Optic Chiasm & Optic Nerves

The optic nerves run from the back of each globe through the optic canals and meet at the chiasm, above the pituitary gland, where fibers from the inner half of each retina cross over.

The optic nerves carry what each eye sees to the brain. At the chiasm the signals are sorted, so each side of the brain receives the opposite half of what both eyes see.

Damage causes radiation-induced optic neuropathy — permanent loss of vision in one or both eyes. These are serial structures with maximum-dose limits, and they come into play with nasopharynx, sinonasal and skull-base targets. QUANTEC describes how the risk rises with maximum dose. In brain treatment they limit targets near the sella and the frontal and temporal lobes, and they sit at the top of the planning priority list for hippocampal avoidance.

Optic nerves — from Brouwer 2015 and the EPTN 2018 atlas:

• Usually 2–5 mm thick and generally clear on CT.
• Contour all the way from the back of the eyeball, through the bony optic canal, to the chiasm.
• MRI is recommended, at least close to the chiasm. EPTN stresses contouring the nerves in continuity with the chiasm, because dose gradients there can be very steep.

Optic chiasm

• Brouwer places it in the suprasellar cistern, typically about 1 cm above the pituitary gland, bordered at the sides by the internal carotid arteries. MRI is recommended.
• EPTN adds that it sits just in front of the pituitary stalk, in the front-lower part of the third ventricle, with the optic nerves in front, the optic tracts behind, and the anterior cerebral and anterior communicating arteries in front of it. It recommends T1 MRI.

Because it is so small, Brouwer 2015 suggests expanding it by 5 mm when calculating DVH data, to make the numbers more consistent (while avoiding overlap with the PTV), and recommends a contrast CT with slices of 2 mm or less.