I have a relative who has cancer and has been taking a particular chemotherapy drug.  It has been very successful; all of the tests and scans have been coming back negative for some time.  Recently I went along to an appointment with a fancy consulting oncologist to get his opinion about how much longer to continue with the drug.  Going into the appointment, I had the idea (based on nothing but what seemed to me like common sense) that there was a tradeoff: more chemo means a higher chance that the cancer won’t reappear, but also means a higher chance of serious side effects, and that we were going there to get his opinion on whether in this case the pros outweighed the cons or vice-versa.  What he said instead was that there was "no evidence" that additional chemo, after there are no signs of disease, did *any* additional good at all, and that the treatments therefore should have been stopped a long time ago and should certainly stop now.  I asked him what was incorrect about the (seemingly) common sense notion that additional chemo might get rid of the last little bits of cancer that are too small to show up on scans, and he said, more or less, that it’s not my idea of common sense that matters, it’s the evidence, and there is no evidence that things work that way.  So then I asked him whether by "no evidence" he meant that there have been lots of studies directly on this point which came back with the result that more chemo doesn’t help, or whether he meant that there was no evidence because there were few or no relevant studies.  If the former was true, then it’d be pretty much game over: the case for discontinuing the chemo would be overwhelming.  But if the latter was true, then things would be much hazier: in the absence of conclusive evidence one way or the other, one would have to operate in the realm of interpreting imperfect evidence; one would have to make judgments based on anecdotal evidence, by theoretical knowledge of how the body works and how cancer works, or whatever.  And good people, maybe I’m being unfair and underestimating this guy, but I swear to you that this fancy oncologist in this very prestigious institution didn’t seem to understand the difference between these two types of "no evidence."  So while he had a very strong and very (generally) laudable instinct that one ought to base one’s medical opinions on evidence rather than instinct, he seemed to be unable to avoid what strikes me as a pretty fundamental mistake.*  I’d love to hear thoughts about this, particularly from doctors who either have something to say about whether this is a common mistake among doctors or who have something to say about the chemotherapy question itself.

*The most generous possible interpretation of what went on, but which would require me to attribute to him a thought process that he did not express at all, is that he understands the difference between the two types of "no evidence" but has come to believe that doctors’ interpretations of imperfect evidence will systematically lead them to over-treat and so has adopted a rule of "do nothing unless there is strong evidence that you should do something" as a second-best optimum.