We're going in for surgery tomorrow. About 90% of the mass is amenable to being removed; the rest is up against parts that make Sara move or talk, and is too dangerous to take out. When we last spoke to the surgeon, his first guess was that it was a primary brain tumor of some sort; however, it had some unusual features on the MRI (not the least of which that Sara is young and immunocompetent) that led him to waffle between craniotomy or a biopsy.
Given where the mass is--the fact that it's not in a place where it'll cause immense damage if removed--Sara's going with surgery: speed is a virtue in this case, and if it is a primary brain tumor, it needs to be out as quickly as possible.
Wish us luck. We'll update everyone as soon as the mass is out.