Masseter Botox: can it be limited to specific portions of the muscle such that slimming/proportions are controlled? (Photo)

I want my Masseter region to be slimmer for a more tapered, angular & masculine jawline more in line with classically attractive proportions. However, I don't want the entire bulk of it to be slimmer, only a certain portion of it. Can the injection of botox be localized to only very specific portions of the muscles such that the slimming effect is restricted to only those highly specific parts? Or does the entirety of the muscle become paralyzed & slimmer even if you inject into a small part?