By Richard P. Holm, MD
Screening for cancer is sometimes like fishing for walleye. If you have the money, you can buy a good fish-finder to see where the fish might be. The finder may advise that there are fish down below your boat or your ice-house, but you don’t know if they might be carp, northern, or weeds . . . and not walleye. If, then, you decide to drop your hook, sometimes you only catch small ones that aren’t keepers and they all go back. Sometimes you catch nothing. Sometimes, however, you catch the big one.
Okay, this is not the perfect metaphor, but screening for cancer often requires advanced and expensive technology that are not always accurate. Once there is an indication that there might be cancer, the next step usually requires the commitment of a significant, sometimes invasive, effort that doesn’t always result in the best good for the patient. Sometimes, however, you catch a dangerous cancer about which you and your doctor can do something to provide a cure or, at least, add significant days of quality time to your life.
We are not talking about a patient coming to the doctor with a cancer that is discovered because the prostate cancer stopped the urine flow, the breast cancer lump grew to an easily feel-able size, the lung cancer induced a cough that turned bloody, or the colon cancer caused blood in the stool. We instead are talking about screening for cancer in perfectly healthy individuals who are not experiencing any symptoms. Examples of this preliminary screening would be a yearly prostatic-specific antigen (PSA) blood test for prostate cancer in 50-plus-year-old men, a yearly mammogram in 50-plus-year-old-women, and a CT scan of the lung one time in all 50-70-year-old smokers.
Experts measure the value of screening in several ways. First is the cost of the test and any further treatment that follows, measured against the benefit. This would be the financial cost to the patient, the insurance company, or the government, but would also include the pain and emotional suffering cost to the patient. Second is the sensitivity of the test: if it isn’t sensitive enough, then it will miss cancers. If it is too sensitive, then it will indicate cancer even when there is not cancer. The PSA test is not encouraged by many experts because it misses a lot of prostate cancer, and, at the same time, it identifies too many non-dangerous tumors that will never cause harm to the patient but would result in treatment that disables the patient without enough benefit. We call this a test with poor sensitivity and poor specificity.
There are screening tests that have good sensitivity and specificity, however, and times when even the PSA is indicated. So, let your doctor be your fishing guide through these difficult and challenging waters.