Radiologically typical pilocytic astrocytoma with histopathological signs of atypia.

Pilocytic astrocytoma (PCA) is the most common CNS tumor in primary school-aged children. Herein, we report the case of a 7-month-old female child with a large cerebellar hypodense tumor on computer tomography occupying nearly the whole cerebellar hemisphere. Magnetic resonance imaging revealed characteristic features of a PCA, depicting a mass with hyperintense solid and cystic areas on T2-weighted images and with marked solid and ring-like enhancement pattern. Histopathological and immunohistochemical analysis showed partially typical features of a PCA, but also atypical features such as a high proliferation rate, hypercellularity, and focally diffuse infiltration pattern were present. However, definite signs of transformation to an anaplastic PCA were not observed. We therefore classified the tumor as pilocytic astrocytoma with signs of atypia instead of simple WHO grade I pilocytic astrocytoma. The case illustrates that neuroimaging features may give very important clues for the definite diagnosis in histopathologically atypical PCA.