Several types of radiation therapy (RT) are currently available, and techniques continue to evolve and emerge with scientific advances. Traditional whole-brain RT, which is used to treat cerebral malignancies, is delivered in fractions over approximately 4–6 weeks. In 3-dimensional conformal RT, a computer helps concentrate radiation in a precise area; this concentration is accomplished through the use of complex asymmetric 3-dimensional shapes. Radiation surgery (radiosurgery) refers to linear accelerator or gamma knife techniques, which are distinct from traditional fractionated RT. Radiosurgery is typically given in a single sitting, using computer-based techniques to focus radiation at the desired regions. These techniques may treat malignancies, occasional inflammatory lesions, and vascular malformations. Tumors in very close proximity to the anterior visual pathway structures may not be suitable targets for single-fraction radiosurgery.
Figure 14-25 Radiation optic neuropathy (RON). A, Axial and B, coronal, T1-weighted, fat-suppressed MRI scans performed after gadolinium injection of a patient with RON. Radiation was performed for a pituitary adenoma, and months after the radiation, the patient experienced vision loss that was greater in the right eye than the left. MRI demonstrates enhancing optic nerves (arrows) in the prechiasmal region.
(Courtesy of Eric Eggenberger, DO.)
Complications resulting from RT directed at the CNS may take several forms and can occur years after therapy. Immediate complications include transient swelling of the involved tissue. Later complications of neuro-ophthalmic interest include radiation necrosis, cranial neuropathies, and ocular neuromyotonia (see Chapter 7). Radiation necrosis involves death of nervous system tissue with attendant edema. This complication may simulate the imaging appearance of recurrent neoplasm on traditional MRI or CT imaging. Occasionally, functional imaging techniques such as PET or magnetic resonance spectroscopy (MRS) are required to separate these entities radiographically; neoplasms generally display a hypermetabolic profile, whereas radiation necrosis is hypometabolic in nature.
Of greater concern to the neuro-ophthalmologist is radiation optic neuropathy (RON). This rare complication typically occurs months after radiation administration and produces subacute optic neuropathy. RON is in part dose-related; it is more likely to occur with higher doses (often >5000 cGy). MRI studies of RON usually demonstrate enhancement of the optic nerve(s) on the postcontrast, T1-weighted sequences (Fig 14-25). Hyperbaric oxygen, corticosteroids, pentoxifylline, and intravenous or intravitreal bevacizumab have all been used for the treatment of RON. Thus far there is only anecdotal evidence regarding the effectiveness of such treatments.
Stroke-like migraine attacks after radiation therapy (SMART) is a syndrome related to delayed complications resulting from brain irradiation for CNS malignancies, typically occurring many years after radiation therapy. Patients present with recurrent attacks of complex neurologic signs and symptoms, including headache, visual disturbances, and seizures. Debilitating stroke-like deficits such as homonymous hemianopia, hemiplegia, and/or aphasia typically resolve over an average of 2 months; however, in some cases, the neurologic recovery is incomplete. MRI findings include unilateral increased T2 signal within the temporal, parietal, or occipital cortices with pronounced gyral cortical gray matter enhancement. As symptoms resolve, the MRI findings disappear.
Stieber VW. Radiation therapy for visual pathway tumors. J Neuroophthalmol. 2008;28(3): 222–230.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.