Radiation optic neuropathy after proton beam therapy for optic nerve sheath meningioma

A 63-year-old woman with a 1-month history of blurred vision in the right eye was found to have a right optic nerve sheath meningioma. She was treated with fractionated proton beam therapy using a total dose of 50.4 cobalt gray equivalent (CGE) in 1.8 CGE fractions, with subsequent improvement in vision. Twenty-seven months later, the patient reported a 6-week history of progressive blurred vision in her right eye. Magnetic resonance imaging revealed enhancement of the right optic nerve consistent with radiation optic neuropathy (RON). We are unaware of any previous reports of RON when radiotherapy doses fall within the current recommended guidelines of <55 CGE fractionated into daily doses <2 CGE.