Patients with a life expectancy of less than 10 years derive little benefit from screening for breast or colorectal cancer, a meta-analysis of randomized trials suggested.

For every 1,000 women screened for breast cancer, almost 11 years would pass before one breast cancer death would be prevented. More than 10 years would pass before a single death from colorectal cancer would be prevented for every 1,000 persons screened, wrote Sei Lee, MD, of the University of California San Francisco, and colleagues in BMJ online.

Increasing the number screened to 5,000 reduced the intervals to 3 and 5 years for prevention of one death by breast or colorectal cancer, respectively, they added.

“Our results suggest that screening for breast and colorectal cancer is most appropriate for patients with a life expectancy greater than 10 years,” they wrote. “Incorporating time-lag estimates into screening guidelines would encourage a more explicit consideration of the risks and benefits of screening for breast and colorectal cancer.”

Clinical guidelines target screening for breast and colorectal cancer to healthy older individuals with a substantial life expectancy, a position backed by the rationale that screening does not provide immediate benefits.

The benefits of cancer screening come from early detection of asymptomatic cancers that would cause symptoms or death years later, according to the authors. As such, screening is associated with a “time lag to benefit.”

When life expectancy is shorter than the time lag, patients are exposed to immediate risks of screening, which has little chance of providing a benefit. However, the life expectancy required to benefit from screening for breast or colorectal cancer remains unclear.

To examine the issue of time lag to benefit, Lee and colleagues performed a survival meta-analysis of major clinical trials of screening mammography and fecal occult blood testing (FOBT). They excluded studies that targeted younger populations.

For screening mammography and FOBT, investigators calculated the number of years required to prevent a single cancer-related death with screening thresholds of 500 to 10,000 patients.

A review of multiple databases identified five mammography trials and four trials of FOBT suitable for meta-analysis.

The mammography trials involved 13,811 to 61,004 patients, and follow-up ranged from 10 to 20 years. Investigators limited their analysis to women ages 55 to 74. The primary outcome of all the trials was breast cancer mortality.

The colorectal cancer screening trials included 30,964 to 150,251 patients, ages 45 to 80, and follow-up ranged from 8 to 19 years. Patients younger than 50 were excluded from analysis.

The authors determined that 2.8 colorectal cancer deaths would be prevented after 5 years for every 10,000 patients screened by FOBT. With a screening threshold of 5,000 patients, the time-lag interval was 4.8 years to prevent a single death from colorectal cancer. The interval increased to 10.3 years per cancer prevented for a threshold of 1,000 patients.

The mammography analyses showed that 5.1 breast cancer deaths were prevented over 5 years for every 10,000 women screened, one death in 3 years for a screening threshold of 5,000 women, and one death prevented every 10.7 years for every 1,000 women screened.

The frequency of serious harm has been estimated at three in 10,000 for breast cancer screening and one in 1,000 for colorectal cancer screening, the authors wrote. As a result, an absolute risk reduction of one in 1,000 would be reasonable as the threshold wherein potential benefit probably outweighs potential risk.

“Therefore, patients with a life expectancy greater than 10 years should be encouraged to undergo screening for colorectal cancer and breast cancer,” they said. “Conversely, patients who life expectancy is less than 3 to 5 years…should be discouraged from screening, since the potential risks probably outweigh the small probability of benefit.”

“Between these extremes is an intermediate zone of small or unclear benefit, in which patient preferences and values should have the dominant role in deciding whether screening is appropriate,” they added.

The analysis had some limitations. All of the studies included multiple rounds of screening so the authors may have underestimated the true time lag to benefit for one screening test. Also, the study focused on cause specific mortality that could have been subject to ascertainment bias.

From: Medpage Today