Ionizing radiation is one of the most effective treatments for complex intracranial pathologies including malignant (e.g. brain metastasis and glioblastoma) and benign tumors (e.g. vestibular schwannomas). Higher doses per fraction techniques and the integration of ionizing radiation with immunotherapies and targeted therapies can lead to changes in neuro-imaging studies that can be difficult to interpret. Differentiating between adverse radiation effects (AREs) some but not all of which may be radionecrosis and underlying tumor recurrence or progression can be difficult for even the most experienced of clinical teams. Moreover, the response need not be binary; in some instances, there may be a combination of pseudoprogression with AREs and active tumor. The term adverse radiation effect suggests that the features seen particularly within the tumor may be detrimental, but they could be a harbinger of favorable radiological and clinical responses to come. AREs could be a manifestation of the radiation cellular inactivation effects and the resulting inflammatory response within the central nervous system; many brain tumor patients exhibiting AREs on MRI are asymptomatic and will eventually demonstrate durable and significant tumor regression (Fig.
1). …
