Presentation
History of headache that gets worse with changes in posture. Blurry vision is also related in the last days. Bilateral papilledema. No abnormal neurology.
Patient Data
There is a large predominantly cystic mass arising from the inferior left cerebellar hemisphere measuring up to 33 mm in the transverse diameter. A solid enhancing component is demonstrated inferiorly. There is considerable associated mass effect with partial compression of the inferior fourth ventricle. The lateral and third ventricles are dilated. There is deformity of the left cerebellar peduncle and distortion of the adjacent brainstem.
Thee cerebral hemispheres are intact and no additional intracranial mass lesion is seen superior to the tentorium. There is no abnormal leptomeningeal contrast enhancement. The basal cisterns are however to some extent effaced.
Centered in the medial aspect of the left cerebellar hemisphere, there is a mixed solid and cystic mass, measuring 3.6 cm x 3.6 cm x 2.9 cm. The solid component is located medially and measures 2.8 cm x 2.7 cm x 2 cm. The solid component enhances vividly and homogeneously. There is a cystic component superiorly within the cerebellum, as well as inferiorly, extending as far as the upper cervical spinal cord. The cystic component is located posterior to the cord here, and there is mild flattening of the cord.
There is peripheral enhancement of the cystic component of the mass. The solid component demonstrates homogeneous T2 hyperintensity. No flow void is identified.
There is mild surrounding vasogenic edema. The mass compresses the fourth ventricle, and there is obstructive hydrocephalus with evidence of mild transependymal CSF flow.
No restricted diffusion or susceptibility artefact. No supratentorial mass is identified.
Single voxel spectroscopy demonstrates elevated choline and metabolite depletion to the right of the spectrum.
MICROSCOPIC DESCRIPTION: Paraffin sections show a moderately hypercellular glial tumor involving cerebellum. Tumor cells are a mixture of plump piloid cells and cells with round and oval nuclei and delicate processes. Both cell types show moderate nuclear pleomorphism. No mitotic figures are identified. There is no microvascular proliferation and no necrosis is seen. Small numbers of Rosenthal fibers are identified. A moderate amount of extracellular Alcian Blue positive myxomatous material is noted throughout the tumor. Tumor gradually merges with surrounding cerebellar parenchyma. Gliosis and patchy loss of Purkinje cells is noted in the cerebellar cortex.
IMMUNOHISTOCHEMISTRY:
- GFAP positive
- Nestin positive (moderate)
- ATRX positive (not mutated)
- IDH-1 R132H negative (not mutated)
- MGMT positive (likely unmethylated)
- p53 negative
- p16 negative
- Topoisomerase labeling index: Approximately 2%.
- The features are of pilomyxoid astrocytoma (WHO Grade II).
DIAGNOSIS: "Posterior cranial fossa tumor": Pilomyxoid astrocytoma (WHO Grade II)
Molecular testing for BRAF-KIAA1549 fusion and BRAF V600E mutation confirmed the diagnosis of a pilocytic astrocytoma.
Case Discussion
This case illustrates a large cystic mass lesion arising from the left inferior cerebellar hemisphere with an enhancing solid component and associated obstructive hydrocephalus. In the patient's age group and with these image features, hemangioblastoma and pilocytic astrocytoma must be considered.
Hemangioblastoma typically occur in the young adult, and although they are the most common posterior fossa mass in a young adult, they are nonetheless uncommon in absolute terms, accounting for only 1-2.5% of all intracranial tumors. They have a peak incidence around 30-60 years of age. One imaging feature that makes this case less likely a hemangioblastoma is the presence of enhancement in the cystic component wall.
Pilocytic astrocytomas are tumors of young people, with 75% occurring in the first two decades of life, typically late in the first decade (9-10 years). There is no recognized gender predisposition.
This case was first considered to favor a pilomyxoid astrocytoma (PMA) diagnosis by the histopathology study. However, after the BRAF genetic tests, the diagnosis was confirmed to be a pilocytic astrocytoma due to the absence of genetic fusion or mutations. PMA was originally reported as arising in the hypothalamus or optic chiasm of an infant or young child. However, it is also known that they may also occur in the posterior fossa and in the spinal cord 1.
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