Screening Mammography Associated With Modest Decline in Breast Cancer Mortality

Roxanne Nelson

September 22, 2010

September 22, 2010 — Another study has found that screening mammography does reduce the mortality rate from breast cancer. However, the reduction was considerably smaller than was expected, and lower than that seen in previous studies.

The data come from Norway, and are reported in the September 23 issue of the New England Journal of Medicine.

Participation in the Norwegian breast cancer screening program was associated with a 10% reduction in the rate of death from breast cancer among women 50 to 69 years of age, said lead author Mette Kalager, MD, from Oslo University Hospital in Norway.

However, only part of this reduction can be attributed to the screening program, she said, because during the period that the study was conducted, Norway built multidisciplinary teams to treat breast cancer, which also had an impact.

"One third of the mortality reduction we observed in the 20-year period — 1986 to 2005 — can be associated with the screening program, while two thirds can be attributed to enhanced breast cancer awareness and improved diagnosis and treatment for breast cancer," she told Medscape Medical News.

Lower Than Previously Seen

Previous studies with a follow-up period of 10 years or less have shown a relative reduction in the rate of death from breast cancer (from 6.4% to 25.0%). The mortality reduction in the current study is also much lower than the 15% to 23% estimated by the US Preventive Services Task Force.

There are several possible explanations for these differences, explained Dr. Kalager. "It is quite plausible that today, the effect of increased breast cancer awareness and improved therapy have outweighed the effect of screening on reducing mortality from breast cancer."

"Thus, screening may be less important than it was 20 years ago," she added. "Further, our study is a population-based cohort study, and sometimes results from the randomized controlled trials are not reached in a population setting."

Even though the reduction in mortality was less than expected, Dr. Kalager emphasized that the screening program reduced death from breast cancer for women 50 to 69 years of age.

Importance of Optimized Care

Another surprise was that in women who were in the nonscreening age group (70 to 84 years), the reduction in breast cancer mortality (about 8%) was largely the same as in the screening group (women 50 to 69 years of age).

"This can be explained by treatment by the multidisciplinary teams of highly specialized radiologists, radiologic technologists, pathologists, surgeons, oncologists, and nurses that managed the care of the patients," Dr. Kalager said.

Thus, the 10% reduction we found in women in the screening age group "is attributed to both the mammograms and management by multidisciplinary teams," she said.

The program began in Norway in 2005, and all women 50 to 69 years of age received an invitation to undergo screening mammography every 2 years. Each county in Norway was required to establish multidisciplinary breast cancer management teams and breast units before enrolling in the national screening program, Dr. Kalager explained.

For women outside that age group (the nonscreening cohort), the change in mortality could be related only to the establishment of multidisciplinary teams, she said. "The importance of optimized patient care is often missed."

A Delicate Decision

In an accompanying editorial, H. Gilbert Welch, MD, MPH, writes that screening mammography has become one of "the most prominent measures of healthcare performance." Dr. Welch is from the Dartmouth Institute for Health Policy and Clinical Practice, Dartmouth Medical School, Lebanon, New Hampshire.

The decision about whether to undergo screening mammography is, in fact, a close call.

Healthcare "report cards" have focused on ensuring that all women undergo the procedure, he writes. However, in highlighting the fact that the mortality benefit is modest, the current study helps confirm "that the decision about whether to undergo screening mammography is, in fact, a close call."

"Many observers will argue that because it is a delicate decision — involving trade-offs among noncomparable outcomes — it must be left to informed individuals to decide," writes Dr. Welch.

However, there are those who will argue that physicians should continue to persuade their patients to be screened, and that the modest benefit is worth any associated harms, he adds.

"No one can argue that screening mammography is one of the most important services we provide in medicine," explains Dr. Welch. "But the time has come for it to stop being used as an indicator of the quality of our healthcare system."

The Norwegian Protocol

In the Norwegian study, 40,075 women received a diagnosis of breast cancer from 1986 to 2005. Of the 4791 women (12%) who died, 423 (9%) received their diagnosis after the screening program was introduced.

The study consisted of 4 groups of women: a screening group of women who lived in counties that had a screening program (1996 to 2005); a nonscreening group of women who lived in counties that did not have a screening program (1996 to 2005); and 2 historic groups from before the implementation of the screening program (1986 to 1995) that mirrored the screening and nonscreening groups.

The analyses showed that the death rate in the screening group was 18.1 per 100,000 person-years, compared with 25.3 per 100,000 person-years in the historic screening group, for a difference of 7.2 deaths per 100,000 person-years (rate ratio, 0.72; 95% confidence interval [CI], 0.63 - 0.81; P < .001). This amounted to a relative reduction of 28%.

In the nonscreening group, the mortality rate was 21.2 per 100,000 person-years, compared with 26.0 per 100,000 person-years in the historic nonscreening group. This amounted to a difference of 4.8 deaths per 100,000 person-years (rate ratio, 0.82; 95% CI, 0.71 - 0.93; P < .001), for a relative reduction of 18%.

Criticism from ACR

The American College of Radiology (ACR) has criticized the study, noting that there are "many problems with the article."

In a statement, Daniel B. Kopans, MD, chair of breast imaging at Massachusetts General Hospital in Boston, notes that although the study authors agree that screening for breast cancer saves lives, the issue is how many lives saved does it take to make screening "worthwhile."

The current study also suggests that most of the decline in breast cancer deaths is due to improvements in therapy, and the contribution from screening is lower. Dr. Kopans points out that "there are large published studies from Sweden and the Netherlands that disagree with these results and show that most of the decrease in deaths is due to screening and not therapy."

Although there are several reasons for the differences in results, a notable one is the extremely short follow-up period of the current study. He questions why the figures are not more up to date, being that they were drawn from a registry.

"It is clear that when screening programs begin, not everyone starts being screened on day 1, but one has to wonder why, if screening began in 1995, they only have an 'average of 2.2 years' of follow-up," he states. "The more important question is why did they stop at 2005? It is now 2010."

The ACR has launched its own interactive source of information — It will be launched this week, along with a series of public service announcements that will be broadcast on television and radio stations nationwide.

The purpose of this campaign, according to the ACR, is to "clear confusion, reduce unnecessary breast cancer deaths, and help women avoid extensive treatment for advanced cancers that went undetected because they did not get annual mammograms."

The study was supported by the Cancer Registry of Norway and the Research Council of Norway. The researchers have disclosed no relevant financial relationships.

N Engl J Med. 2010;363:1203-1210; 1276-1278.