Distinguishing atypical focal areas of signal intensity from probable low-grade gliomas in the posterior fossa of children with neurofibromatosis type 1

Focal areas of signal intensity (FASI) in the posterior fossa are common in children with neurofibromatosis type 1 (NF1). These can be difficult to distinguish from tumour processes when they are atypical. This study aimed to describe the initial radiological features of atypical posterior fossa FASI, depending on the clinical and radiological evolution of the lesion. We conducted a multicentre retrospective study of NF1 children with atypical FASI in the posterior fossa. The lesions were classified according to their evolution over time: 'non-neoplastic FASI' if they regressed, or 'possible neoplastic FASI' if they remained stable or progressed. Forty-two lesions from 31 patients were analyzed. The mean follow-up period between the first and last MRI scans was 7.1 years. Twenty-one atypical FASI were classified as 'non-neoplastic FASI' and 21 as 'possible neoplastic FASI'. Younger age at diagnosis, pontine location, and blurred contours were more indicative of 'non-neoplastic FASI' on the initial MRI, whereas symptomatic, cystic or exophytic lesions, the presence of enhancement after gadolinium injection, sharp contours, and clear mass effect were more indicative of a 'possible neoplastic FASI'. Unexpectedly, abnormal initial spectroscopy and small mass effect could be found in 'non-neoplastic FASI', as could the presence of hypointensity on T1-weighted images. This study is important in specifying which posterior fossa hyperintensities on T2-weighted MRIs require specific clinical and radiological follow-up.