Embryonal brain tumours develop from cells left over from when the embryo was forming in the womb, but have remained in the brain after the child has been born. The cells should be harmless, but can sometimes become cancerous. Unfortunately they then have a tendency to spread via the cerebrospinal fluid (CSF) to other parts of the brain and spinal cord, which together form the central nervous system (CNS).

These tumours were previously called primitive neuro ectodermal tumours (PNETs), but as scientists have studied them in more detail they have noticed important differences that have led to new sub-types being identified and hence re-named. This is an important step forward that enables clinicians to adapt existing treatments to the individual patient’s risk, and researchers to explore new ways to tackle these challenging tumours.

Embryonal tumours represent 20% to 25% of primary paediatric central nervous system (CNS) tumours, with medulloblastoma accounting for the majority of these tumours. If medulloblastoma are considered separately, the incidence of non-medulloblastoma embryonal tumours drops to around 2% of paediatric brain tumours.

Embryonal tumours are up to ten times more likely to be diagnosed in a child under 9 years old than they are to be diagnosed in an adult.

Embryonal tumours are grouped into a number of different tumour types, defined in the 2016 report by the World Health Organisation (WHO) entitled “Classification of Tumours of the Central Nervous System (CNS).”

All brain tumours are classified based on their characteristics identified by histology (how they look under a microscope), and by molecular profiling (information gained via genetic testing on a sample of the tumour removed during a biopsy).

Embryonal tumours include:

Medulloblastoma

There are a number of different types of medulloblastoma, so we have covered this tumour type in detail on a separate page of our website.

Atypical teratoid / rhabdoid tumour (AT/RT)

AT/RT tumours are covered in detail on a separate page on our website.

Embryonal tumour with multilayered rosettes (ETMR), C19MC-altered

This embryonal tumour is identified by amplification (extra copies of a gene) on the C19MC region on chromosome 19 (19q13.42). It tends to be diagnosed primarily in children under 4 years old, but is so rare that no incidence statistics are available.

The category of C19MC-amplified embryonal tumours has only been recognised by the World Health Organisation (WHO) since 2016 and includes those previously known as embryonal tumours with abundant neuropil and true rosettes (ETANTR), ependymoblastoma and medulloepithelioma (MEPL).

The most common location is in the cerebral hemisphere (at the front of the brain), but they have also been found in the brainstem and spinal cord. The most common age of diagnosis is 2 to 3 years old.

ETMR is classified as grade 4, meaning that it is an aggressive form of brain tumour, with a prognosis of approximately 12 months despite treatment with surgery and radiotherapy.

Embryonal tumour with multi-layered rosettes (ETMR), NOS

“NOS” stands for “Not Otherwise Specified”. This category represents tumours that look like an embryonal tumour with multi-layered rosettes (ETMR) under a microscope, but lack the C-19MC amplification. The NOS designation thus represents tumours that need to be researched in more detail before further refinements in classification can be made.

Medulloepithelioma

Medulloepitheliomas can form anywhere in the brain, spinal cord, or nerves just outside the spinal column. They are aggressive, fast-growing tumours that occur most often in very young children.

Prognosis is poor for patients with medulloepithelioma, with 5-year survival rates ranging between 0% and 30%.

CNS neuroblastoma

Neuroblastomas form in certain types of nerve cells (neuroblasts) left behind from a baby’s development in the womb. Neuroblastoma outside the CNS usually begins as a tumour in the abdomen that then spreads to other parts of the body. CNS Neuroblastomas can develop in the cerebral nerve tissue (at the front of the brain) or the layers of tissue that cover the brain and spinal cord (meninges).

Primary CNS neuroblastomas, meaning those that begin in the CNS, are extremely rare and so no definite prognosis is currently available, although they tend to be aggressive grade 4 tumours. If the tumour is large and has spread to other parts of the brain or spinal cord, the prognosis is poor. However if complete tumour removal is possible, and if the child is older than 3 years, including radiotherapy amongst the treatment options may improve the outlook.

CNS ganglioneuroblastoma
CNS ganglioneuroblastomas contains elements of both malignant neuroblastoma and benign ganglioneuroma. Neuroblastomas, ganglioneuroblastomas and ganglioneuromas all develop from primordial neural crest cells that form the sympathetic nervous system, which triggers our “flight or fight” stress response to perceived dangers.

Pure ganglioneuromas are often discovered incidentally because they are so slow growing that they rarely cause symptoms, but in this case they are combined with elements of an aggressive neuroblastoma, which makes their behaviour hard to predict.

CNS embryonal tumour, NOS

“NOS” stands for “Not Otherwise Specified,” which means that there is insufficient information to assign a more specific code or category. In some instances this refers to tumours that have not been fully tested for relevant genetic markers, but can also include tumours that have been tested, but do not show the diagnostic genetic alterations commonly seen within this tumour type.

In other words, “NOS” does not define a specific type of tumour; rather it designates a group of lesions that cannot be classified into any of the more narrowly defined groups. An NOS designation thus represents tumours that need to be researched in more detail before additional refinements in classification can be made.