Specific Imaging Findings

The classic imaging appearance of a pilocytic astrocytoma (PA) is a well circumscribed cerebellar/fourth ventricle mass composed of a cyst and intensely enhancing mural nodule. This appearance is, however, absent in many cases of PA and is common with other neoplasms in this location and age group. The cysts are frequently absent, but the solid portions are characteristically hypodense on CT, T1 hypointense and of very high T2 signal. PAs typically exhibit avid contrast enhancement, which may be partial and patchy, and non-enhancing solid portions may also be present. The tumors ar3e bright on ADC maps, approaching the CSF signal. This very high diffusion is essentially pathognomonic for pediatric posterior fossa PA. Peritumoral edema is typically minimal or absent. PAs also occur in the brainstem, usually as well circumscribed, exophytic lesions. Supratentorial PAs commonly involve the optic pathways and hypothalamus. Perfusion imaging reveals relative cerebral blood volume to be only mildly elevated or similar to the normal brain. PAs are also found in adults and these tumors show the same imaging characteristics as in the pediatric population.