It occurs most often around 1.5 years after treatment and results in irreversible and severe vision loss, which may also be associated with damage to the retina (radiation retinopathy). This is thought to be due to damage to dividing glial and vascular endothelial cells. RON can present with transient visual loss followed by acute painless visual loss in one or both eyes several weeks later. The risk of RON is significantly increased with radiation doses over 50 Gy. Optic neuritis
Optic neuritis is inflammation of the optic nerve, which is associated with swelling and destruction of the myelin sheath covering the optic nerve. Young adults, usually females, are most commonly affected. Symptoms of optic neuritis in the affected eye include pain on eye movement, sudden loss of vision, and decrease in color vision (especially reds). Optic neuritis, when combined with the presence of multiple demyelinating white matter brain lesions on MRI, is suspicious for multiple sclerosis. Several causes and clinical courses are possible for the optic neuritis. It can be classified in: 

Single isolated optic neuritis (SION)
relapsing isolated optic neuritis (RION)
chronic relapsing inflammatory optic neuropathy (CRION)
the neuromyelitis optica (NMO) spectrum disorder
multiple sclerosis associated optic neuritis (MSON)
unclassified optic neuritis (UCON) forms.Medical examination of the optic nerve with an ophthalmoscope may reveal a swollen optic nerve, but the nerve may also appear normal. Presence of an afferent pupillary defect, decreased color vision, and visual field loss (often central) are suggestive of optic neuritis. Recovery of visual function is expected within 10 weeks.
Genetic mutations in mitochondrial DNA, vitamin depletion, alcohol and tobacco abuse, and use of certain drugs can cause derangements in efficient transport of mitochondria, which can cause a primary or secondary optic neuropathy. Nutritional optic neuropathies
A nutritional optic neuropathy may be present in a patient with obvious evidence of under-nutrition (weight loss and wasting). Months of depletion are usually necessary to deplete body stores of most nutrients. Undernourished patients often have many vitamin and nutrient deficiencies and have low serum protein levels. However, the optic neuropathy associated with pernicious anemia and vitamin B12 deficiency can even be seen in well-nourished individuals. Gastric bypass surgery may also cause a vitamin B12 deficiency from poor absorption. Patients who have nutritional optic neuropathy may notice that colors are not as vivid or bright as before and that the color red is washed out. This normally occurs in both eyes at the same time and is not associated with any eye pain. They might initially notice a blur or fog, followed by a drop in vision. While vision loss may be rapid, progression to blindness is unusual. These patients tend to have blind spots in the center of their vision with preserved peripheral vision. In most cases, the pupils continue to respond normally to light. Toxic optic neuropathies
The most recognized cause of toxic optic neuropathy is methanol poisoning. This can be a life-threatening event that normally accidentally occurs when the person mistook or substituted, methanol for ethyl alcohol.