A case of Neurofibromatosis Type I "complicated" by a malignant cerebellar expanding process with atypical CT/MR features [Un caso di Neurofibromatosi di Tipo 1 "complicata" da processo espansivo cerebellare maligno con aspetti TC/RM atipici]

A 13 year-old boy, previously diagnosed as having Neurofibromatosis Type 1 (NF1) but otherwise healthy, was referred to our Department for headache, vomiting, ataxia and VI-VII left cranial nerve palsy. Emergency head CT scan was performed, immediately followed by contrast-enhanced MR scan with Echo-Planar (EPI) water diffusibility study. A large ovalar mass lesion was evident in the left cerebellar hemisphere, hyperdense on CT, hypointense in T2-weighted images, isointense to grey matter in FLAIR and T1-weighted images, with clear mass effect on the surrounding structures, but only mild peripheral contrast-enhancement in the anterior part of the mass, these features resembling those of a desmoplastic medulloblastoma. However, the EPI Diffusion-weighted images (DWI) and corresponding Apparent Diffusion Coefficient (ADC) maps consistently showed increased water diffusivity, a pattern not compatible with the dense cell-packing typical of medulloblastomas. The pathological analysis of the surgical specimen indeed showed focal neoplastic cell clusters interspersed in a diffusely altered cerebellar cortex. Neoplastic cells exhibited large eosinophilic cytoplasma, hyperchromatic and multinucleolated nuclei, and frequent mitoses, a pattern indicative of anaplastic astrocytoma. The case presented supports the evidence that NF1 is associated with increased risk of developing several different neoplasms, beside the well-known pilocytic astrocytomas of the optic pathway. As these neoplasms may exhibit misleading imaging features at CT or " conventional" MRI study, the assessment of water diffusivity may provide useful data for reaching a correct pre-operative diagnosis, since it reflects the histological architecture of the neoplastic tissue