to be screened. Several professional associations and expert groups recommend shared decision making (SDM), informing men of the pros and cons of screening and encouraging them to participate in the decision about whether to be screened.62-67

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