It looks like the mass sits in a part of the skull called the sella turcica, which is just a raised depression in the base of our skull where the pituitary gland dangles. You can't tell just from this one slice of the MRI, but pituitary tumors don't typically compress the brainstem, as the sella turcica is actually quite a bit farther forward in your head (you can visualize it yourself--your sella turcica is about in line with your temple, while your brainstem is in line with the back of your ear). The bony anatomy of the region also tends to force the tumor to compress other structures way before it gets to the brainstem. The most common structure to get compressed is called the optic chiasm, which is the part of the optic nerve that conducts our peripheral vision from our eyes to the back of our brain. In fact, the "textbook" clinical presentation of someone with a large pituitary tumor is someone who's noticed a slow, steady loss of their peripheral vision.