I also have heard that very rarely a brain tumor can spread to the spine. But of course back pain in general is very common for other reasons. I also want to point out that a brain tumor can cause the sensation of pain in other areas of the body which themselves do not have a problem, it's just the brain tumor causing the feeling. My husband has had carpal tunnel type pain and excruciating RLS type neurological pain, that were not carpal tunnel or RLS. Neurontin was helpful for him.
I agree with everybody--try to put it out of your mind until you have results. This is a good skill to learn for brain tumor families since things like this always seem to happen sadly enough. You haven't heard anything bad yet, so it's a good day if you can just find a way to let it be one.