Key-words:
Astrocytoma - neurosurgery - pediatrics
Introduction
Pilomyxoid astrocytoma (PMA) is a very rare tumor that merits recognition as a unique
and specific entity. PMA shares the same features as pilocytic astrocytoma (PA), the
most common central nervous system (CNS) tumor in the pediatric population. However,
some pathological differences have been described to make part between these two entities.
Previous studies have shown PMA to behave more aggressively than PA, with shorter
overall survival as well as a higher rate of recurrence and dissemination.[[1]],[[2]] PMA is considered as a pediatric tumor, mainly involving hypothalamic and chiasmatic
regions.[[3]],[[4]] Only few studies reported the cerebellum as a localization of this lesion.[[5]],[[6]] This review summarizes the clinical, radiographic, prognosis, and current therapeutic
options of cerebellar PMA through three pediatric cases.
Cases Reports [[Table 1]]
Cases Reports [[Table 1]]
Case 1
A 1-year-old boy with normal background history presented with signs of intracranial
hypertension with increased head circumference.
Table 1: Summary of the features concerning the patients reported in this article
Brain magnetic resonance imaging (MRI) shows a large 51 mm diameter well-circumscribed
intra-axial heterogeneous tumor in the left cerebellar lobe. The tumor was hyperintense
on T2-weighted images (WIs) and hypointense T1-WI with peripheral contrast enhancement
[[Figure 1]]. A hydrocephalus was also present due to the obstruction of the 4th ventricle.
Figure 1: Brain MRI: (a) axial section on a T2-weighted image; (b) axial section on a T1-weighted
image with contrast product injection; (c) sagittal section on a T1-weighted image
with contrast product injection. They show a large 51 mm diameter well-circumscribed
intra-axial heterogeneous tumor in the left cerebellar lobe. The tumor is hyperintense
on T2-weighted image and hypointense T1-weighted image with peripheral contrast enhancement.
A hydrocephalus is associated due to the obstruction of the 4th ventricle
The patient underwent a total resection of the tumor via left suboccipital craniotomy.
The tumor was pale pink and the cyst contained yellowish turbid fluid.
Postoperative course was uneventful. The patient showed clinical improvement, as well
as regression of hydrocephalus on control brain computed tomography scan.
Histological examination revealed monomorphous bipolar cells with loose myxoid background.
No biphasic pattern, Rosenthal fibers, or eosinophilic granular bodies were noticed.
The tumor cells were arranged around vessels in a pattern resembling pseudorosettes
seen in ependymomas, with no evidence of neovascularization, significant pleomorphism,
abnormal mitoses, or necrosis [[Figure 2]].
Figure 2: Histological examination of the tumor revealing monomorphous bipolar cells with loose
myxoid background. No biphasic pattern, Rosenthal fibers, or eosinophilic granular
bodies were noticed. The tumor cells were arranged around vessels in a pattern resembling
pseudo rosettes seen in ependymomas, with no evidence of neovascularization, significant
pleomorphism, abnormal mitoses, or necrosis
A 2-year follow-up showed no signs of recurrence. No adjuvant treatment was proposed.
Case 2
A 7-year-old boy, with no pathologic background, presented with signs of intracranial
hypertension (headache and vomiting) for 1 year with aggravation in the last 2 weeks.
Clinical examination found a fully conscious patient with wide-based ataxic gait.
Neither papillary edema nor cutaneous stigmata of neurofibromatosis had been found.
Brain MRI showed a large 60 mm diameter well-circumscribed intra-axial solid and cystic
tumor located in the vermis and left cerebellar lobe. The tumor was hyperintense in
T2-WI and hypointense in T1-WI with contrast enhancement [[Figure 3]]. Surgical procedure was the same as the first case with total resection confirmed
with postoperative brain scan [[Figure 4]]. Histological features were the same as for the first patient. Two-year follow-up
showed no signs of recurrence and no adjuvant treatment was needed.
Figure 3: Brain MRI: (a) axial section on a T2-weighted image; (b) axial section on a T1-weighted
image with contrast product injection; (c) sagittal section on a T1-weighted image
with contrast product injection. They show a large 60 mm diameter well-circumscribed
intra-axial solid and cystic tumor located in the vermis and left cerebellar lobe.
The tumor was hyperintense in T2-weighted image and hypointense in T1-weighted image
with contrast enhancement
Figure 4: Axial sections of a postoperative brain CT scan (a) without injection of contrast
product; (b) with injection of contrast product. They show a complete exeresis of
the tumor
Case 3
A 9-year-old girl with no abnormal background consulted with progressive gait disturbance
and frequent falls for several days added to signs of high intracranial hypertension
for 1 year. Wide-based ataxic gait was observed with no other signs in clinical examination.
Brain MRI found a large 70 mm diameter well-circumscribed intra-axial solid and cystic
tumor located in the vermis and left cerebellar lobe [[Figure 5]]. A total resection of the tumor via suboccipital craniotomy was performed. The
histological features of the tumor were the same as the two previous cases. Two-year
follow-up showed no signs of recurrence and no adjuvant treatment was performed.
Figure 5: Brain MRI: (a) axial section on a Tl-weighted image with contrast product injection;
(b) axial section on a T2-weighted image; (c) sagittal section on a T1-weighted image
with contrast product injection. They show a large 60 mm diameter well-circumscribed
intra-axial solid and cystic tumor located in the vermis and left cerebellar lobe.
The tumor was hyperintense in T2-weighted image and hypointense in T1-weighted image
with discreet peripheral contrast enhancement
Discussion
PMA has been defined as a distinct entity among brain tumors, but on the last 2016
World Health Organization (WHO) classification for CNS tumors, no definite grade assignment
was proposed for these lesions.[[1]],[[2]] PMAs have been reported with overwhelming majority in children aged between 2 months
and 4 years.[[7]] These tumors are known to be mostly located in the chiasmatic-hypothalamic region,
but many other locations were reported, including the spinal cord, temporal lobe,
occipital lobe, and sellar-suprasellar region.[[8]],[[9]],[[10]] In the adulthood, PMAs may be located in different regions from those of the childhood,
such as the deep temporal area,[[11]] spinal cord,[[12]] and fourth ventricle.[[13]]
In our pediatric series, the posterior fossa localization was found in all three patients.
In the literature, only few studies reported the cerebellum as a possible localization
for PMAs.[[5]],[[6]] Despite many pathological similarities with PAs, PMAs have some specific features
in histology, leading to their identification as independent type of glioma. These
characteristics include monomorphous growth of piloid cells, with an angiocentric
pattern, rich in myxoid background, and lacking of Rosenthal fibers or eosinophilic
granular bodies.[[14]] Furthermore, necrosis is more commonly associated with PMA, whereas cystic formations,
calcifications and perilesional oedema are more common in classic PAs.[[14]] Immunohistochemical studies proved that PMAs stains are strongly positive for glial
fibrillary acidic protein and vimentin and are negative for neuronal markers.[[14]] PMAs and PAs share also some radiological similarity: they both show isointensity
on T1-WI sequences, hyperintensity on T2-WI, and FLAIR sequences. However, PMAs are
often solid, rarely with peripheral edema. 40% of PMAs show homogenous enhancement
and 30%–60% display heterogeneous enhancement.[[13]],[[15]] Radiological differential diagnosis between PMAs and PAs can be made using arterial
spin labeling imaging, which shows low perfusion parameters in PAs.[[16]],[[17]]
Yeom et al.[[16]] showed that the maximal relative tumor blood flow of high-grade tumors (Grades
III and IV) was significantly higher than that of low-grade tumors (Grades I and II).
In another report by Komotar et al.,[[11]] 76% of patients with PMAs exhibited local recurrence, versus 50% for those with
PAs, with an increased rates of leptomeningeal dissemination (14%). Furthermore, PMAs
have significantly decreased mean progression free time and overall survival.
All of our three patients underwent a total surgical resection. No adjuvant treatments
were proposed. All of our patients showed a positive posttherapeutic evolution, having
shown no signs of clinical or radiological recurrence. In the literature, there is
still no consensus about the management of PMAs. Nevertheless, some reports showed
that gross total resection was the primary treatment strategy and the most reliable
predictor of outcome in children with low-grade gliomas where surgery can be performed
without excessive morbidity.[[18]] Adjuvant chemo- or radiotherapy is restricted to cases with subtotal excision or
recurrence.[[5]]
Conclusions
PMAs are classified as low-grade tumor, but must be distinguished from PAs. The WHO
classification defines PMA as a subgroup of PAs with more aggressive attributes. A
close clinical and radiological follow-up is needed due to an increased risk of recurrences
and dissemination. Gross total resection is the most important factor to predict the
outcome of PMA.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms.
In the form the patient(s) has/have given his/her/their consent for his/her/their
images and other clinical information to be reported in the journal. The patients
understand that their names and initials will not be published and due efforts will
be made to conceal their identity, but anonymity cannot be guaranteed.
