Radiation -induced brain injury
In the central nervous system (CNS), the effects of radiation can be roughly divided into effects on vascular endothelial cells and direct effects on neuroglial cells, in particular the oligodendroglial cells
On the basis of the time of expression, radiation-induced injury can be divided into three phases: acute, early delayed, and late delayed. Acute and early delayed injuries are usually transient and reversible, whereas late delayed injuries are generally irreversible.
To correctly interpret imaging studies, radiologists should maintain familiarity with the expected imaging appearances after RT and carefully distinguish them from tumor recurrence. Keys to recognizing radiation-induced changes at follow-up imaging are knowledge of (a) the amount of time elapsed since RT, (b) the location of the target lesion, and (c) the amount of normal structures included.
At imaging, radiation-induced leukoencephalopathy is characterized by cerebral white matter high signal intensity on T2-weighted or fluid-attenuated inversion-recovery (FLAIR) images, usually without enhancement or significant mass effect. It typically exhibits diffuse and symmetric involvement after whole-brain RT, with relative sparing of the subcortical U-fiber, corpus callosum, and gray matter. White matter lesions usually develop around the periventricular white matter at the beginning and progress to diffuse white matter changes with varying degrees of cerebral atrophy over months or years.
Currently, the only method of distinguishing pseudoprogression and true tumor progression is to perform follow-up examinations of the patient because conventional MRI does not allow differentiation of the two conditions. Imaging may be regularly performed at 2–3-month intervals throughout the follow-up period, although the frequency of imaging can be variable across institutions. In clinical practice, the following features can be helpful: (a) presence of symptoms and (b) methylation status of the MGMT gene promoter.
Prompt diagnosis of radiation-induced spinal cord myelopathy can be difficult because symptoms can vary, and MRI findings are nonspecific and can vary depending on the timing of MRI with respect to radiation exposure. Some imaging features may be useful in incorporating radiation-induced spinal cord myelopathy in the differential diagnosis, such as the longitudinally extensive cord signal intensity pattern corresponding to the radiation field and demonstration of T1-weighted hyperintense marrow signal changes in vertebrae included in the radiation field.
