Figure 1: This posterior fossa PA has a large cyst with an adjacent mass (nodule). (Left) Axial image showing that this patient's cystic tumor is compressing and deviating the fourth ventricle, resulting in hydrocephalus. (Right) Sagittal postcontrast T1-weighted image showing the sometimes very heterogeneous enhancement apparent in the solid portion of the lesion that belies its underlying low-grade histology.
Figure 2: Axial (left) and sagittal (right) postcontrast T1-weighted images demonstrate a cystic lesion with a brightly enhancing mural nodule/mass. This appearance is very typical for juvenile PAs but must be distinguished from the similar-appearing hemangioblastoma when in the posterior fossa, particularly when in older children and young adults.
- Slow-growing, well-circumscribed tumor arising from astrocytic precursors
- Most common pediatric primary intracranial neoplasm
- WHO grade I
- Biphasic pattern of astrocytes (Rosenthal fibers and multipolar cells)
- Syndromic or sporadic
- Optic nerve/chiasm pilocytic astrocytomas (PAs) associated with neurofibromatosis type 1
- Patients aged 5 to 15 years most common
- No gender predilection
- Overall good prognosis
- Common presenting signs/symptoms
- Headaches, nausea, vomiting, ataxia, seizures, visual loss if optic pathways involved
- Cerebellar: tumor resection; adjuvant chemoradiation is rarely recommended, as residual tumors rarely grow and may even regress spontaneously
- Optic pathway: slow-growing tumors often monitored without treatment; tumor debulking, radiation, and/or chemotherapy for symptomatic or rapidly enlarging tumors
- Usually well-circumscribed cerebellar hemispheric lesion but can also be supratentorial
- Mass effect, effacement of fourth ventricle common
- May arise from optic nerve, optic chiasm, or hypothalamus
- Minimal surrounding edema
- Most often cystic mass with mural nodule but can be completely solid, particularly in older patients
- Mixed solid-cystic mass with minimal surrounding edema
- Solid component isodense to hypodense to gray matter
- Can have calcification; hemorrhage uncommon
- Strong mural nodular enhancement on contrast-enhanced CT images
- Nonenhancing cystic component, but may show cyst wall enhancement
- Contrast accumulation in cystic component on delayed images
- Cyst: isointense to hyperintense to cerebrospinal fluid but hypointense to parenchyma
- Solid: isointense to hypointense to gray matter
- Cyst: hyperintense to brain parenchyma
- Solid: usually hyperintense to parenchyma
- Hyperintense optic pathway lesions
- FLAIR: hyperintense solid and cystic components
- DWI: typically no diffusion restriction
- T1WI+C: enhancing solid component ± cyst wall enhancement; can show optic pathway enhancement
- MRS: aggressive features; elevated choline and lactate, decreased NAA
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