PILOCYTIC ASTROCYTOMA

Large cystic lesion in left cerebellar hemisphere extending into the vermis with mild left tonsillar herniation. Post contrast there is intense homogenous enhancement of the eccentric mural nodule situated towards the vermis. Cyst wall didnot show any enhancement.





·        Four predominant imaging patterns of pilocytic astrocytoma have been described:

1.     Mass with a nonenhancing cyst and an intensely enhancing mural nodule

2.     Mass with an enhancing cyst wall and an intensely enhancing mural nodule

3.     Necrotic mass with a central nonenhancing zone

4.     Predominantly solid mass with minimal to no cystlike component

·        Hence, two-thirds of all cases demonstrate the classic imaging manifestation of a cystlike mass with an enhancing mural nodule.

·        Although most cyst walls do not enhance, some may enhance intensely, even as much as the mural nodule.

·        Differential diagnosis : hemangioblastoma

JUVENILE PILOCYTIC ASTROCYTOMA	HEMANGIOBLASTOMA
Hypoattenuating cystic componenet with hypo to isoattenuating mural nodule with avid contrast enhancement	Hypoattenuating cyst and isodense nodule with avid contrast enhancement
Well demarcated lesion	Well demarcated lesion
Solid component: iso to hypointense relative to normal brain	Nodule peripherally located near pial surface and isointense to gray matter on T1
Cystic component follows fluid signal intensity	Iso or slightly hyperintense on T1 compared with CSF
Nodule: hyperintense with minimal surrounding T2WI signal	Nodule : hyperintense
Cyst follows fluid signal	High signal intensity
Avid enhancement of mural nodule	Avid enhancement of solid component
No reported abnormal flow voids	Peripheral in location due to vascular supply from pia mater with nodule located near pial surface. May see abnormal vessels/ flow voids
Low signal on DWI and high on ADC	Low signal on DWI and high on ADC
Rarely calcifies	May bleed into cyst leading to fluid levels
Rarely hemorrhage into the nodule or into subarachnoid space	If hemorrhage into nodule, appearance depends on age of hemorrhage

References :

1.     Pilocytic Astrocytoma: Radiologic-Pathologic Correlation, November 2004 RadioGraphics, 24

2.     Practical differential diagnosis for CT and MRI.