What is the interpretation of an incidence rate of 15 breast cancer diagnoses per 1000 individuals per year, detected by mammogram?

Interpretation of Incidence Rate: 15 per 1000 per Year

The incidence rate of 15 breast cancer cases per 1000 women per year (or 1.5%) represents the number of new breast cancer diagnoses detected by mammography screening during the one-year follow-up period in this population.

Understanding the Calculation

• Incidence rate measures the occurrence of new disease cases in a defined population over a specified time period 1
• In this scenario: 15 new cases ÷ 1000 women = 15 per 1000 per year = 1.5% annual incidence 1
• This can also be expressed as 1500 per 100,000 women per year, which is the standard epidemiological format 1

Clinical Context and Comparison

This observed rate of 15 per 1000 is substantially higher than typical population-based breast cancer incidence rates:

• The age-adjusted annual incidence in Europe (2012) was approximately 94.2 per 100,000 (or 0.94 per 1000) 1
• The crude incidence in the European Union was reported as 109.9 per 100,000 (or 1.1 per 1000) 1
• The U.S. incidence in 2003 was 124.2 per 100,000 (or 1.24 per 1000) 1

The elevated detection rate of 15 per 1000 in this study likely reflects:

• Cancer detection rate (CDR) in a screening context rather than true population incidence 1
• This aligns with mammographic screening CDRs reported in elevated-risk populations, where MRI screening shows CDRs of 8-29 per 1000 1
• Abbreviated MRI screening demonstrates CDRs of 15 per 1000 in women with dense breasts and variable risk profiles 1

Important Caveats

This rate should not be confused with baseline population incidence:

• The 15 per 1000 likely represents a prevalent screen (first screening round) that detects both incident and pre-existing undiagnosed cancers 1
• Subsequent screening rounds typically show lower CDRs (3.7-5.3 per 1000) as prevalent cases are removed 1
• The population studied may have enriched risk factors (dense breasts, family history, or other risk factors) that elevate detection rates above average-risk populations 1

The study population characteristics matter significantly:

• Age distribution affects incidence, with steep age gradients showing <5% of cases before age 35 and approximately 25% before age 50 1
• Breast density and other risk factors substantially modify detection rates 1
• The 10-year risk for breast cancer varies from 1 in 69 at age 40 to 1 in 29 at age 60 1