Nice article in the New York Times about the "overdiagnosis" problem in cancer screening. The particular case is thyroid cancer in South Korea.
There are a number of things about any form of screening tests that one should always bear in mind:
- Death rate is measured as the number of deaths divided by the number of people with the disease. The latter number increases with better diagnosis techniques.
- Better diagnosis techniques for cancers inevitably identify tiny tumors, almost all of which will never develop into cancer during anyone's lifetime. Peggy Orenstein had a fabulous article a year ago about the same issue in breast cancer. In that case, those tiny tumors weren't even called cancer until the diagnosis movement sprang alive.
- Once a tumor is labeled cancerous, patients will opt to fix it. Because these tumors would never have killed the patients, they inflate the number of diagnosed cases without increasing the number of deaths. So just by virtue of increased diagnoses, the death rate is brought down.
- The immediate outcome of a nationwide screening program is to dramatically increase cases. The article is a little unclear about whether it is the number or the rate of death that did not fall. It doesn't really matter; either case leads us to conclude that the screening has failed to improve health.
- One must not forget that screening tests, subsequent confirmatory tests, treatments, etc. all cost money, so there is a financial incentive to over-diagnose and over-treat.
- In addition to the financial incentive, there is the issue that I raised in Chapter 4 of Numbers Rule Your World (link). A false negative is a very public error on the part of the medical establishment while a false positive (followed by say removal of the thyroid) is an unobservable error. So there is a statistical incentive to over-diagnose and over-treat.