Case of the Week
Section Editors: Matylda Machnowska1 and Anvita Pauranik2
1University of Toronto, Toronto, Ontario, Canada
2BC Children's Hospital, University of British Columbia, Vancouver, British Columbia, Canada
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October 21, 2021
Pilomyxoid Astrocytoma
- Background:
- Pilomyxoid astrocytoma (PMA) is an aggressive variant of pilocytic astrocytoma.
- Pilomyxoid astrocytomas typically occur in younger children and carry a higher risk of recurrence.
- CSF spread is more common with meningeal enhancement concerning for metastasis.
- The suprasellar region is the most common location of occurrence; however, it can also be found in the cerebral hemispheres, cerebellum, basal ganglia, and fourth ventricle.
- PMA is composed of monomorphic bipolar cells with an angiocentric arrangement in a diffuse myxoid matrix lacking Rosenthal fibers or eosinophilic granules.
- PMAs commonly have activation of the RAS/BRAF pathway, similar to the closely related pilocytic astrocytoma (PA). About 60% of PMAs harbor KIAA1549-BRAF fusions. Unlike PA, PMAs overexpress genes critical for normal development (H19 and DACT2) as well as genes that code for collagens of the extracellular matrix (COL2A1 and COL1A1).
- PMAs were historically assigned a WHO grade 2 classification due to prior reports of increased risk of CSF dissemination, though this grade 2 designation is currently under debate.
- Clinical Presentation:
- Symptoms of increased intracranial pressure including headache, vomiting, vision changes, seizures, etc.
- Key Diagnostic Features:
- Usually located in the hypothalamic/optic chiasm region
- Can be large in size (>6 cm) with an “H-shape” configuration due to growth around the sella
- Obstructive hydrocephalus of the lateral ventricle
- T1 hypointense, T2 hyperintense, FLAIR hyperintense, no peritumoral edema, T2* hemorrhagic blooming more common, heterogeneous contrast enhancement, and rare calcifications
- Differential Diagnoses:
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Pilocytic astrocytoma: Can be indistinguishable on imaging; however, PAs typically occur in older children, have a greater cystic component, calcifications are more common, and hemorrhage is rare. Genetically, it is due to a duplication at chromosome 7q34 with KIAA1549-BRAF fusion.
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Suprasellar germinoma: Typically demonstrates uniform enhancement with multiple intralesional cysts and restricted diffusion; may have an enlarged infundibulum; can also be seen in the pineal region; subependymal spread is more common.
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Hypothalamic hamartoma: Classically seen in the tuber cinereum of the hypothalamus; signal is isointense to gray matter without enhancement; can be variable in size with a pedunculated or sessile morphology
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Treatment:
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In areas amenable to resection, a complete resection is curative in most cases.
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In deep or sensitive locations, radiation is considered most effective.
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Due to late effects on cognition and endocrine function, many chemotherapy options are considered, such as carboplatin or vinblastine.
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Identification of BRAF mutations provides an option for targeted therapy, which may spare young patients acute and long-term effects of chemotherapy.
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