Interval Breast Cancer: What a 6.5-Million–Mammogram Study Teaches Us About Missed Opportunities in Screening
Interval Breast Cancer (IBC) remains one of the most important quality markers in breast cancer screening programs. These are cancers diagnosed after a negative screening mammogram but before the next scheduled screen—and they represent the uncomfortable gray zone between biology, technology, and human interpretation.
A recent large-scale study from Taiwan, analyzing 6.5 million screening mammograms in 2.88 million women, provides one of the most comprehensive insights into why interval cancers happen and how we can reduce them.
1. Dense Breasts: Still the Biggest Blind Spot
The study confirms the firm association between breast density and the risk of IBC:
Extremely dense breasts (BI-RADS D): IBC risk = 1.15 per 1000 person-years
Fatty breasts (BI-RADS A): IBC risk = 0.29 per 1000 person-years
That’s nearly a 4X increase.
Dense breast tissue not only hides cancers but is also biologically more prone to aggressive tumors. For clinicians, this reinforces the need for:
Supplemental screening (US / MRI)
Shorter imaging intervals
Risk-stratified screening pathways
2. Family History Matters—Even After a Negative Screen
Women with a family history of breast cancer had:
1.8X higher risk of IBC
Higher cancer detection rates overall
This mirrors real-world clinical practice: these women often need personalized surveillance rather than waiting for biennial screening.
3. Radiologist Performance Is a Predictor of IBC Risk
This is the bold, practice-changing part of the study.
Taiwan’s national screening program uses an audit score based on:
Recall Rate (RR)
Positive Predictive Value (PPV1)
Interval cancer occurrence
Minimal cancer detection
Penalty points for poor performance
Radiologists with audit scores ≥5 had:
Higher interval cancer risk: 1.23 per 1000 person-years
Lower cancer detection rates
Higher proportion of missed cancers
Even more striking:
Radiologists with low recall and low CDR had the highest proportion of IBCs — 53.8%.
This indicates that ultra-low recall isn't a sign of excellence—it may be a sign of under-calling.
4. What Did the Missed Cancers Look Like?
In the reassessment of negative mammograms that later turned into IBC:
Asymmetry was the most common missed finding (22%)
Followed by masses and calcifications
Most 2019 negative screens were correctly classified, showing improvement
Asymmetry remains one of the trickiest findings in breast imaging—subtle, subjective, and easy to discount.
5. Takeaway: IBC Is Not Just Biology; It’s Also Quality
IBC rates reflect:
Tumor aggressiveness
Breast density
Patient risk
Radiologist performance
Screening program structure
This study highlights that IBC must be treated as a quality indicator, not an unavoidable phenomenon.
What Should Radiology Residents & Breast Imagers Take Away?
Know your recall rate and PPV—both too high and too low can harm patients.
Learn to recognize subtle asymmetries; compare with priors meticulously.
Don’t hesitate to recall when in doubt—undercalling is more dangerous than overcalling in population screening.
Encourage high-risk women to opt for shorter-interval or supplemental screening.
Think of your BI-RADS 1 or 2 as a clinical decision: negative doesn’t mean "no cancer"—it means "no actionable finding today."
The Future: Risk-Based, Not One-Size-Fits-All Screening
This mega-analysis paves the way for:
Personalized screening intervals
Radiologist performance auditing
Integrating breast density & family history into automated risk models
Expanding supplemental modalities—especially in Asian populations with high density prevalence
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