Pilomyxoid astrocytoma

Pilomyxoid astrocytomas are an aggressive variant of pilocytic astrocytoma with unique clinical and histopathologic characteristics.

Pilomyxoid astrocytomas are usually encountered in young children and infants (mean age of 10 to18 months), however, adults cases have been described 3,4. As most of these tumours have been previously classified as pilocytic astrocytomas it is uncertain whether they have distinctive epidemiology 6

There is no characteristic clinical presentation and the symptoms will be related to the mass effect and the tumour location, usually symptoms of raised intracranial pressure or parenchymal compression 1-4

Grading

Although it has been suggested that pilomyxoid astrocytomas should be considered WHO grade II tumours (compared to pilocytic astrocytomas which are WHO grade I), in the 2016 update to the WHO classification of CNS tumours a formal grade has not been given 6

Location

Pilomyxoid astrocytomas were originally reported as arising in the hypothalamus or optic chiasm, which is the most common location, however, they may also occur elsewhere within the brain, including the posterior fossa, as well as in the spinal cord 1,6

Microscopic features

Pilomyxoid astrocytomas are composed of monomorphous bipolar cells with a prominent angiocentric pattern. They are embedded within a profoundly myxoid background 6. Generally, these tumours are well circumscribed without infiltration 6

Features typically found in pilocytic astrocytomas, such as Rosenthal fibres, eosinophilic granular bodies and calcification are uncommon or absent in pilomyxoid astrocytomas 3,5,6. These tumours also lack the biphasic appearance (dense cellular areas alternating with loose cystic areas) usually present in pilocytic astrocytomas 5

Vascular proliferation and foci of necrosis are only occasionally encountered 6

Immunophenotype

Generally these tumours show astrocytic immunohistochemistry 6

These tumours are usually large, well-circumscribed, lobulated and may have solid or cystic components. They are typically described as H-shaped as they expand from the midline to both temporal lobes. Haemorrhagic components may be present.

MRI

Reported signal characteristics include:

  • T1: isointense  
  • T2: usually hyperintense, reflecting its myxoid matrix
  • T2*: approximately 20% show intratumoral haemorrhage
  • T1 C+ (Gd): common and is usually in the solid component but can be also peripheral
  • DWI/ADC: usually high ADC values (facilitated diffusion), reflecting its myxoid matrix

Compared to pilocytic astrocytomas, pilomyxoid astrocytomas have been generally thought to behave more aggressively and have a poorer prognosis, although this may be in part at least due to their predilection for the hypothalamus and optic chiasm, which makes complete resection impossible 6

The total resection of the tumour is the most reliable predictor of a favourable outcome 3

Pilocytic astrocytoma is the main differential diagnosis, and no reliable imaging features can distinguish between them, although certain features are more common in pilomyxoid astrocytomas, including:

  • haemorrhage
  • hypothalamic-chiasmatic involvement
  • very young age (<2 years of age)​
Astrocytic tumour
Share article

Article information

rID: 15828
Section: Pathology
Tag: cases
Synonyms or Alternate Spellings:
  • Pilomyxoid astrocytoma (PMA)
  • Pilomyxoid astrocytomas

Support Radiopaedia and see fewer ads

Cases and figures

  • Drag
    Case 1
    Drag here to reorder.
  • Updating… Please wait.
    Loadinganimation

    Alert accept

    Error Unable to process the form. Check for errors and try again.

    Alert accept Thank you for updating your details.