Why does a pituitary tumour take away the outer half of vision in both eyes? · Grey Matter

The pituitary sits just below the optic chiasm, and a tumour growing upward out of the sella turcica presses on the chiasm's centre from below.

Why does a pituitary tumour take away the outer half of vision in both eyes?

The pituitary sits just below the optic chiasm, and a tumour growing upward out of the sella turcica presses on the chiasm's centre from below. The fibres crossing in that centre come from the nasal half of each retina, which, because the lens inverts the image, sees the outer (temporal) half of the visual field. Damaging them removes the right half-field of the right eye and the left half-field of the left eye, leaving a central band, the pattern called bitemporal hemianopia.