If i had a brain tumor on my upper left side would they need to shave all of my hair?

Not usually: They will usually need to shave the area where the entry into your brain will be. The entire scalp is not usually shaved from what i've seen of pts coming to me for radiation after their tumor has been removed.

Not all: If you had a tumor on the left side of your brain, the usual neurosurgical approach would be from the left side of the head, and so some of the hair in the area of the incision would typically be shaved. There would be no need to shave off 'all of' your hair.