Magnetic Resonance Imaging Findings of Radiation-induced Changes After Treatment of Malignant Gliomas and Metastases with a Particular Emphasis on Radiation Necrosis

New contrast-enhancing lesions discovered on routine follow-up brain imaging at or near the site of previously treated primary or metastatic brain tumours represent a clinical and radiological challenge, as radiation necrosis and tumoral recurrence often present at standard magnetic resonance imaging (MRI) with overlapping imaging features. Since the advent of combined therapy, including surgery and/or radiotherapy and chemotherapy, the post-treatment radiological assessment has been made earlier, with a subsequent higher incidence of early radionecrosis and a potential risk of mistaking it for disease progression. Therefore, both clinical and imaging follow-up are essential. This article briefly describes and illustrates the temporal patterns and spectrum of MR findings of radiation-induced brain injury and considers practical aspects of conventional and advanced MR sequences (diffusion-weighted imaging, perfusion MR and MR spectroscopy), with a particular emphasis on the distinction between tumoral recurrence and radiation necrosis.