Length bias

Sir Patrick. I know your Cutler Walpoles and their like. Theyve found out that a man’s body’s full of bits and scraps of old organs he has no mortal use for. Thanks to chloroform, you can cut half a dozen of them out without leaving him any the worse, except for the illness and the guineas it costs him. I knew the Walpoles well fifteen years ago. The father used to snip off the ends of people’s uvulas for fifty guineas, and paint throats with caustic every day for a year at two guineas a time. His brother-in-law extirpated tonsils for two hundred guineas until he took up women’s cases at double the fees. Cutler himself worked hard at anatomy to find something fresh to operate on ; and at last he got hold of something he calls the nuciform sac, which he’s made quite the fashion. People pay him five hundred guineas to cut it out. They might as well get their hair cut for all the difference it makes ; but I suppose they feel important after it. You cant go out to dinner now without your neighbor bragging to you of some useless operation or other.

Gorge Bernard Shaw The Doctors Dilemma

What is the appropriate interval for breast cancer screening in 40 – 49-year-old women? Could this be a part of the “government” conspiracy to limit early detection of breast cancer in an effort to further the “culture of death“? Conversely, are certain mammograms akin to removing the “nuciform sac” in that they accomplish nothing other than extracting money from patient’s health insurance? What’s a doctor (and a patient) to do?

If only it were an easy problem. Mammograms, as it turns out, are not a “prevention” technology but are used for “early detection”. The US Preventive Services Task Force, a group of scientists (some of whom I consider my friends) who specialize in looking at the risks and benefits for various preventive and early detection methods that the average citizens (not patients or people with known problems) should be exposed to. It is because of the task force that we know the benefits of early detection for hypertension (very high benefit, very low risk) and as opposed to those of screening for heart disease (very little benefit to early detection, lots of risk if positive screening turns out to be untrue (a false positive)). They have critically looked at the literature and have created a tool that allows one to determine which prevention and early detection measures are right for them.

Why the problem with early breast cancer detection? The New England Journal of Medicine points out that direct problems include the very low risk from the radiation exposure, along with pain, anxiety, and psychological distress related to the procedure. False positive results are particularly problematic in this age group, with one study suggesting that for every case of breast cancer detected in women 40 to 49 years of age, 556 women have mammography, 47 have additional imaging, and 5 have biopsies. Part of the problem is the very low rate of breast cancers in these women, with only 15/1000 developing cancer between 40 and 49, 2 dying of cancer, and 21/1000 dying of everything else. This is half of the rate in subsequent decades. The other problem is that cancers are less likely to be detected in younger breasts (false negative tests) and some cancers are so rapidly growing in this age group. Even mamograms every 6 months wouldn’t result in any benefit from early detection for women with rapidly growing tumors and mamograms every other year would be sufficient for those with slower growing tumors (length bias, see picture above).

Turns out that what we want to do well is prevent disease (hence that push for immunizations). It is also important to detect and intervene in cases where the person would develop disease if left to their own devices (such as smokers, and people with early hypertension). While we would like to believe that early detection is always preferable to late detection with regards to treatment, that is not always the case. It is clear that the average woman of average risk benefits from early detection when older than 50 until about 70. For women from 40-49 it turns out that knowing the patient’s  family history and genetics and as a physician-patient team making sure that the correct test is ordered is equally important. Another arguement for the medical home?

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