Commentary

Screening mammography starting at age 40: Still relevant

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Screening mammography is not a perfect test, but it still plays an important role for women even in their 40s, when the incidence of breast cancer is low but the risk of a tumor being aggressive is especially high.

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SCREENING DETECTS CANCER EARLY

The goal of screening mammography is to reduce breast cancer deaths by detecting cancers early, when treatment is more effective and less harmful.

Mammography detects tumors when they are smaller: the median size of breast cancers found with high-quality, two-view screening mammography is 1.0 to 1.5 cm, whereas cancers found by palpation are 2.0 to 2.5 cm.1 In general, tumors found when they are smaller require less treatment, and patients are more likely to survive.

Moreover, about 10% of invasive cancers smaller than 1 cm have spread to lymph nodes at the time of detection, compared with 35% of those 2 cm in size and 60% of those 4 cm or larger. Women who have a positive lymph node at the time of diagnosis usually undergo more intensive treatment with chemotherapy and more radical surgery than those who do not. The 5-year disease-free survival rate is more than 98% for breast cancer with a tumor smaller than 2 cm that has not spread to lymph nodes (stage I), compared with 86% for stage II disease (tumors 2.1–5 cm or one to three positive axillary lymph nodes).2

The median size of breast cancers found by mammography is 1.0–1.5 cm; by palpation, 2.0–2.5 cm

Treating breast cancer early is also less expensive. In a study of women enrolled in a health maintenance organization in Pennsylvania, 14% of those not screened presented with advanced breast cancer (stage III or IV) compared with 2% who had been screened. The cumulative cost of treating advanced breast cancer was two to three times that of treating early breast cancer (stage 0 or I), not accounting for time lost away from work and family, in addition to pain and suffering.3

SCREENING SAVES LIVES

Multiple prospective, randomized controlled trials have been conducted to assess whether inviting women between ages 40 and 74 to undergo screening mammography reduces the rate of death from breast cancer.4,5 Such trials tend to underestimate the effect of screening because not all women invited to be screened actually are screened, and some in the control group may undergo screening on their own.6

The Canadian National Breast Screening Study (NBSS) had additional problems that underestimated the benefits of screening. The quality of mammography came under question, and an issue with randomization became evident after the first round of screening, as the group invited to be screened had an excess of women presenting with palpable lumps and advanced breast cancer.6–8 Despite these issues, a meta-analysis of randomized controlled trials of screening mammography, including the NBSS data, found a 15% reduction in deaths.9 When the NBSS data were excluded, the reduction was 24%.10

In 2009, the United States Preventive Services Task Force (USPSTF)11 recommended against mammographic screening for women ages 40 to 49. Using results from trials including the NBSS, they estimated that the number of women needed to be invited to screening to prevent one breast cancer death was:

  • 1,904 for ages 39 to 49
  • 1,339 for ages 50 to 59
  • 377 for ages 60 to 69.

But if the NBSS study were excluded, these results would be 950, 670, and 377, respectively.6

In a review on screening mammography, Feig12 points out that the USPSTF selected the number of women invited to be screened rather than the number that were actually screened to measure the absolute benefit of screening.

Hendrick and Helvie13 reported that the number of women who needed to be screened to prevent one cancer death was:

  • 746 for ages 40 to 49
  • 351 for ages 50 to 59
  • 253 for ages 60 to 69.

The benefit of screening, if analyzed by number of life years gained rather than number of deaths prevented, is even more favorable to younger women with longer life expectancy. The number needed to be screened per life year gained is:

  • 28 at ages 40 to 49
  • 17 at ages 50 to 59
  • 16 at ages 60 to 69.12

These data provide additional support for screening women starting at age 40.

Observational studies, which provide a better measure of effectiveness because only women who actually undergo routine mammography are compared with those who do not, also support this conclusion. An observational study in Sweden with 20 years of follow-up found that women of all ages who participated in screening had a 44% lower risk of death from breast cancer than with those who were not screened; for women in their 40s, the risk reduction was 48%.14 Similarly, an observational study conducted in British Columbia15 found a 40% decrease in deaths in women screened annually between ages 40 and 79, and a 39% decrease in deaths in women first screened between ages 40 and 49.

LOW RATE OF FALSE-POSITIVE RESULTS

Like many screening programs, screening mammography does not benefit all women equally.

Only about 1% of the women screened underwent an unnecessary biopsy

False-positive results occur, for which women need additional imaging or a biopsy for findings that turn out not to be cancer. But the false-positive rate is not high: for every 1,000 women screened in the United States, 80 to 100 (10% or less) are recalled for additional evaluation, 15 (1.5%) undergo biopsy, and 2 to 5 have a cancer, so only about 1% of the women screened underwent an unnecessary biopsy.16

False-positive test results can provoke unnecessary anxiety, but evidence indicates that this tends to be a temporary effect, and even women who had a false-positive result tend to support mammography. In a report by Lerman et al,17 when mood was assessed 3 months after mammography, worry was reported by 26% of women who had had a false-positive report, compared with 9% of women who had had a normal mammogram. Another report addressing the consequences of false-positive mammograms found that although short-term anxiety increased, long-term anxiety did not.18­ In a random telephone survey, 98% of adults who reported having had a false-positive cancer screening result stated that they were nevertheless glad that they had undergone screening.19

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