The question is whether "people with headaches" is the right reference class. If the headache is unusually severe or persistent, it makes sense to look deeper. Also, a doctor can ask for details about the headache before prescribing the expensive tests.
More precisely, the question is whether or not the right reference class is one in which cancer tests are worth while. The headaches would have to be very unusually severe to get enough evidence.
Also, a doctor can ask for details about the headache before prescribing the expensive tests.
It was never mentioned whether or not the doctor asked for details. It's also possible that none of those reference classes are worth looking into, and she'd need headaches and something else.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.