Bitemporal hemianopia is due to lesion of :

A chiasmatic lesion (often owing to a pituitary tumor or a lesion around the sella turcica) can injure the decussating axons of retinal ganglion cells within the optic chiasm. These axons originate in the nasal halves of the two retinas. Thus, this type of lesion produces bitemporal hemianopsia, characterized by blindness in the lateral or temporal half of the visual field for each eye. Ref : Waxman S.G. (2010). Chapter 15. The Visual System. In S.G. Waxman (Ed),Clinical Neuroanatomy, 26e.