Discussion
Eberhart and colleagues first described this new histologic category of
infant brain tumors and called it embryonal tumors with abundant
neuropil and true rosettes (ETANTR).1 Over the years,
this was described with various names/acronyms including embryonal
tumors with multilayered/ ependymoblastic rosettes, MEPL, EBL and
supratentorial Primitive Neuroectodermal Tumors
(sPNET).11 Until 2010, when Paulus and Kleihues
proposed “ETMR” as a unifying entity to these embryonal
tumors12 displaying multilayered rosettes as a common
feature. In 2009, Pfister and colleagues discovered the novel oncogenic
microRNA locus (C19MC) as the diagnostic genetic hallmark of this
tumor.5 These studies led to the designation of
ETMR-C19MC altered as a distinct new entity in the 2016 WHO CNS
neoplasms.[13] C19MC amplification is present in about 90% of all
ETMR regardless of histology.9,14 Transcriptomic
profiling of C19MC-altered tumors revealed enrichment of these tumors
for early neural and pluripotency genes including LIN 28/LIN 28B,
reflecting their primitive nature. In 2013, Spence et al interrogated a
large cohort of 450 paediatric tumors to assess the diagnostic
specificity of C19MC amplification and LIN 28A.8 Their
study demonstrated that C19MC alteration is seen in a subset of
CNS-PNETs with high LIN 28 expression, however a small subset of other
tumors such as rhabdoid brain tumors, malignant gliomas, atypical
teratoid rhabdoid tumor (ATRT), germ cells tumors also express LIN 28A.
While LIN 28A immunopositivity is not specific for the diagnosis of
ETMR, we excluded our cohort for ATRT, germ cell tumors, high grade
glioma by performing INI-1, Oct 3/4 and GFAP in all cases. LIN 28A
expression was consistently expressed in all cases in our cohort.
Additionally, to substantiate the above findings, we also performed LIN
28A immunohistochemistry in a case of ATRT (with INI-1 loss) featuring
multilayered true rosettes. LIN 28A expression was seen within these
rosettes as has been reported earlier in ATRTs.15Although LIN 28A is a surrogate marker for ETMR, its expression needs to
be interpreted in context of histology. ATRT may often contain
multilayered rosettes and presents a diagnostic challenge; INI-1 is
extremely helpful in reliably distinguishing the two. All the tumors in
our cohort readily expressed INI-1. Recently, using DNA methylation
profiling and transcriptomic analysis Lambo et al,9have demonstrated that approximately 5% of ETMR negative for C19MC
amplification contain biallelic DICER mutations (ETMR,DICER- altered) and these mutations are mutually exclusive to
C19MC amplification.10 It is interesting to note how
mutually exclusive DICER mutations and C19MC amplification leads
to activation of oncogenic pathways. A common link is LIN 28A/Let 7
signalling which brings about reprogramming of stem cells. High
expression of LIN 28A leads to depletion of Let-7 miRNA and subsequent
upregulation of Let-7 targets.16 ETMR have low
expression of Let-7 miRNA and typically have high expression of LIN 28A.
The tumor predominantly arises in the supratentorial region in 70% of
cases 3,7,17 as was also observed in our cohort.
Tumors in infratentorial region comprise 30% 3,17, 18with rare reports of these tumors arising in the
spine.17,19 A trend for infratentorial location of
C19MC- altered ETMR has been observed. ETMR has traditionally been
described as a tumor of infants and young children, three patients in
our cohort were more than 18 years of age.
ETMR encompass a wide spectrum of histology from tumors with abundant
neuropil (ETANTR) with tumors with low neuropil content (EBL, MEPL).
Multilayered rosettes are a distinctive feature, recognition of which
amidst sheets of acellular neuropil (Figure 3B) or within cellular
islands is critical. These are best recognized at low magnification,
Ki67 is helpful in highlighting these mitotically active,
undifferentiated cells with high N/C ratio surrounding central lumen
(Figure 3C). In a single case, a continuum of morphological features was
observed with tumor cells exhibiting ependymoblastic rosettes along with
multilayered rosettes. MEPL comprising papillary and tubular structures
reminiscent of embryonic neural tube was identified in the spinal tumor
of adolescence girl. Another case of undifferentiated embryonal tumor
with occasional rosettes was encountered with cells strongly expressing
LIN 28A and retained nuclear expression for INI-1 (thereby excluding
ATRT). ETMR are known to show neuronal and glial differentiation
post-therapy.20,21 The tumor recurrence after 9 months
of primary diagnosis in case #8 demonstrated areas of glial and
rhabdoid differentiation. Interestingly, recurrent tumors on second
surgery are known to transition from one histological variant to
another.7
On imaging, these tumors usually appear large and well demarcated.
Though 75% of them are supratentorial, posterior fossa lesions are well
described. Large size of the lesions causes significant mass effect on
the adjoining brain parenchyma, which is more marked in the
infratentorial compartment, often with compression on peduncles. These
solid masses are usually hypointense on T1-weighted and hyerintense on
T2-weighted MR images showing diffusion restriction and mild patchy
contrast enhancement. There is usually surrounding oedema and
significant mass effect. The cerebral blood flow has been reported to be
low on arterial spin labelling (ASL) sequence. However, the cerebral
blood volume (CBV) on dynamic susceptibility contrast imaging is usually
high. These features help differentiating ETMR from other childhood
tumors. Pilocytic astrocytoma shows strong contrast enhancement with no
diffusion restriction. Medulloblastomas and ATRT usually enhance well
and have high cerebral blood flow using ASL. Furthermore, ATRT’s usually
demonstrate central necrotic areas. Ependymomas often have increased T2
signal like ETMR, but have a variable diffusion restriction
pattern.22
Complete resection undoubtedly translates into better survival; however
certain location particularly brain stem and posterior fossa structures
may not be amenable to complete resection. Possible benefits of second
resections on survival following application of chemotherapy are also on
record.21 Consensus guidelines for ideal chemotherapy
regimen and radiotherapy dose- volume (CSI versus focal radiotherapy)
remains poorly defined. Treatment strategies are based on results in CNS
PNETs and other high-risk CNS-embryonal tumors. Various combinations of
chemotherapeutic drugs that have been used so far include lomustine,
vincristine, cyclophosphamide, etoposide and
carboplatin.19
ETMR are aggressive pediatric brain tumors with universally poor
prognosis and a median overall survival (OS) of only 12 months or less
in most of the studies.7,8 The 5-year survival rate is
0-30%.14 The overall median survival observed in our
cohort was 6.4 months and was worst for ETANTR (1.3 months) followed by
EBL pattern (9.2 months); however, this did not differ significantly
between the three histological patterns, similar to observations by
Korshunov et al.7 As these tumors have recently been
classified, there is limited data to guide prognostication and
treatment.23 Cumulative data suggest C19MC patients
who received chemotherapy with radiotherapy had significantly longer
survival compared to untreated patients (median survival of 13 months
versus 0.6 months, respectively).8 Studies have
indicated that higher overall survival is seen in completely resected
tumors with patients treated with radiotherapy and high-dose
chemotherapy with autologous stem cell rescue.17,24
In conclusion, our study emphasizes the histological diversity inherent
to ETMR. LIN 28A while being less specific, is a sensitive
immunohistochemical marker. Prognosis of ETMR remains dismal with median
survival of 6.4 months and this does not differ significantly amongst
the three histological patterns.