Understanding Breast Cancer Incidence: 15 Cases per 1000 Person-Years
The correct interpretation is that 15 out of 1000 women developed breast cancer during the observation period, which represents an incidence rate of 15 per 1000 person-years (or 1.5%), NOT 15%. 1
What This Number Actually Means
The incidence rate of 15 per 1000 person-years represents the number of new breast cancer cases detected during one year of observation in this population 1. This can also be expressed as:
- 15 per 1000 women per year
- 1500 per 100,000 women per year 1
- 1.5% annual incidence (not 15%)
Clinical Context: Why This Rate Is Elevated
This detection rate of 15 per 1000 is substantially higher than typical population incidence rates and likely reflects a screening context rather than true population incidence 1:
- Average population incidence in Europe: approximately 1.1 per 1000 (109.9 per 100,000) 2, 1
- U.S. population incidence: approximately 1.24 per 1000 (124.2 per 100,000) 1
The elevated rate of 15 per 1000 suggests this represents a cancer detection rate (CDR) in a screening program, particularly in an enriched risk population 1. This is consistent with:
- High-risk screening populations: CDRs of 8-29 per 1000 have been reported in BRCA mutation carriers and women with elevated risk 2, 1
- Abbreviated MRI screening: demonstrates CDRs of 15 per 1000 in women with dense breasts and variable risk profiles 2, 1
- Prevalent screening rounds: first-time screening detects both incident and pre-existing undiagnosed cancers, yielding higher detection rates than subsequent rounds 1
Important Caveats About This Detection Rate
Prevalent vs. Incident Screening
This 15 per 1000 rate likely represents a prevalent screen (first screening round) rather than steady-state incidence 1:
- First screening round: detects accumulated prevalent cases (10-15 per 1000) 2, 1
- Subsequent screening rounds: typically show lower CDRs of 3.7-5.3 per 1000 as prevalent cases are removed 1
Population Risk Factors
The American College of Radiology suggests this population likely has enriched risk factors 2, 1:
- Dense breasts: relative risk 1.2 for heterogeneously dense, 2.1 for extremely dense 2
- Family history: 19% of similar screening populations report first-degree relatives with breast cancer 2
- Age distribution: breast cancer incidence increases steeply with age, with approximately 25% of cases occurring before age 50 1
Reservoir of Undetected Disease
To sustain a detection rate of several cancers per 1000 women annually, there must be a substantial reservoir of undetected breast cancers growing in the population 3. Mathematical modeling demonstrates that for every 3 breast tumors reaching 2 cm, there are approximately 57 additional tumors (39 smaller invasive carcinomas and 18 DCIS) that remain undetected "below the surface" 3.
Comparison to Standard Screening Outcomes
For context, standard mammography screening in average-risk populations yields much lower detection rates 2:
- Digital breast tomosynthesis (DBT): 6 cancers per 1000 in women with dense breasts 2
- Screening ultrasound: incremental CDR of 3.3 per 1000 overall, 5.1 per 1000 in dense breasts 2
- Sestamibi MBI: incremental CDR of 7.7 per 1000 in women with dense breasts 2
The rate of 15 per 1000 is most consistent with high-sensitivity screening modalities (like MRI) in elevated-risk populations 2, 1.