Brain tumors are classified based on numerous factors. These may be related to the exact location of the tumor, the nature of the tissues that have turned malignant or other factors.
Classification of brain tumors
Brain tumors are classified based on whether they are malignant (cancerous) or benign (non-cancerous).
Brain tumors are also classified based on whether they have originated within the brain tissue or spread from cancers elsewhere. These are respectively known as primary brain tumors or metastatic tumors.
The most common basis for classification is based on histopathology of the tumor. This is essentially the nature of the tissues from which the cancer has originated that help neurologists classify the tumor.
The World Health Organization in 1993 laid out an uniform classification of brain tumors. The classification system is based on the principle that the abnormal cell growth that leads to the brain tumor behaves in a manner that is determined by the cell of origin and sometimes location.
The classification also states that the therapy plan, as well as the prognosis of the tumor, depends upon the exact histopathological classification of the tumor. (1, 2)
Grading of brain tumors
Once tumors are classified they need to be graded. There are numerous grading systems that are based on the microscopic appearance of the tumor. The grading of a single tumor, however, may vary with different grading systems.
Thus, while diagnosing specification of the grading system is of vital importance. According to the World Health Organization, the St. Anne/Mayo grading system best correlated with the predictability of survival of a brain tumor compared to the earlier used Kernohan grading system.
The Kernohan grading system can be used on invasive tumors of astrocytic tumors. It is similar to WHO grading system. (2)
Simply explained grading may be shown as: (3)
- Grade I – This is the lowest grade. This means the tumor grows slowly. The cells under the microspope appear almost normal. This type of tumor usually does not spread and may be removed by surgery.
- Grade II – The tumor grows slowly but may spread or recur after therapy. These may transform into higher grade tumors.
- Grade III – These are fast growing tumors that may spread. The cells do not resemble normal cells.
- Grade IV – These are also fast growing and spreading tumors. The tumors may be surrounded by a circle of dead necrotic tissues. They are difficult to treat.
Examples of brain tumor classification
In adults, the commonest brain tumors are either gliomas or meningiomas. Gliomas originate from cells called glial cells. These include a subclassification of cells called astrocytes (with tumors nomenclatured as astrocytomas), oligodendrocytes (called oligodendral tumors) and ependymal cells. Thus there are three major types of gliomas (1)
- Astrocytic tumors that may be further classified as –
- Astrocytoma - non-cancerous, WHO grade II
- Non invasive astrocytomas WHO grade I
- brain stem
- Cancerous (anaplastic astrocytomas) WHO gade III These may further be classified as –
- brain stem
- Glioblastomas multiforme (WHO grade IV) that are the most aggressive forms of primary tumors. There are variants of this form called gliosarcomas.
- Oligodendroglial tumors that may be made up of a combination of astrocytes and oligodendrocytes. These are thus called mixed gliomas. These may be classified as Oligodendroglioma (WHO grade II) or Anaplastic (malignant) oligodendroglioma (WHO grade III).
Other types of common brain tumor in adults are meningioma and schwannoma. These affect people aged between 40 and 70 years and are usually benign. Meningiomas affect women more commonly while schwannomas may affect both sexes alike.
The tumors may, however, lead to severe complications due to compressive symptoms on other vital and sensitive areas of the brain leading to life threatening or fatal complications. Some of these tumors may also be cancerous and aggressive. (1)
There are also other types of brain tumor, but these are rarer. These include:-
- ependymomas (WHO grade II)
- anaplastic ependymoa (WHO grade III) or Myxopapillary ependymoma or subependymoma
- primary central nervous system lymphomas
- pituitary (adenomas, carcinomas, cranipharyngiomas) and pineal gland tumors (Pineocytoma, Pineoblastoma or Mixed pineocytoma/pineoblastoma)
- primary germ cell tumors in the brain
There are also tumors that may affect the Choroid plexus like papillomas or carcinomas, Gangliocytoma, Olfactory neuroblastoma, Neuroblastoma, Retinoblastoma etc. (1, 2)
Stage of brain tumor
After classification of the tumor the patient’s stage of the cancer could be determined for appropriate treatment planning.
For staging the patient needs to be assessed clinically as well as radiologicaly with imaging tests. These determine the extent of spread of the tumor as well as its aggression. Staging determines the prognosis of the cancer as well and provides uniformity of care.
Staging however, is different in other cancers compared to those in central nervous system tumors since most of the other tumors spread rapidly throughout the body.
Cancers and tumors in the brain on the other hand, almost never metastasize. Their prognosis, however, may still be poor because of their ability to grow and pressurize vital areas of the brain to impair functions.
As a consequence of the lack of spread of the CNS tumors there are no formal systems that outline staging of brain tumors.
Factors that may help determine the prognosis of brain tumors in ways akin to staging systems include the patient’s age, level of interference with normal brain activities and functions, type, location, size and grade of the tumor.
Other factors include the resectibility of the tumor. This means that a tumor that can be largely removed by brain surgery carries a better prognosis or outcome. If the tumor has spread to other parts of the brain through the cerebrospinal fluid (CSF that bathes the brain and spinal cord at all times) or has spread to other parts of the body the prognosis is usually poor. (4)