Not sure exactly when they check for this, but my oncologist checked it last week and I was deficient. I started researching B-12 deficiency and it has wide ranging consequences.
This can cause alot of the problems we all suffer from, and think it is from our treatments. It is quite interesting to see all the things this can cause, and it makes you wonder why we are not tested for it. Might be worth asking for them to test when they do your blood work.