I don't know how many (if any) tumor's are missed. However, I do know about MS looking lesions. My step-mother has MS. These lesions are "scar" tissue on the brain and as far as I know don't look like tumors. My neurologist says that there are other ways to tell if there is a tumor, movement tests, eye tests and a lot of the things you mentioned to rightcoastsurf in the other message thread. Have you asked your neurologist about the liklihood these can be missed?
I would ask the members of the MS forum about the MS looking lesions. They will know a lot more than me. MS is a difficult disease to diagnose but headaches are a symptom.