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  • Pediatric brain tumors are masses of abnormal cells that generally occur in children, and

  • result from the uncontrolled growth of those cells within the brain.

  • OK - let's start with some basic brain anatomy.

  • First off, there's the cerebral cortex which is the part of the brain that's supratentorial

  • or above the tentorium, and the cerebellum, which is infratentorial or below the tentorium.

  • And the brain has four interconnected cavities called ventricles, which are filled with cerebrospinal

  • fluid - a fluid that helps provide buoyancy and protection, as well as metabolic fuel

  • for the brain.

  • Highest up, are two C-shaped lateral ventricles that lie deep in each cerebral hemisphere.

  • The two lateral ventricles drain their cerebrospinal fluid into the third ventricle, which is a

  • narrow, funnel-shaped, cavity at the center of the brain.

  • The third ventricle makes a bit more cerebrospinal fluid and then sends all of it to the fourth

  • ventricle via the cerebral aqueduct.

  • The fourth ventricle is a tent-shaped cavity located between the brainstem and the cerebellum.

  • After the fourth ventricle, the cerebrospinal fluid enters the subarachnoid space, which

  • is the space between the arachnoid and pia mater, two of the inner linings of the meninges

  • which cover and protect both the brain and the spine.

  • So this makes it possible for cerebrospinal fluid to also flow through the central canal

  • of the spine.

  • Now, focusing in on cells within the brain - there are many different types with specialized

  • functions.

  • For example, neurons communicate neurologic information through neurotransmitter regulated

  • electrical impulses.

  • Then there are cells that secrete hormones into circulation and regulate the functions

  • of other cells throughout the body.

  • These cells are found in glands, like the supratentorial pineal gland which is located

  • just behind the third ventricle.

  • Or the infratentorial pituitary gland located near the front of the third ventricle.

  • There is also a category of cells called neuroglial cells that help support brain homeostasis,

  • and neuronal functions.

  • These include astrocytes which have cellular processes coming off their cell body, giving

  • them a star-shaped appearance.

  • Astrocytes are found throughout the brain and spinal cord, and their main roles include

  • maintaining the blood-brain barrier, providing nourishment to neurons, and recycling neurotransmitters.

  • Ependymal cells are also neuroglial cells, and they're cuboidal-to-columnar - so square

  • to rectangular shaped - ciliated cells that line the ventricles and central canal.

  • One of their main roles is to regulate the circulation of cerebrospinal fluid.

  • Some brain cells have a limited ability to be replaced, especially during injury, and

  • they do it by having undifferentiated stem cells - called embryonic stem cells - in the

  • brain activate and mature into a specialized cell.

  • Now, a tumor develops if there's a DNA mutation in any of these cell types that leads to uncontrolled

  • cell division.

  • Typically these are mutations in proto-oncogenes which results in a promotion of cell division,

  • or mutations in tumor suppressor genes which results in a loss of inhibition of cell division.

  • You can think of proto-oncogenes as the accelerator or gas pedal and tumor suppressor genes as

  • the brakes.

  • Too much acceleration or an inability to brake can lead to runaway cell division.

  • As a result, the mutated cells can start piling up on each other and can become a tumor mass.

  • Some of these tumors are benign and stay well contained or localized.

  • But some become malignant tumors or cancers, and these are the ones that break through

  • their basement membrane and invade nearby tissues.

  • Malignant tumor cells can get into nearby blood or lymph vessels, and travel from the

  • primary site to establish a secondary site of tumor growth somewhere else in the body

  • - and that's called metastasis.

  • Brain tumors can be categorized by their primary site location as either supratentorial, or

  • infratentorial tumorsthough some tumors can form in either.

  • They are typically named by the cell type involved, so for example an astrocytoma is

  • a tumor formed by mutated astrocytes.

  • But their severity is classified, or graded by the World Health Organization's (WHO)

  • scale.

  • The scale goes from I to IV based on the morphologic and functional features of the tumor cells;

  • a grade IV tumor being the most abnormal looking cells that also tend to be the most aggressive.

  • But not all tumors have all four grades because some tumors are basically always more benign,

  • whereas others are more aggressive.

  • So let's start with tumor types that are generally infratentorial tumors, because they

  • make up the majority of pediatric brain tumors.

  • Overall, the most common malignant tumor is a medulloblastoma, which typically forms in

  • or around the cerebellum, adjacent to the fourth ventricle.

  • Medulloblastomas originate from embryonic stem cells and they tend to be extremely aggressive.

  • One relatively unique feature is that they metastasize through the cerebrospinal fluid

  • in a process called drop metastasis where the tumor spreads to the base of the spine.

  • Because of this, medulloblastomas are typically only classified as grade IV.

  • Histologically, medulloblastomas often have a feature called Homer-Wright rosettes, which

  • are dense tangles of neurons and neuroglial cells, surrounded by ring-like structures

  • formed by tumors cells.

  • Now, while medulloblastoma may be the most common malignant tumor, the most common primary

  • pediatric tumor is a type of astrocytoma called a juvenile pilocytic astrocytoma.

  • Because astrocytes are found through the brain and spinal cord, astrocytomas can form throughout

  • those locations, but juvenile pilocytic astrocytomas are mostly infratentorial tumors in the cerebellum

  • or near the brainstem.

  • And while astrocytomas can be graded I through IV, juvenile pilocytic astrocytomas are only

  • grade I because they are generally benign and slow-growing.

  • Histologically, they can have cysts, or sacs filled with fluid; bodies of granular material;

  • and Rosenthal fibers, which are fibers that clump together in the cytoplasm of the astrocyte

  • and look a bit like a worm or a corkscrew.

  • Among the fibers is the structural protein glial fibrillary acidic protein that is typically

  • found in astrocytes.

  • Finally, another common infratentorial tumor is an ependymoma.

  • Now, again, they can form in the brain and spinal cord because that's where ependymal

  • cells are found, but pediatric ependymomas tend to form in the fourth ventricle.

  • Now, there are a few types of ependymomas and they're WHO graded I through III.

  • In grade II, classic ependymomas, tumor cells have a regular, round to oval nucleus.

  • Histologically, a prominent feature are their perivascular pseudorosettes, which are ring-like

  • structures formed by tumors cells with rod-shaped ependymal processes, like cilia, surrounding

  • a centralized blood vessel.

  • Ok, so, now let's focus on supratentorial pediatric tumors.

  • The most common is a craniopharyngioma, which forms near the pituitary gland.

  • During development, some cells migrate down from the brain and form the posterior pituitary;

  • while some cells migrate up from the back of the throat, called the oropharynx, first

  • forming the Rathke's pouch and then eventually forming the anterior pituitary gland.

  • Now, remnants of Rathke's pouch that do not mature can form craniopharyngiomas.

  • They're typically only classified as WHO grade I tumors because they are typically

  • benign and slow growing.

  • Histologically, it's a pretty complex tumor with lots of prominent features including

  • multiple cysts filled with a thickmotor-oil”-looking fluid; and cells that stratify, or arrange

  • themselves in layers.

  • The innermost layer tends to have loosely packed cells that retain their nuclei despite

  • being filled with keratin, a strong protein, creating what's called a “wet keratin

  • appearance.

  • A rare supratentorial pediatric tumor is a pinealoma, which forms in the region of the

  • pineal gland.

  • Pinealomas primarily emerge from the endocrine cells of the pineal gland, and they can be

  • grade I through IV.

  • Histologically, prominent features include large, round tumor cells resembling germline

  • tumors; or small cells forming Homer-Wright rosettes.

  • Now, the most common symptoms of brain tumors include headaches, nausea, vomiting, and seizures

  • - and they are a result of the compression and destruction of healthy brain tissue.

  • In addition, it's important to consider the cell type that's involved.

  • So, for example, a pinealoma may lead to increased secretion of the hormone beta human chorionic

  • gonadotropin that can cause an early onset of puberty.

  • In addition, as the tumor grows in size, it can compress nearby cells and structures,

  • interrupting their normal functions.

  • For example, as pinealomas, medulloblastomas, and ependymomas enlarge - the mass of the

  • tumor can compress nearby ventricles blocking the flow of cerebrospinal fluid which causes

  • swelling, called hydrocephalus.

  • Generally, the diagnosis of central nervous system tumors includes medical imaging, like

  • CT scans but more commonly MRIs.

  • But definitive diagnosis needs to be made based on the histologic and molecular characteristics

  • of a tissue biopsy.

  • Treatments depend on the tumor type, grade, and symptoms.

  • And can include surgical removal, radiotherapy, or chemotherapy - frequently in a combination.

  • But specific courses of treatment are guided by the molecular characteristics of the tumor

  • based on the biopsy.

  • Finally, the chance of recurrence gets higher in high grade tumors and in tumors that have

  • not been fully removed or destroyed.

  • Ok, quick recap: Pediatric brain tumors can be infratentorial and supratentorial and form

  • from a variety of cells.

  • Tumor types are classified using WHO grading based on histologic and functional features.

  • Diagnosis includes medical imagining, with a definitive diagnosis being made with a tissue

  • biopsy.

  • Treatments are largely dependent on the molecular characteristics and tumor grade, and can incorporate

  • surgical removal and some combination of radiotherapy

  • and chemotherapy.

Learning medicine is hard work!

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B2 H-INT US tumor brain ventricle fluid grade pediatric

Pediatric brain tumors - causes, symptoms, diagnosis, treatment, pathology

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    keep seeing   posted on 2019/04/26
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