Annual mammography in women aged 40 to 59 years was no more effective than was physical examination or usual care for reducing breast cancer mortality when adjuvant therapy for breast cancer is readily available, according to 25-year findings from the Canadian National Breast Screening Study.
During a 5-year screening period beginning in 1980, 666 invasive breast cancers were detected in 44,925 women randomized to receive annual mammography and annual physical breast examinations, and 524 were detected in 44,910 women randomized to receive a single physical breast examination followed by usual care in the community. Of these, 180 and 171 in the mammography and control arms, respectively, died of breast cancer during up to 25 years of follow-up (mean of 22 years), Dr. Anthony B. Miller of the University of Toronto, and his colleagues reported.
The 25-year cumulative mortality from breast cancer detected during the screening period was similar between the women in the mammography arm and the control arm (hazard ratio, 1.05), the investigators reported Feb. 11 in BMJ.
After the screening period, an additional 2,584 breast cancers were detected in the mammography arm and 2,609 in the control arm during the follow-up period. Overall, 1,005 women died from breast cancer during follow-up, including 351 who were diagnosed during the screening period.
Of the breast cancers detected in the mammography arm during the initial screening period, 484 were screen detected, 176 were interval cancers, and 6 were lacking data. The mean size of cancers diagnosed in the mammography arm was 1.91 cm vs. 2.10 cm in the control arm, they said.
"In the mammography arm, 30.6% of cancers were node positive and 68.2% were palpable. In the control arm, 32.4% of the cancers were node positive and all were palpable ...on average, palpable cancer were larger than cancers that were detected only by mammography (2.1 cm v 1.4 cm) and were more likely to be node positive (34.7% vs. 16.5%)," the investigators wrote (BMJ 2014 Feb.11;348 [doi:10.1136/bmj.g366]).
Although 25-year survival was better for those with tumors less than 2 cm vs. greater than 2 cm, those with breast cancer detected in the mammography vs. the control arm, and for those in the mammography arm with nonpalpable vs. palpable tumors (HR, 0.46, 0.79, and 0.58, respectively), the overall 25-year cumulative mortality from breast cancer was similar between women in the mammography arm and control arm, (HR, 0.99), the investigators wrote.
The findings were nearly identical for those aged 40-49 years, and those aged 50-59 years, the investigators noted.
Notably, at the end of the screening period, an excess of 142 breast cancers were detected in the mammography group, compared with the control group, and at 15 years, that excess remained at 106, implying that 22% of the screen-detected invasive cancers in the mammography arm were overdiagnosed, meaning that the cancer might not otherwise have become clinically apparent during the patient’s lifetime.
"This represents one overdiagnosed breast cancer for every 424 women who received mammography screening in the trial. Assuming that nearly all overdiagnosed cancers in the Canadian National Breast Screening Study were nonpalpable, 50% of mammogram detected, nonpalpable cancers were overdiagnosed," they said.
The Canadian National Breast Screening Study was supported by the various Canadian health agencies and advocacy groups. Dr. Miller was supported in part by a national health scientist award from Health and Welfare Canada.