Do screening mammograms cause spread of existing breast cancer?

Mammograms Do Not Spread Breast Cancer

Screening mammography does not cause spread of existing breast cancer—this is a myth without scientific basis. The compression and radiation used in mammography do not cause cancer cells to disseminate or metastasize.

Evidence Supporting Safety of Mammography

Mortality Reduction Demonstrates Safety

• Mammography is the only screening modality proven to decrease breast cancer mortality, with randomized controlled trials showing at least a 22% reduction in breast cancer deaths 1
• The Swedish Two-County trial demonstrated a 27-31% reduction in breast cancer mortality after 29 years of follow-up in women aged 40-74 years 1
• Observational studies show even larger mortality reductions of 40% in women who actually underwent screening 1, 2
• If mammography caused cancer spread, we would see increased mortality in screened populations—the opposite is observed 2

Screen-Detected Cancers Have Better Outcomes

• Screen-detected tumors are typically lower stage (smaller and more likely to be node-negative) compared to cancers detected by palpation, which decreases treatment morbidity 1
• This finding directly contradicts any theory that mammography spreads cancer—if compression caused spread, screen-detected cancers would be more advanced, not less 1
• Screening reduces the risk of being diagnosed with advanced breast cancer by 45% for tumors >2cm in women aged 40-49 years 2

Radiation Risk is Minimal and Does Not Cause Spread

• Modern digital mammography delivers a glandular dose of approximately 2.5 mGy per 2-view examination 3
• The theoretical risk is cancer induction over decades, not spread of existing cancer 3
• The ratio of radiation-induced incidence over baseline incidence is approximately 1.6‰ for biennial screening in women aged 50-74 years, which is negligible compared to screening benefits 3
• Multiple mammograms starting before age 37 showed no increased cancer risk after 8-26 years of follow-up (RR 1.12; 95% CI: 0.83-1.51) 4

Long-Term Follow-Up Confirms Safety

• The UK Age Trial with 23 years of follow-up showed a 25% reduction in mortality from breast cancers diagnosed during screening, with no evidence that mammography caused harm or spread 5
• There was no increase in other-cause mortality (RR 1.02; 95% CI: 0.97-1.07), which would be expected if mammography caused cancer spread or other serious harm 5
• Among elderly breast cancer survivors, surveillance mammography was associated with reduced breast cancer-specific mortality (OR: 0.80,95% CI: 0.70-0.92), not increased mortality that would occur if mammography spread cancer 6

Common Pitfalls to Avoid

Do not confuse overdiagnosis with cancer spread. Overdiagnosis refers to detection of cancers that would never have become clinically apparent during a woman's lifetime (estimated at 1-10% of screen-detected cancers), not spread caused by the screening procedure itself 1.

Do not confuse false-positive recalls with harm from spreading cancer. Approximately 10% of screening mammograms result in recall for additional imaging, but less than 2% lead to biopsy recommendations—these are diagnostic clarifications, not evidence of cancer spread 1.

Clinical Bottom Line

Mammography screening saves lives by detecting cancer earlier when it is more treatable, not by causing spread. Annual screening mammography for women aged 40-84 years decreases mortality by 40% (12 lives saved per 1,000 women screened) 1, 2. The American College of Radiology recommends annual screening beginning at age 40 years for average-risk women 1.