Pathology of Embryonal Tumors

Embryonal tumors of the central nervous system (CNS) include a heterogeneous group of immature-appearing neoplasms that are highly cellular and mitotically active, thus having a superficial resemblance to the developing nervous system. ^[1] These highly malignant tumors most often arise in children; they not only invade nervous tissue but also can disseminate in the cerebrospinal fluid (CSF) and subarachnoid space.

The most common CNS embryonal tumor is the medulloblastoma, which is discussed in a separate article. This article addresses neoplasms that are designated by the World Health Organization (WHO) as "other CNS embryonal tumors" and includes atypical teratoid/rhabdoid tumors (AT/RTs) and some less common CNS embryonal tumors that can arise outside of the cerebellum.

Atypical teratoid/rhabdoid tumor

AT/RT is a highly malignant CNS neoplasm that primarily affects infants and young children. It contains a distinctive tumor cell type (the rhabdoid cell) that displays multilineage antigen expression. ^[1] Although adult AT/RT is rare, a sellar region AT/RT occurs primarily in adults; the latter may represent a clinicopathologic and potentially genetically distinct variant of AT/RT. ^[2] Most AT/RTs are defined by loss of heterozygosity for the SMARCB1 (INI1) gene. ^{[3, 4, 5, 6, 7]} SMARCA4 (BRG1) mutations may rarely occur in AT/RT.

The histologic spectrum of AT/RT overlaps significantly with other forms of CNS neoplasia, including medulloblastomas, germ cell tumors, sarcomas, and choroid plexus carcinomas. ^[1] It is important to distinguish AT/RT from other CNS embryonal tumors because of the relatively poor prognosis of AT/RT and because of the need for accurate stratification of patients for therapeutic protocols.

The term "malignant rhabdoid tumor" was first used to describe rare pediatric renal tumors that contained tumor cells with vesicular nuclei, prominent nucleoli, and cytoplasmic inclusion-like structures composed of whorls of intermediate filaments. Neoplasms with such rhabdoid features were subsequently identified in other locations, including the CNS. At least three different molecular subgroups of AT/RT (ATRT-TYR, ATRT-SHH, and ATRT-MYC) have been identified at the time of this writing. ^[5]

Cribriform neuroepithelial tumor (CRINET) has been introduced as a provisional entity in the 2021 WHO Classification of Tumors of the Central Nervous System to describe a non-rhabdoid brain tumor with cribriform growth pattern characterized by alterations in the SMARCB1 (INI1) gene, similar to AT/RTs. ^[8] Studies suggest a relatively favorable outcome for this neoplasm relative to AT/RT. ^[9]

Other genetically defined CNS embryonal tumors

A rare group of embryonal CNS neoplasms that arise outside of the cerebellum and pineal gland were previously called "central nervous system primitive neuroectodermal tumors" (CNS PNETs). Such tumors may arise in the cerebral hemispheres, brainstem, or spinal cord, and they are composed of immature neuroepithelial cells that may express neuronal and glial antigens. ^[10] All of these neoplasms are WHO grade 4 by definition. The histologic characteristics of high cellularity and mitotic activity—along with a potential to show glial, neuronal, and even ependymal differentiation—are found in this group. ^[10]

The WHO classifies this group of tumors as "other CNS embryonal tumors" ^[8] and includes genetically defined entities such as the "embryonal tumor with multilayered rosettes" (see the following image) with C9MC alteration or DICER1 mutation, "CNS neuroblastoma, FOXR2-activated," and the "CNS tumor with BCOR internal tandem duplication" ^[11] as well as other CNS embryonal tumors without a defined molecular alterations. ^[8]

Atypical teratoid/rhabdoid tumor (AT/RT)

Loss of heterozygosity and mutation of the retained allele of a putative tumor suppressor gene SMARCB1 (INI1) located on chromosome 22q11.2 is the defining molecular characteristic of AT/RT. SMARCB1 (INI1) protein normally functions in chromatin remodeling, ^[3] and its loss is associated with several other pediatric renal and soft-tissue tumors as well as with central nervous system (CNS) AT/RT.

In some tumors, SMARCB1 protein loss may lead to reduction of p16 and increased cell proliferation via modulation of an Rb (retinoblastoma gene) controlled pathway. Constitutional loss or inactivation of the SMARCB1 (INI1) gene (rhabdoid tumor predisposition syndrome) occurs in a few rare families. Tumors seen in this autosomal-dominant syndrome with incomplete penetrance include AT/RTs and other CNS embryonal tumors, choroid plexus carcinomas, and rhabdoid tumors of the kidneys and extrarenal tissues. In addition, germline nonsense mutation and somatic inactivation of SMARCA4/BRG1 has also been described in a family with rhabdoid tumor predisposition syndrome. ^[12]

AT/RT may arise in either supratentorial or infratentorial locations. The cerebral hemispheres are the preferred site above the tentorium, with occasional cases arising in the pineal region or, rarely, as suprasellar lesions. The cerebellum is the most common location in the infratentorial compartment where the hemispheres are most commonly involved. In this location, medulloblastoma will be a part of the differential diagnosis. Such tumors may also be found in the cerebellopontine angle, brainstem, and, rarely, in the spinal cord.

AT/RTs may involve multiple nervous system sites at presentation; dissemination along cerebrospinal fluid (CSF) pathways is identified in more than 20% of patients at the time of presentation.

Other genetically defined CNS embryonal tumors

Embryonal tumor with multilayered rosettes (ETMR)

An ETMR  having amplifications or fusions in the C19MC locus of chromosome 19q13.42 is now a World Health Organization (WHO) defined entity. ^[13] Rare ETMRs with DICER1 mutations have also been identifiied. ^[8] The term "ETMR" now unifies a group of histologically similar CNS embryonal tumors that were previously called "ependymoblastoma," "CNS medulloepithelioma," and "embryonal tumor with abundant neuropil and true rosettes" because the defining C19MC alterations could be found in all of the latter neoplasms. ETMR typically affects the cerebral hemispheres. Infratentorial locations including the cerebellum is less frequent.

CNS neuroblastoma, FOXR2-activated (CNS NB-FOXR2) and CNS tumor with BCOR internal tandem duplication (CNS-BCOR)

The CNS NB-FOXR2  and the CNS-BCOR  were discovered in a multi-institutional, large-scale DNA methylation profiling study of tumors that had previously been classified as CNS primitive neuroectodermal tumors (CNS PNETs)." ^[11] Although many tumors in the study were subsequently reclassified as known tumor types, such as malignant gliomas, medulloblastomas, and ependymomas, the above new tumor entities emerged. These are now recognized by the most recent WHO brain tumor classification. ^[8] CNS NB-FOXR2 arise mostly in the supratentorial compartment, whereas the CNS-BCOR may arise in the cerebral or cerebellar hemispheres.

Younger children have a higher incidence of central nervous system (CNS) embryonal tumors, whereas immature-appearing tumors in older patients are generally of glial origin. ^[14]

Atypical teratoid/rhabdoid tumor (AT/RT)

Most patients with AT/RT are younger than 3 years, and these tumors account for 1-2% of pediatric brain tumors. A male predominance varying from 3:2 to 2:1 has been reported. ^[1] AT/RTs account for over 10% of CNS tumors in infants. ^[3] As noted (see the Pathophysiology and Etiology section), rhabdoid tumor predisposition syndrome (with constitutional loss or inactivation of the INI1 gene) may occur, but it is rare.

Other genetically defined CNS embryonal tumors

It is difficult to determine the true incidence of embryonal tumors with multilayered rosettes (ETMRs) because of varied terms used in the past. However, over 100 cases have been reported. ETMRs primarily affect children younger than age 4 years, with most cases occurring in the first two years of life. ^[13] The CNS neuroblastoma, FOXR2-activated (CNS NB-FOXR2) and CNS tumor with BCOR internal tandem duplication (CNS-BCOR) ^[11] are even less common than the ETMR and tend to occur in children older than 3 years.

The clinical and imaging features of atypical teratoid/rhabdoid tumors (AT/RTs) and other genetically defined central nervous system (CNS) embryonal tumors are briefly reviewed in this section.

Atypical teratoid/rhabdoid tumor

Signs and symptoms of AT/RTs depend on patient age, tumor location, and obstructive effects, and they may include lethargy, vomiting, and failure to thrive. Headaches and hemiplegia tend to be more common in children older than 3 years. Children with posterior fossa tumors may develop head tilt and/or cranial nerve palsies.

Imaging studies of AT/RTs usually reveal an aggressive-appearing lesion that enhances with contrast administration and shows restricted diffusion (see the image below). The latter feature is also a characteristic of other CNS embryonal brain tumors, especially the medulloblastoma when the cerebellum is involved. These tumors may contain areas of apparent necrosis or cystic change. Nodular leptomeningeal dissemination will be present in around 20% of patients.

Other genetically defined CNS embryonal tumors

Focal signs and symptoms of embryonal tumor with multilayered rosettes (ETMR); CNS neuroblastoma, FOXR2-activated (CNS NB-FOXR2); and CNS tumor with BCOR internal tandem duplication (CNS-BCOR) are related to the location in which the tumor arises; generalized effects due to mass and increased intracranial pressure may also occur. Seizures are not uncommon, and, in infants, there may be an increase in head circumference before closure of the cranial sutures. The tumors are usually large and contrast-enhancing (see the image below). The ETMR and CNS-BCOR may occur in the cerebellum, thus mimicking a medulloblastoma.

The differential diagnosis for embryonal tumors is specific to atypical teratoid/rhabdoid tumors (AT/RTs) and other genetically defined central nervous system (CNS) embryonal tumors.

Atypical teratoid/rhabdoid tumor

The aforementioned histologic variability displayed by AT/RTs creates significant overlap with the appearances of other embryonal tumors (see the section on Microscopic Findings).

In patients with posterior fossa tumors, medulloblastoma is the most important differential diagnosis to consider, especially the large cell/anaplastic histologic type. In addition, supratentorial CNS embryonal tumors (see below), choroid plexus carcinoma, and anaplastic ependymoma should also be considered. Immunohistochemistry for INI1 will be negative in AT/RTs, whereas this antigen will be reactive in the other tumors mentioned. ^[2]

Other genetically defined CNS embryonal tumors

In infants and very young children, the immature teratoma should be considered in the differential diagnosis of embryonal tumor with multilayered rosettes (ETMR). Immature teratomas typically contain extensive immature neuroepithelial features, including neural tube-like elements. A careful search for immature epithelial and mesenchymal elements can be helpful in the diagnosis of immature teratoma, especially in fetuses and neonates with congenital CNS tumors.

The differential diagnosis for CNS neuroblastoma, FOXR2-activated (CNS NB-FOXR2) and CNS tumor with BCOR internal tandem duplication (CNS-BCOR) includes pediatric-type diffuse high-grade gliomas, highly cellular ependymomas (ependymomas are discussed in a separate article), and glioneuronal and neuronal tumors such as ganglioglioma and central neurocytoma in tumors with extensive neuronal differentiation. Distinct clinical, histologic, and, more importantly, molecular features should help in correctly classifying these tumors.

Predominantly astrocytic features occasionally emerge after therapy in tumors that originally showed typical features of CNS embryonal tumors. Virtually any highly cellular, small blue cell process could enter the differential diagnosis if one considers a wide patient age group; thus, lymphoma, metastatic small cell carcinoma, or even a meningeal sarcoma should be excluded.

The gross features of atypical teratoid/rhabdoid tumors (AT/RTs) and other genetically defined central nervous system (CNS) embryonal tumors are briefly reviewed in this section.

Atypical teratoid/rhabdoid tumor

AT/RTs are similar to other CNS embryonal tumors in their gross appearance. They are typically soft, fleshy, and pink-gray with necrosis or hemorrhage. There may be evidence of multifocality as well as leptomeningeal spread. The authors have observed cases in which the tumors spread in a striking manner through the spinal foramina to extensively involve the pleura.

Other genetically defined CNS embryonal tumors

Embryonal tumor with multilayered rosettes (ETMR); CNS neuroblastoma, FOXR2-activated (CNS NB-FOXR2); and CNS tumor with BCOR internal tandem duplication (CNS-BCOR) are rather soft, gray-tan tumors with solid and sometimes necrotic-appearing areas. These masses may have a deceptively circumscribed border with adjacent brain tissue.

The microscopic features of atypical teratoid/rhabdoid tumors (AT/RTs) and other genetically defined central nervous system (CNS) embryonal tumors are briefly reviewed in this section.

Atypical teratoid/rhabdoid tumor

Although the rhabdoid cell is considered to be a unifying histologic feature of AT/RTs, several microscopic features significantly overlap with those of other embryonal tumors. ^{[1, 3]} Rhabdoid cells have vesicular nuclei that contain prominent nucleoli and, in the classic case, have an eosinophilic inclusion–like cytoplasm that is composed of intermediate filament whorls (see the images below).

Typical rhabdoid elements are usually combined with a variety of other morphologies, including so-called small blue cell types (see the following image).

The tumor cells may also appear pale, show cytoplasmic vacuolation, or present a spindle cell or fascicular growth pattern (see the image below). Moreover, the tumor cells may be arranged in cords or trabeculae that are embedded within a myxoid extracellular matrix.

In a series of six adult patients with sellar AT/RT, histologic features included an uncommon AT/RT vascular pattern that appears to be characteristic for sellar AT/RT: a hemangiopericytoma-like stag-horn vasculature within a dense, diffuse proliferation of jumbled cells and a small number of scattered rhabdoid cells (not shown). ^[2]

Embryonal tumor with multilayered rosettes (ETMR); CNS neuroblastoma, FOXR2-activated (CNS NB-FOXR2); and CNS tumor with BCOR internal tandem duplication (CNS-BCOR)

ETMR

The ETMR is defined histologically by the presence of immature-appearing multilayered rosette-like structures that occur within less cellular neuropil-like areas (see the following image).

The "multilayered rosettes" of ETMR may present immature tubular, trabecular, or sometimes papillary arrangement of neuroepithelial cells that resembles the appearance of the embryonic neural tube (see the image below). ^[15]  In the past, this feature led to diagnoses of medulloepithelioma.

In addition to the characteristic neural tube-like formations and areas of nonspecific embryonal-like histology, the ETMR may contain a range of neoplastic cells that appear to be differentiating along neuronal, astrocytic, oligodendroglial, and ependymal lines. Mitoses occurring at apparent luminal aspects of tubular structures are reminiscent of the developing neural tube. Multilayered rosettes were once considered to be a feature of a tumor called ependymoblastoma, which is now included under the definition of ETMR. ^[15] Such rosettes often have a central lumen lined by the apical regions of tumor cells that contain basal bodies of cilia (blepharoplasts). See the following image.

CNS NB-FOXR2

The CNS NB-FOXR2 is characterized by embryonal (small blue cell) morphology, with areas of differentiation including neuropil-like areas, neurocytic cells, and/or ganglion cells frequently seen (see the images below). In some cases, perivascular anuclear zones ("perivascular pseudorosettes"), nuclear palisades, and Homer Wright rosettes are identified. This group encompasses tumors previously classified as CNS neuroblastoma and CNS ganglioneuroblastoma in the 2007 and 2016 World Health Organization (WHO) Classification of CNS Tumors. ^{[10, 11]}

CNS-BCOR

The CNS-BCOR shows a predominantly solid growth pattern as well as demonstrates spindle to oval cells with round to oval nuclei and fine chromatin (not shown). Ependymoma-like perivascular pseudorosettes are a prominent feature of the tumor. Areas with glioma-like fibrillarity are commonly seen. CNS-BCOR tumors may display pseudopalisading necrosis and a rich capillary vascular network. However, true microvascular proliferation is not a histologic feature of these tumors. ^[16]

Immunohistochemical (IHC) features are specific to atypical teratoid/rhabdoid tumors (AT/RTs) and other genetically defined central nervous system (CNS) embryonal tumors.

Atypical teratoid/rhabdoid tumor

AT/RTs show variable multilineage antigen expression by IHC. Most tumors are reactive for epithelial membrane antigen (EMA) (see the first image below), vimentin (see the second image below), and smooth muscle actin (although desmin is usually negative) while being negative for germ cell markers.

Regional immunopositivity for glial fibrillary acidic protein (GFAP) or synaptophysin (in the presence of an embryonal component) may be seen. The defining AT/RT deletion of the INI1/SMARB1 gene correlates with a loss of INI1 protein immunoreactivity, ^{[3, 17]} in contrast with most other forms of CNS neoplasia, which show strong nuclear immunoreactivity for INI1 (see the following image).

Fluorescence in situ hybridization (FISH) may be useful diagnostically in some cases (see the image below).

Embryonal tumor with multilayered rosettes (ETMR); CNS neuroblastoma, FOXR2-activated (CNS NB-FOXR2); and CNS tumor with BCOR internal tandem duplication (CNS-BCOR)

ETMR

The embryonal component and the multilayered rosettes in ETMR are immunoreactive for nestin and vimentin. Small cell areas and true rosettes may focally express cytokeratins, EMA, and CD99 but are negative for neuronal and glial markers. The neuropil-like areas, including neoplastic neurons, are strongly immunoreactive for neuronal lineage markers (synaptophysin, neurofilament protein, Neu N). GFAP immunoreactivity is seen in scattered reactive-appearing glial cells as well as in some embryonal cells. LIN28A has been proposed as a diagnostic marker for ETMR. ^[13] These tumors show strong and diffuse cytoplasmic immunoreactivity for LIN28A, with more intense positivity in multilayered rosettes and small cell areas whereas neuropil-like areas show focal reactivity.

CNS NB-FOXR2

CNS NB-FOXR2 tumors are immunoreactive for OLIG2 and synaptophysin. Expression of other neuronal markers such as Neu N may be seen. These tumors are negative for GFAP. See the following images.

CNS-BCOR

CNS-BCOR demonstrates variable immunoreactivity for OLIG2 and Neu N, and it is negative for synaptophysin. Immunoreactivity for GFAP is focal to absent. EMA is either negative or shows faint granular cytoplasmic staining; however, the perinuclear dot-like immunoreactivity for EMA characteristic of ependymomas is not seen in CNS-BCOR. These tumors show diffuse strong nuclear positivity for BCOR protein. ^[16]

CNS embryonal tumors show a variable Ki-67 (MIB-1) labeling index, with areas of high cellularity and primitive cellular elements displaying elevated proliferation indices. ETMR, CNS NB-FOXR2, and CNS-BCOR show retained nuclear immunoreactivity for INI-1 and are thereby readily distinguishable from AT/RT, which is negative for this antigen.

The prognosis of atypical teratoid/rhabdoid tumors (AT/RTs) and other genetically defined central nervous system (CNS) embryonal tumors are specific to these tumors.

Atypical teratoid/rhabdoid tumor

Unfortunately, the median survival of patients with AT/RT is less than 2 years (around 17 months). Aside from the biologically aggressive nature of this tumor, radiation therapy cannot be safely used in the young patient population affected (age < 3 y). Chemotherapy with bone marrow or stem cell transplantation has had some success. ^{[18, 19]}

Other genetically defined CNS embryonal tumors

The underlying molecular genetic differences between genetically defined CNS embryonal tumors and medulloblastomas probably underlie the differences in prognosis and survival between these two neoplasms. CNS embryonal tumors have an overall poor survival (20-30% at 5 years) when compared with the 50-70% 5-year survival rate for medulloblastomas.

Hope for improved outcomes in young patients with CNS embryonal tumors has been provided by some protocols using myeloablative chemotherapy followed by hematopoietic stem cell rescue. Radiation therapy is typically reserved for older children, but its utility in younger children is under evaluation.

The prognosis for ETMR is poor, with reported survival averaging 12 months after combination therapies. Patients commonly experience local recurrences. CSF dissemination and systemic metastases may occur.

The prognosis for CNS NB-FOXR2 and CNS-BCOR is not well known; however, reports with limited clinical follow up suggest a uniformly poor prognosis. The identification of characteristic genetic alterations in these tumors indicate the possibility of developing targeted therapies.