LAUSR

R adiation optic neuropathy (RON) is a rare complication of external beam radiation therapy (RT) in which exposure of the anterior visual pathway (AVP) to radiation results in acute, painless vision loss in one or both eyes, months to years after treatment (1). The diagnosis of RON is supported by constant enhancement on MRI in an involved area of previous radiation treatment without evidence of tumor involvement. No treatment has proven effective in reversing this vision loss; therefore management is individualized. Maximum radiation dose to the optic apparatus is the most significant known risk factor for RON. Clinical and experimental data support a dose tolerance for the AVP of 50 Gy with the incidence of RON approaching 0% below this threshold (2). Dose prescriptions for central nervous system and head-and-neck tumors often are constrained by the sensitivity of the optic apparatus to radiation. Most RT protocols therefore limit maximum point doses to the optic pathway to 54–55 Gy in 1.8–2 Gy fractions (2). Whole brain radion therapy (WBRT) is a palliative intervention for patients with multiple brain metastases. The most common WBRT regimens deliver a total dose of 30 Gy in 3 Gy fractions or 35 Gy in 2.5 Gy fractions, which corresponds to a biologically equivalent dose at 2 Gy fractions (EQD2) of 37.5 or 39.4 Gy, respectively (using the linear quadratic model and assuming an a/b ratio [tissue repair capacity] of 2 for the optic apparatus) (3). Biologically equivalent doses allow for comparison of WBRT regimens with differing fractions. The biologically equivalent dose in these regimens is well below the dose-tolerance threshold of the AVP even accounting for larger fraction size. A lower, palliative dose for brain metastases is essentially a compromise between toxicity and tumor control, resulting in control of disease in approximately 50% of patients at 6 months (4). Multiple long-term neurological sequelae after WBRT have been described, including dementia, more subtle cognitive deficits in memory and attention, ataxia, incontinence, and death (5,6). Here, we report 3 cases of RON following WBRT.