What does it mean to say a cancer is rare?
[Plugging in the US Census Bureau figure of 310,838,574 for the U.S. population, a type of cancer is considered rare in the U.S. if it affects less than 6.4 out of 10,000.]
Where we get into trouble is when we define "rare" using incidence for one geographic location and prevalence in another, because only prevalence is affected by life expectancy.
- Incidence = # patients who are newly diagnosed with same disease in a given year
- Prevalence = # patients who are living with same disease at a given moment in time
If life expectancy is long, the prevalence rises. For example, relatively few men are diagnosed with testicular cancer each year. But because the disease is frequently cured, the population of survivors of testicular cancer is sizeable, i.e., the prevalence is high.
If life expectancy is short, the prevalence is low. For instance, relatively many people are diagnosed with small cell lung cancer (SCLC). But because the disease is usually fatal fairly quickly, the population of SCLC survivors is small, i.e., the prevalance is low.
My next post will discuss why it matters.