SORRY to hear the you are diagnosed with mass in brain stem.
Now coming to the question, the BRAIN STEM is the area where nerves from all the parts of the body leaves the brain and other nerves enters from the same portion of the brain, it has center/ control for various vital system and its the most complicated portion ever to deal with, to put in to visual understanding, have you ever seen a circuit board of computer , its similar to that but far more complicated structure.
There are very few changes to do biopsy
without doing some damage to any other system, hence their are many changes to have a adverse outcome if trying to go for the biopsy.
Performing even a needle biopsy is not without out danger, I mean their are higher chances to win lottery then to get the biopsy in brain stem with out our any adverse result.
Hope it helps, again very sorry to hear about the mass.
Dr. JAY PATEL.