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.
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.
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.
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.
Generally these tumours show astrocytic immunohistochemistry 6:
- GFAP: positive
- S100: positive
- vimentin: positive
- synaptophysin: variable
- NFP: negative
- chromogranin-A: negative
- BRAF (V600E-mutant): negative
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.
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
Treatment and prognosis
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:
- hypothalamic-chiasmatic involvement
- very young age (<2 years of age)
- WHO classification of CNS tumours
- WHO grading of CNS tumours
- VASARI MRI feature set
- diffuse astrocytoma grading
- grade I:
- grade II:
- grade III
- anaplastic astrocytoma
- anaplastic oligodendroglioma
- grade IV:
- glioblastoma vs cerebral metastasis
- radiation-induced gliomas
- gliomatosis cerebri (growth pattern)
- specific locations
- treatment response
- Stupp protocol
- glioma treatment response assessment in clinical trials
- multicentric glioblastoma
- multifocal glioblastoma
- prognostic genetic markers
- 1. Komotar RJ, Mocco J, Carson BS et-al. Pilomyxoid astrocytoma: a review. MedGenMed. 2004;6 (4): 42. MedGenMed (link) - Free text at pubmed - Pubmed citation
- 2. Linscott LL, Osborn AG, Blaser S et-al. Pilomyxoid astrocytoma: expanding the imaging spectrum. AJNR Am J Neuroradiol. 2008;29 (10): 1861-6. doi:10.3174/ajnr.A1233 - Pubmed citation
- 3. Pereira FO, Lombardi IA, Mello AY et-al. Pilomyxoid astrocytoma of the brainstem. Rare Tumors. 2013;5 (2): 65-7. doi:10.4081/rt.2013.e17 - Free text at pubmed - Pubmed citation
- 4. Pruthi SK, Chakraborti S, Naik R et-al. Pilomyxoid astrocytoma with high proliferation index. J Pediatr Neurosci. 2013;8 (3): 243-6. doi:10.4103/1817-1745.123694 - Free text at pubmed - Pubmed citation
- 5. Azad S, Kudesia S, Chawla N et-al. Pilomyxoid astrocytoma. Indian J Pathol Microbiol. 2010;53 (2): 294-6. doi:10.4103/0377-4929.64283 - Pubmed citation
- 6. Louis DN, Ohgaki H, Wiestler OD et-al. The 2007 WHO classification of tumours of the central nervous system. Acta Neuropathol. 2007;114 (2): 97-109. Acta Neuropathol. (full text) - doi:10.1007/s00401-007-0243-4 - Free text at pubmed - Pubmed citation