Update
Mary Ann should be coming out of surgery shortly.
Her tumor is situated between the cerebellum at that lower back of her head and her brainstem. One of my areas of expertise is neuropsych, so I know a little bit about such things, perhaps too much. The cerebellum is attached to the rear of the brainstem, coordinates muscle movement and helps process sensory information. The cerebellum enables nonverbal/sensory learning and memory. It helps us judge time, modulate our emotions, and discriminate sounds and textures. It also coordinates voluntary movement, and is implicated in such things as balance and gait.
The brainstem is the oldest part of the brain and is responsible for automatic survival functions. Its components are the medulla (which controls heartbeat and breathing), the pons (which helps coordinate movements), and the reticular formation (which affects arousal).
There is a greater chance that the tumor is malignant than otherwise, but we won't know that until after the surgery and following the results of further testing. The surgical team was optimistic about the procedure, and don't foresee any complications. The tumor may be a primary condition or an indication that there are problems in other parts of her body. She'll be treated for a time with radiation therapy and steroids, with plenty of doctor visits in the first month post-surgery. She'll likely be discharged by the middle of next week.
I spoke with my mother this morning, and she told me that she had the same experience as I did, that Mary Ann somehow seemed oddly relieved after getting the bad news. As I've commented earlier, my mother has been all over my sister about seeing a doctor, even for a simple physical, for most of her life. Although my Mom is distraught, and extremely saddened that it has come to this, she accepts that she couldn't have done anything else.
I'm doing the best I can, and am hopeful that this will have a good outcome.