Screening mammography may not decrease overall mortality in women ages 40-59

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1. A 5 year mammography screening program did not result in a difference in mortality compared to breast examination alone/usual care in women 40-59.

2. 22% of all mammography-detected invasive cancers were considered to be over-diagnoses.

Evidence Rating Level: 2 (Good)

Study Rundown: The use of mammography is a mainstay in breast cancer screening in women >50 years old. However, the exact magnitude of the benefit provided by breast cancer screening has been controversial. The authors of this article presented the results of a 25-year follow-up of a large randomized control trial, which compared 5-year mammography screening to control (breast examination/usual care). Women enrolled were ages 40-59, had no mammograms in the 12 months leading up to the study, had no history of breast cancer, and were not pregnant. In a cohort of over 89,000 participants, the trial demonstrated no difference in 25-year breast cancer specific or all-cause mortality between the mammography and control group. Given this result, the excess number of breast cancer diagnosis made by mammography was considered over-diagnosis of clinically insignificant pathology. Overall, 22% of all screen detected invasive cancers were categorized as over-diagnoses. The results of this study suggests a decreased value of annual mammography screening for women age 50-59 and may lead to diagnosis of cancer which may not be clinically relevant. The strength of this paper is in its high patient compliance as well as the long-term follow-up; however, there are some methodological short-comings that may affect the validity of the trial. In the mammography arm, only 32% of all breast cancer detected was via mammography only. Furthermore, the analysis is based on data from the Canadian National Breast Screening Study, which has been cited by some researchers as flawed for using poor quality mammography machines. In addition, all women received a clinical breast exam before randomization, which may have contaminated the groups so that patients with larger, more palpable tumors were assigned to mammography.

Click to read the study in BMJ

Click to read an accompanying editorial in BMJ

Relevant Reading: Overdiagnosis in screening mammography in Denmark: population based cohort study

In-Depth [randomized-controlled trial]: This study analyzed results from the Canadian National Breast Screening Study. 89,835 women were followed for incident breast cancer and survival for up to 25 years (mean time of follow-up: 21.9 years). Using a Cox proportional hazards model, there was no statistically significant difference found in the 25 year cumulative mortality from all causes between the mammography and control groups (1.02 95% CI: 0.98-1.06; p=0.28). There was also no statistically significant difference found in breast cancer specific mortality between the two groups (0.99, 95%CI: 0.88-1.12; p=0.87). There was a statistically significant difference in 25 year survival between mammography and control groups, although this was attributed to lead-time and length-time bias. At the end of the 5 year screening, 484 cases of invasive cancer were diagnosed by mammography alone. At 15 years follow-up, there was an excess of 106 diagnoses cases between the mammography and control group. Given similar survival, the authors translated the excess cases to a 22% (106/484) rate of over-diagnosis.

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