Screening mammography starting at age 40: Still relevant
OVERDIAGNOSIS OCCURS BUT IS LIKELY UNCOMMON
Overdiagnosis of breast cancer is a possible drawback of screening mammography. Cancers may be detected that would not have become clinically apparent in a person’s lifetime20 or have affected ultimate prognosis,18 and so would not have needed to be treated.
Overdiagnosis from screening mammography usually refers to finding ductal carcinoma in situ (DCIS) on breast biopsy. Because no randomized controlled study has been done in which breast cancer was diagnosed and not treated, evidence of the danger from DCIS comes from retrospective reviews of 130 cases in which excised tissue initially interpreted as benign was actually cancerous. Over 10 to 30 years, 11% to 60% of these patients developed invasive breast cancer in the same quadrant from which tissue had been excised.21 This rate of cancer development could lead to underestimation of the invasive potential of DCIS because the patients studied all had low-grade DCIS; further, some of the baseline biopsies involved complete removal of the tumor, thereby preventing the development or progression of cancer.
All DCIS is not the same. An ongoing trial22 found a 5-year recurrence rate of 6.1% after surgery for low-grade or intermediate-grade DCIS, and 15% after surgery for high-grade DCIS. Swedish trials23 have shown that most women who die of “early” breast cancer have high-grade DCIS. These findings suggest that although screening mammography may result in overdiagnosis and overtreatment of low-grade DCIS, high-grade DCIS can be lethal and should be treated. Thus, overdiagnosis likely represents a small fraction of all breast cancers.
Most important, it is not yet possible to accurately predict the biologic behavior of an individual tumor. Current clinical practice is to treat patients with DCIS similar to the way we treat patients with early-stage breast cancer, as we cannot determine which types of DCIS may remain indolent and which ones may become invasive.
HOW FREQUENTLY SHOULD YOUNGER WOMEN BE SCREENED?
The frequency of screening mammography has been another area of controversy, but we believe that annual screening offers the greatest benefit, especially for younger women.
The optimum screening frequency depends on how fast breast cancer grows and spreads. Data suggest that tumors in younger women tend to be biologically aggressive and grow and spread more quickly, making the benefit of yearly mammography more dramatic for younger women. A model based on data from Swedish studies24–26 predicted that the mortality reduction from breast cancer in women ages 40 to 49 would be 36% with annual screening, 18% with screening every 2 years, and 4% with screening every 3 years. For women in their 50s, the model estimated a reduction of 46% for yearly mammography, and 39% and 34% for screening every 2 or 3 years, respectively.6
In a prospective cohort study of the Breast Cancer Surveillance Consortium,27 in women ages 40 to 49 with extremely dense breasts, screening every 2 years was associated with a higher risk of advanced-stage disease (IIb or higher) and large tumors (> 2 cm) than with annual screening. For women ages 50 to 74, screening every 2 years vs every year did not increase the odds of advanced-stage or larger tumors.
AN INFORMED DECISION
In agreement with the current recommendations from the American Cancer Society, the American College of Radiology, and the American Congress of Obstetricians and Gynecologists, we support starting breast cancer screening with mammography at age 40.
Not all cancers are visible on mammography (false negatives), as they may be masked by mammographically dense breast tissue. Women should be informed of the importance of seeking medical attention for breast symptoms, even if mammography is normal. We need to inform women of the benefits and risks of screening mammography, including the risk of false-positive results that could lead to additional imaging and anxiety, and the uncertainties related to the potential for overdiagnosis and overtreatment. This information, offered in an easily understandable format, can help the patient make an informed decision regarding screening mammography, based on her values and preferences.
