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KEY FACTS

Terminology
- Pilomyxoid astrocytoma (PMA): More aggressive, myxoid variant of pilocytic astrocytoma (PA) - High risk of local recurrence, CSF dissemination
Imaging
- 60% suprasellar (large, bulky, H-shaped mass in hypothalamus/optic chiasm, medial temporal lobes)
- 40% outside diencephalon (hemisphere, ventricles)
- Grossly well circumscribed, little/no edema
- Enhances strongly
- 20% show intratumoral hemorrhage
Top Differential Diagnoses
- PA
- High-grade glioma
- Germinoma
Pathology
- WHO grade 2 (typical PA is WHO grade 1)
Clinical Issues
- Typical: Infants, young children (< 4 years)
- Less common: Older children, young adults
- 5-10% of cases initially diagnosed as PAs may actually be PMAs - Especially if tumor is hemorrhagic, presents in very young child, or shows CSF dissemination
Diagnostic Checklist
- Consider PMA if - Infant or young child has large/bulky or hemorrhagic H-shaped suprasellar mass - Presumed PA in any patient who has atypical imaging (e.g., hemorrhage, metastases)

TERMINOLOGY

Abbreviations
- Pilomyxoid astrocytoma (PMA)
Synonyms
- Myxoid variant of pilocytic astrocytoma (PA)
Definitions
- Tumor with monomorphic piloid cells dispersed in mucopolysaccharide-rich matrix - More aggressive tumor than PA with high risk of local recurrence, dissemination

IMAGING

General Features

Best diagnostic clue

- Infant or young child with large, bulky, H-shaped mass in hypothalamus/optic chiasm, medial temporal lobes

Location

- 60% suprasellar
        - Optic chiasm, hypothalamus
        - Extension into adjacent structures is common with larger tumors
        - Deep gray nuclei, temporal lobes, adjacent white matter often involved
- 40% centered **outside** diencephalon
        - Cerebral hemispheres
                  - 2nd most common general location
                  - Temporal lobe most common
                  - May be purely cortical
        - Less common sites reported
                  - Midbrain
                  - Cerebellum
                  - 4th ventricle
                  - Spinal cord

Size

- Variable
        - Mean: 4 cm
        - Often large, bulky (up to 12 cm)

Morphology
```
- Grossly well circumscribed
```

CT Findings

NECT

- Uniform hypodensity most common
- 20% show intratumoral hemorrhage
        - Hyperdense; mixed hypo-/hyperdense
- Ca⁺⁺ occurs but uncommon

CECT

- Strong, but inhomogeneous enhancement
- Irregular central nonenhancing area in 1/3

MR Findings

T1WI

- Typical: Uniformly hypointense (almost 2/3 of cases)
- Less common: Mixed hypo-/hyperintensity (10-15%)
- Uncommon: Blood-fluid level

T2WI

- 70% uniformly hyperintense
- 15% inhomogeneously hyperintense
- 10% hypointense center, hyperintense rim

FLAIR

- 50% uniformly hyperintense
- 33% heterogeneously hyperintense
- Relatively well-demarcated margins
- Little or no peritumoral edema

T2* GRE

- Intratumoral hemorrhage in 20%
- May be strikingly hypointense

DWI

- Typically does not restrict
- ADC signal in solid component usually significantly ↑ compared to brain parenchyma

PWI

- ASL and DSC PWI may help distinguish PMA from PA
        - ASL: Mean tumor:GM cerebral blood flow (CBF) ratio = 1.3 in PMA vs. 0.4 in PA
        - DSC: Relative cerebral blood volume (rCBV) is 2 in PMA vs. 1.5 in PA

T1WI C+

- Strong but heterogeneous enhancement
        - 50% heterogeneous (i.e., rim)
        - 40% solid, homogeneous
        - 10% no enhancement
- Basilar/spinal meningeal enhancement is common and indicates CSF dissemination

Other Modality Findings
- MRS - ↑ Cho, ↓ Cr and NAA ± lactate - Some authors report low-metabolite pattern with ↓ Cho, Cr, NAA

Imaging Recommendations

Best imaging tool

- MR with T1 C+, DWI, T2* (GRE or SWI), MRS

Protocol advice

- Thin-section sagittal, coronal pre- and postcontrast T1WI
- Whole-brain FLAIR
- Thin-section T2WI through hypothalamus, chiasm
- GRE or SWI (to look for hemorrhage)
- Optional: Add DWI, MRS

DIFFERENTIAL DIAGNOSIS

Pilocytic Astrocytoma
- Older children (mean age at diagnosis: 6 years)
- In hypothalamus, typically enhances strongly/uniformly
- Occasionally calcified
- Clinically indolent, rarely aggressive
- PMA usually more hyperintense on T2/FLAIR (mucoid matrix)
- Hemorrhage, CSF dissemination uncommon
- Paradoxically aggressive MRS pattern: ↑ Cho, ↓ Cr, ↓ NAA - Some PMAs show low metabolite concentrations
High-Grade Glioma (Anaplastic Astrocytoma, Glioblastoma)
- Hemorrhage, necrosis common
- Hypothalamus rare location
- Patients usually older
- May arise from lower grade astrocytoma
Germinoma
- Most often in suprasellar, pineal, or basal ganglia regions
- Numerous microcysts common
- Diffusion restriction of solid components reflects high cellularity

PATHOLOGY

General Features

Etiology

- Unknown
- Some tumors demonstrate synaptophysin reactivity, suggesting PMAs may be of mixed glioneuronal origin
- May also originate from tanycytic cells

Genetics

- PMA has significant differences in gene expression vs. PA
        - *H19*, *DACT2*, extracellular matrix collagens, *IGF2BP3*(*IMP3*) overexpressed in PMAs
        - Variable tendencies toward maturation to PA

Associated abnormalities

- Few cases associated with neurofibromatosis type 1 have been reported

Staging, Grading, & Classification
- WHO grade 2 (PA is WHO grade 1) - Grade 3 if anaplastic features - Malignant transformation to glioblastoma (grade 4) rare but does occur
- MIB1 generally low (1-2%) but higher in anaplastic pleomorphic xanthoastrocytomas
Gross Pathologic & Surgical Features
- Large, grossly well-circumscribed mass
- Necrosis, hemorrhage may be present
Microscopic Features
- Lacks classic biphasic pattern seen in PAs - Alternating solid and loose areas interspersed with microcysts not seen - Rosenthal fibers, eosinophilic granular bodies absent
- Consists of monomorphic piloid tumor cells - Embedded in myxoid (mucopolysaccharide-rich) matrix - GFAP (+), vimentin (+)
- Conspicuous angiocentric growth pattern (perivascular rosettes) - Vascular proliferation may be marked - Infiltration of tumor cells into adjacent brain common - Necrosis rare

CLINICAL ISSUES

Presentation

Most common signs/symptoms

- Signs of ↑ intracranial pressure
        - Headache
        - Nausea, vomiting
- Delayed development
- Failure to thrive (so-called diencephalic syndrome)
- Visual disturbances
- Hypothalamic dysfunction

Other signs/symptoms
```
- Seizures
- Focal neurologic deficit
```

Demographics

Age

- Typical: Infants, young children (< 4 years)
- Less common: Older children, young adults
- Rare: Middle-aged adult (up to 46 years)

Sex
```
- Slight male predominance (M:F = 4:3)
```

Epidemiology

- Rare; represent < 1% of astrocytomas
- 5-10% of cases initially diagnosed as PAs may be PMAs, especially if hemorrhage is present or tumor presents in very young child

Natural History & Prognosis
- Higher recurrence rate than PA
- CSF dissemination common
- Bimodal pattern - Can mature to PA - May dedifferentiate into GBM
Treatment
- Partial resection with adjuvant therapy may prolong survival

DIAGNOSTIC CHECKLIST

Consider
- PMA if - Infant or young child has large/bulky or hemorrhagic suprasellar mass - Imaging atypical for PA (i.e., hemorrhage)
- If PA with repeated recurrences, CSF dissemination, review histopathology and consider PMA
Image Interpretation Pearls
- H-shaped suprasellar mass may be PMA

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References

Selected References

Images

Selected Images

Coronal graphic depicts a pilomyxoid astrocytoma (PMA). Note the large, bulky, H-shaped mass centered in the hypothalamic/chiasmatic region and extending into both temporal lobes. The tumor is relatively well circumscribed and shows little/no edema. Glistening myxoid matrix is typical. Hemorrhage occurs in ~ 20% of PMAs but is unusual in pilocytic astrocytoma (PA).

Coronal T1 C+ MR in a 20-month-old with a PMA shows a large, heterogeneously enhancing hypothalamic mass .

Sagittal T2 MR in a 7-month-old with a PMA shows a large mass centered in the hypothalamus and optic chiasm. Note the pituitary pressed down along the floor of the sella. The large size of the tumor and the very young age of the patient are typical of a PMA.

Axial ADC MR in the same patient shows almost universal increased signal in the tumor compared to brain parenchyma, which is typical for low-grade astrocytic tumors, such as PMAs.

Sagittal T1 C+ MR in a 2-year-old shows an avidly enhancing hypothalamic tumor . This is a typical appearance for PMA.

Axial SWI in the same patient shows multifocal areas of signal loss , consistent with intratumoral microhemorrhage. Microhemorrhage is more common in PMAs compared to PAs. For this reason, it is helpful diagnostically to include SWI in evaluation of hypothalamic tumors.

Sagittal T1 C+ MR in a 23-month-old demonstrates a homogeneously enhancing midbrain lesion . Pathology revealed a PMA. Regardless of location, PMAs should be considered for any likely low-grade glioma in a very young child.

Coronal T1 C+ MR in a 2-year-old with a uniformly enhancing mass centered in the right cerebellar hemisphere is shown. Note the associated hydrocephalus . While the cerebellum is the most common location for PAs, it is an uncommon location for PMAs.

Axial T1 C+ MR in 9-year-old shows a rim-enhancing mass with lack of adjacent edema. Enhancement patterns of PMAs vary greatly from solid homogeneous enhancement to peripheral enhancement, as seen here. Some degree of enhancement is almost universally present.

Sagittal T1 C+ MR in a 3-year-old with an avidly enhancing intramedullary tumor , found to be a PMA on pathology, is shown. While uncommon, PMAs have been reported to occur in the spine.

Additional Images

Sagittal T2 MR in the same patient shows a large, lobulated tumor centered in the hypothalamus with exophytic extension of the tumor into the 3rd ventricle and prepontine cistern . A suprasellar location is the most common location for a PMA.

Axial ADC map in the same patient shows hyperintense signal within the solid components of the tumor , which is characteristic of hypothalamic gliomas. There is significant overlap in imaging features of PAs and PMAs.

Sagittal T1 C+ MR in a 9-month-old with macrocephaly shows a large, centrally necrotic mass centered within the hypothalamus with extension into the sella and 3rd ventricle.

Axial T2 GRE MR in the same patient shows small foci of signal loss centrally within the lesion, suggesting areas of microhemorrhage. Approximately 20% of PMAs show hemorrhage, which is rare in PAs. Whenever hemorrhage is identified in a hypothalamic glioma, a PMA should be suggested.*

Sagittal T2 MR in a 23-month-old with a PMA shows a homogeneously hyperintense lesion centered in the midbrain. A PMA can occur anywhere, including the brainstem.

Axial T2 MR in a 9-year-old shows a markedly hyperintense mass centered in the right basal ganglia, causing obstruction at the foramina of Monro with obstructive hydrocephalus. Pathology revealed a PMA.

Sagittal T1 C+ MR in a 9-year-old shows an enhancing mass projecting into the 3rd ventricle from the thalamus. PMA was diagnosed on biopsy.

Anteroposterior MRA in the same patient shows encasement of the right anterior cerebral artery and elevation of the middle cerebral artery compared to the normal left side. No neovascularity was seen. A PMA was found at surgery.

Coronal T1 C+ MR in a 3-year-old shows a large, inhomogeneously enhancing, suprasellar mass encasing the carotid bifurcation .

Sagittal T2 MR shows a large, hyperintense hypothalamic/optic chiasm mass . This could be either a pilomyxoid or PA on the basis of imaging findings. A PMA was found at surgery.

Coronal T2 MR in an infant with a large head shows markedly enlarged lateral ventricles and a lobulated, hyperintense suprasellar mass.

Axial FLAIR MR in the same patient shows the large mass completely fills the suprasellar cistern, elevating and encasing both middle cerebral arteries . The mass is mildly hyperintense relative to cortex.

Axial T1 C+ SPGR MR in the same patient shows the mass enhances intensely and quite uniformly. The H-shaped configuration centered in the suprasellar cistern is classic for PMAs, which was confirmed at surgery. (Courtesy M. Thurnher, MD.)

Axial FLAIR MR in a 3-year-old shows a large, H-shaped suprasellar mass with extension into the basal ganglia and both medial temporal lobes. The tumor is quite well delineated despite its size and shows no evidence for surrounding edema.

Axial T1 C+ MR in the same patient shows intense, uniform enhancement.

High-power mucin stain shows the mucinous matrix (blue) with embedded glial nuclei. MIB1 was elevated. Final diagnosis was a PMA (WHO grade II). (Courtesy R. Hewlett, MD.)

Micropathology biopsied from the same patient shows neoplastic, bipolar, "pilocytic" cells. No Rosenthal fibers are seen.

Axial T2 MR in a 20-month-old shows a huge, bulky suprasellar and medial temporal lobe mass. Scattered foci of T2 shortening within the mass may represent hemorrhage (no T2 imaging was performed.)*

Axial T1 C+ MR in the same patient shows mixed solid and rim enhancement. Biopsy disclosed elongated "piloid" cells in a mucin-rich matrix, consistent with a PMA. (Courtesy R. Hewlett, MD.)

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KEY FACTS

Terminology

Imaging

Top Differential Diagnoses

Pathology

Clinical Issues

Diagnostic Checklist

TERMINOLOGY

Abbreviations

Synonyms

Definitions

IMAGING

General Features

Best diagnostic clue

Location

Size

Morphology

CT Findings

NECT

CECT

MR Findings

T1WI

T2WI

FLAIR

T2* GRE

DWI

PWI

T1WI C+

Other Modality Findings

Imaging Recommendations

Best imaging tool

Protocol advice

DIFFERENTIAL DIAGNOSIS

PATHOLOGY

General Features

Etiology

Genetics

Associated abnormalities

Staging, Grading, & Classification

Gross Pathologic & Surgical Features

Microscopic Features

CLINICAL ISSUES

Presentation

Most common signs/symptoms

Other signs/symptoms

Demographics

Age

Sex

Epidemiology

Natural History & Prognosis

Treatment

DIAGNOSTIC CHECKLIST

Consider

Image Interpretation Pearls

References

Selected References

Images

Selected Images

Additional Images

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