Abstract
Differentiating between radiation necrosis and true tumour progression after radiotherapy is challenging due to overlapping imaging appearances. This review outlines useful techniques and imaging features for making this distinction, as well as potential pitfalls. Both radiation necrosis and true tumour progression commonly manifest as peripherally enhancing lesions on post-contrast T1-weighted imaging, but the enhancing rim should be thin in radiation necrosis, while more discrete nodular enhancement suggests active tumour. Other features on post-contrast MRI that suggest radiation necrosis include enhancing lesions across anatomical boundaries, clustering of enhancing lesions and a change in lesion shape. Central diffusion restriction corresponding to the central necrotic area favours radiation necrosis, but there are potential pitfalls to be aware of, including hypercellular tumours, coagulative necrosis due to bevacizumab and intra-lesional haemorrhage. Radiation necrosis typically results in small, clustered foci of magnetic susceptibility on susceptibility-sensitive sequences, and the absence of such foci should raise concern for active tumour. When uncertainty remains, ancillary techniques such as MR perfusion and amino acid PET can improve confidence. Atypical appearances of radiation necrosis can occur, for example, cystic radiation necrosis or radiation necrosis occurring after radiotherapy to adjacent structures. It is also important for the radiologist to be aware of additional factors that may increase the likelihood of either radiation necrosis or tumour necrosis or influence patient management.