Posts Tagged ‘screening’

Five Common Misunderstandings About Screening

May 25, 2010

 This blog posting is taken largely from the chapter, “Breast Cancer Screening,” which comes from the book, Calculated Risks: How to Know When Numbers Deceive You by Gerd Gigerenzer. I highly recommend this book. It provides important insights into risks and how they can be misinterpreted. This blog posting takes no position on screening. Its objective is to provide an accurate understanding of risks. Although the specific topic is breast cancer, for the most part it applies to disease screening in general.

Screening tests are not meant for patients with known symptoms. Screening is intended for people without symptoms. The objective is early detection.

Screening does not reduce the incidence of breast cancer. That is the purpose of preventive measures. Early detection does not lead to prevention, although it is hoped that it leads to a cure and mortality reduction.

Early detection does not necessarily imply mortality reduction. For example, if there is no effective therapy, early detection has no effect on mortality. In these cases, early detection does not increase life expectancy. Instead it increases the time the patient consciously has to live with cancer.

All breast cancers do not progress. Sometimes mammography detects a form of breast cancer called “ductal carcinoma in situ.” In fact, the majority of cancers found in the screening of younger women are “ductal carcinomas in situ.” Half or more of these lesions do not seem to progress.

Consequently, early detection is not always beneficial. If a cancer would not progress, or would not progress enough to kill the woman during her life, early detection is harmful rather than beneficial. Invasive treatments such as mastectomy or lumpectomy with radiation with radiation might severely reduce her quality of life. Prostate cancer can progress very slowly in some men. So slowly that a man can die from something else before he even knows he has prostate cancer. Consequently, some men have undergone surgery that resulted in incompetence and/or impotence for a cancer that they might have outlived had they not undergone the surgery.