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There is a large midline posterior fossa lesion in the midline and centred on the cerebellar vermis measuring 3.9 x 4.4 x 4.2 cm. The lesion is cystic, predominantly high T2 signal and with low T1 signal, but has heterogenous signal within it. Post contrast there are nodular areas of enhancement, but also peripheral enhancement. No meningeal enhancement demonstrated.
The lesion is exerting mass effect with effacement of the fourth ventricle and mild to moderate hydrocephalus. No transependymal oedema. There is slight inferior displacement of the cerebellar tonsils around the brainstem.
A linear enhancing structure in the right temporal lobe is consistent with a venous angioma. No other focal lesions.
Predominantly cystic, nodular and rim enhancing midline posterior fossa lesion with mass effect. The lesion carries a differential, but given the demographics and the cystic appearance a pilocytic astrocytoma would be favoured. A haemangioblastoma or atypical medulloblastoma would be the differentials.