The brain and spinal cord constitute the CNS, the system that complements the senses to perform their functions, control/manage the activity of other systems, builds memory, govern the behavior, and perform many other vital functions. The brain is protected by the skull and the spinal cord is protected by the vertebral column. The brain and spinal cord are covered by 2 membranes known as meninges that further protect the CNS from outside shocks. The space between the 2 meninges and hollow space within the brain (the ventricles) and spinal cord are filled with a liquid known as cerebrospinal fluid.
1) Cerebrum, the largest hemispherical part made-up of 2 halves, responsible for voluntary muscle movements, thought, emotion, memory, thinking, reasoning, and interpretation of sensory signals from the sensory organs;
2) Cerebellum, present below the cerebrum in the posterior fossa and help in maintaining balance and coordination in body movements; and
3) Brain stem, connects the brain to the spinal cord and have various centers for controlling vital functions like breathing, heartbeat, swallowing, gastric reflux, and others.
There are 12 cranial nerves that connect different body parts with the brain via brain stem and without the involvement of the spinal cord. The spinal cord consists of long nerve fibers that control the function of bowel, bladder, and muscles in the limbs.
This type of tumors arises from the glial cells, the neuron supporting cells, are known as gliomas. There are 3 types of glial cells and consequently 3 types of tumors arising from them as described below:
a) Astrocytomas: As the name indicate these types of tumors arise from the astrocytes, the supporting cells for neurons. The astrocytes form a scar tissue in response to an injury to the brain tissue and help in repairing the damage. Astrocytomas account for about 20% of all CNS cancers/tumors and can quickly spread throughout the CNS. They frequently involve nearby brain tissue and rarely spread outside of the CNS. Pilocytic astrocytoma, subependymal giant cell astrocytomas (SEGAs), diffuse astrocytomas, pleomorphic xanthoastrocytomas (PXAs), and Optic gliomas are some less aggressive forms of astrocytomas. Anaplastic astrocytoma and glioblastoma are aggressive forms of astrocytoma.
b) Oligodendrogliomas: This type of tumors arises from the oligodendrocytes, the supporting cells that synthesize a fatty substance known as myelin. The myelin covers and insulates the axon of various neurons and help in faster signal transmission. The aggressive form is known as anaplastic oligodendroglioma. Oligodendrogliomas account for about 2% of all CNS cancers/tumors.
c) Ependymomas: This type of tumors arises from the ependymal cells, the cells that line the ventricles and secrete CSF. Ependymomas generally spread via CSF route and may sometimes lead to hydrocephalus, a condition characterized by the ventricle’s enlargement due to blockage in the path of CSF. The aggressive form is known as anaplastic ependymoma. They account for about 2% of all CNS cancers/tumors.
This type of tumors arises from the meninges, the membranes that cover the brain and consist of the CSF within the space between them (the subarachnoid space). Meningiomas account for about 30% of all CNS cancers/tumors and are the most common brain tumors in adults. Although, the benign form is mostly observed, meningiomas may occur as aggressive form (anaplastic or malignant) that tend to grow and spread quickly.
This type of tumors arises from the neuroectodermal cells. Medulloblastoma is the most common type of embryonal tumor that arises in the cerebellum. These tumors generally involve nearby brain tissue and can spread very quickly via CSF route. They were referred to as primitive neuroectodermal tumors (PNETs) in the past. They are generally observed in children and very rare in adults. They account for about 2% of all CNS cancers/tumors.
This type of brain tumors involve both glial cells and neurons. They are generally benign. Examples of such tumors include dysembryoplastic neuroepithelial tumors (DNETs) and ganglioglioma. Gangliogliomas are more common in children than adults.
As the name indicate, these tumors arise from the germ cells in the brain. The presence of germ cells in the brain is abnormal and these cells are normally present in the egg cells in women and sperm cells in men. However, in some individuals, these cells may be present at certain abnormal locations within the brain (most common region of germ cells’ location in the brain include pineal gland or above the pituitary gland) and give rise to germ cell tumors. Germinoma, choriocarcinoma, embryonal carcinoma, teratoma, and yolk sac tumor are examples of germ cells tumors. These tumors are generally seen in children and are rare in adults. They can sometimes be diagnosed with the help of tumor markers present in the CSF.
Schwannomas or neurilemmomas are the type of CNS tumors that arise from the Schwann cells surrounding the cranial or spinal nerves. They account for about 8% of all CNS cancers/tumors. They are generally benign but can cause symptoms related to the nerve affected. For example, if they affect cranial nerve responsible for hearing, they can cause loss of hearing.
This type of tumors is believed to arise from the epithelial cells at the junction of the infundibular stalk and the pituitary gland (residual cells of the Rathke pouch). The tumor is generally slow-growing and is mostly associated with a proteinaceous fluid-filled cyst. Both tumor and cyst can press on the nearby structures causing symptoms. The pituitary gland and the optic nerve are mostly involved, and thus, these tumors are frequently associated with hormonal imbalance and vision problems. These are more common in children than adults.
This type of cancers arises from the bones at the base of the skull or at the lower end of the spine. They are usually slow-growing but can grow in nearby soft tissues. They have a high recurrence rate.
The NHLs that starts in the CNS are called primary CNS NHLs. The primary CNS NHLs generally originate from the B-cells and are generally aggressive. These cancers frequently invade nearby brain tissue and usually present as a multifocal disease. These are mostly observed in immunocompromised individuals, such as those with an organ transplantation or patients with AIDS. They account for about 3% of all CNS cancers/tumors.
Almost all pituitary tumors are glandular and known as pituitary adenomas. These tumors mostly start in the anterior lobe of the pituitary gland and are mostly benign. These tumors can cause a problem of hormonal imbalance (hyper- or hyposecretion of anterior pituitary hormones) or can press on other structures, such as optic chiasm. They account for about 10% to 15% of all CNS cancers/tumors. Pituitary adenomas are further classified as functional and non-functional tumors based on their ability to secrete hormones.
These tumors mostly arise in the cerebellum and may occur in some other locations like the brain stem, spinal cord, or cerebrum. Most cases of hemangioblastomas are associated with von Hippel-Lindau disease, a hereditary syndrome caused by a germline mutation in the VHL gene. The tumors are highly vascular and are usually associated with a fast-growing cyst which is responsible for early symptoms. These tumors mostly affect individuals aged between 30 to 40 years.
These tumors mainly include choroid plexus papilloma (benign form) and choroid plexus carcinoma (malignant/cancerous form). The choroid plexus carcinomas are mostly associated with Li-Fraumeni syndrome, a hereditary syndrome caused by a germline mutation in the TP53 gene. These are rare forms of CNS cancers/tumors and mostly affect children below the age of 12 years. These tumors mostly occur in lateral ventricles and may lead to blockage of CSF flow, and thus, hydrocephalus. They generally involve nearby brain tissue and can spread via CSF route.
The treatment approach for brain tumors mainly depend on the type and location of the tumor and hence diagnosis of the exact location and type of brain tumor is the key before starting the treatment. There are some other rare brain tumors not included in above list due to a low occurrence.
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