Interpretation of Breast Cancer Incidence
The correct interpretation is D) 15 people out of 1000 population will develop breast cancer in one year—this represents an incidence rate of 15 per 1000 person-years (or 1500 per 100,000 per year). 1
Understanding Incidence vs. Prevalence
Incidence measures new cases developing over a specific time period in a defined population at risk, which is exactly what this study describes—15 new breast cancer cases among 1000 people followed for one year 1
This is not prevalence (option A suggesting "15% have breast cancer"), which would measure existing cases at a single point in time 2
The rate is expressed as 15 per 1000 person-years, which can also be written as 1500 per 100,000 women per year 1
Clinical Context: This is an Elevated Detection Rate
This observed rate of 15 per 1000 is substantially higher than average population incidence and likely represents a high-risk screening population or prevalent screen:
Average population incidence in Europe is approximately 1.1 per 1000 (109.9 per 100,000), which is more than 10-fold lower than the observed rate 1
The U.S. population incidence is approximately 1.24 per 1000 (124.2 per 100,000) 1
High-risk screening populations (such as BRCA mutation carriers or women with dense breasts) show cancer detection rates of 8-29 per 1000, which aligns with this observed rate of 15 per 1000 1
Why This Rate is Elevated
Several factors explain detection rates this high:
Prevalent screen effect: First-round screening detects both new cancers and pre-existing undiagnosed cancers, with subsequent screening rounds typically showing much lower rates (3.7-5.3 per 1000) 1
High-sensitivity screening modalities: Abbreviated MRI screening demonstrates detection rates of 15 per 1000 in women with dense breasts and elevated risk profiles 1
Enriched risk factors: The population likely includes women with dense breasts (relative risk 1.2-2.1), family history (19% of similar populations), or other risk factors 1
Common Pitfalls to Avoid
Do not confuse incidence with prevalence: The 15 cases are new diagnoses during the observation period, not the total number of women living with breast cancer 1
Do not assume this rate applies to average-risk populations: This is 10-15 times higher than typical population incidence and represents either high-risk women or a prevalent screening round 1
Do not extrapolate linearly: The rate of 15 per 1000 per year does not mean 150 per 1000 over 10 years, as the denominator changes as cases are diagnosed 1
Age matters significantly: Less than 5% of breast cancers occur before age 35, and approximately 25% occur before age 50, so age distribution of the cohort substantially affects observed rates 1, 3