Types of Tumours
Primary tumours
Tumours that start from cells that are normally found in the brain are called “primary”, as opposed those that may have started somewhere else in the body and spread to the brain.
The two main cell types in the brain are the nerve cells or “neurones” and the supporting cells called “glial cells”, of which there are a number of different subtypes. There are many more glial cells than neurones, and the neurones are genetically very stable rarely undergoing cell division in the adult, so tumours from glial cells are much more common than those from neurones.
Although clues as to the subtype might be apparent in the initial scans to know the diagnosis for sure usually requires some form of tissue sampling either via biopsy or surgical resection. More recently, as we learn more about the genetic variations of individual tumours, analysis of tissue by a neuropathologist is critical since it is the presence or absence of various genetic variations that help grade the tumours, identify the exact subtype, and predict how the tumour will behave and respond to treatment (so-called “prognosis”). Our knowledge of brain tumours is growing all the time and so the names and classification systems periodically get updated and more refined to reflect this increasing understanding.
1. Gliomas
Tumours that arise from the glial or supporting cells of the brain are called gliomas, and these are by far the most common type of primary tumour of the brain in adults. There are many different subtypes that are based and our understanding of the cell of origin as well as the genetic variations that the cell may undergo in the process of becoming a tumour. Many of these subtypes are further sub-divided based on how quickly growing or active they are. This is often referred to as the tumour grade. Although we use terms such as benign and malignant to refer to low grade or high grade gliomas respectively they mean different things for gliomas than for other tumours outside the brain since it is extremely rare for even the highest grade glioma to spread outside the brain to other parts of the body. This is where so-called “brain cancer” is very different from many other types of cancer which commonly spread through the body in a process known as metastatic spread. Nonetheless, gliomas do tend to infiltrate through the substance of the brain often along pre-existing anatomical pathways through the white matter.
i) Astrocytoma
This is the most common type of glioma and arises from the cells known as astrocytes due to the star-like nature of all of the appendages arising from the cell body.
These can be further graded from grade I lesions (which are very slow growing, benign, and often curable) through to grade IV lesions.
Grade I and II tumours are sometimes referred to as low grade glioma. Grade I lesions are somewhat less common in adults but can be curable by resection. Grade II lesions (confusingly sometimes also called “adult low grade glioma”) present a problem since over time they can become more aggressive and turn into a higher grade lesion. Various treatments may be able to slow this process down and delay the time it takes for progression to occur.
Grade III and IV tumours are referred to as high grade glioma. Grade III tumours are also known as anaplastic astrocytoma, and Grade IV is called glioblastoma (previously glioblastoma multiforme). These are more rapidly growing tumours which can be aggressive and challenging to treat but nonetheless are often able to be controlled for a period of time with appropriate combined treatment strategies.
ii) Oligodendroglioma
This type of glioma arises from cells known as oligodendrocytes due to the relatively fewer number of appendages to the cell body. Like astrocytoma it is graded into low and high grades, but as more has been learned about these tumours it is now recognised that there are in effect only two main subtypes: grade II and grade III (or low grade oligodendroglioma and anaplastic oligodendroglioma respectively). Generally speaking oligodendroglioma is said to have a better prognosis than astrocytoma since these tumours are often less aggressive, more amenable to treatment and more controllable over a longer period of time than an astrocytoma of the equivalent grade.
iii) Others
Less common gliomas include so-called ependymoma, choroid plexus tumours, and tumours of mixed or uncertain cell lines such as pineal region tumours. Like the more common types each of these needs to be carefully assessed to work out the best treatment strategy in the individual case.
An understanding of the genetic changes that a tumour cell has undergone is revolutionising our knowledge of brain tumours and the best treatments to offer. Often, we may describe the tumour by these variations so that although the above sub-typing is still used an individual tumour may also be given almost indecipherable names reflecting these genetic flags (for example “IDH-1 mutated, ATRX preserved, 1p-19q non co-deleted low grade glioma”). The important thing is to understand as much about an individual tumour as possible to help determine the best treatment in each case.
Treatment for gliomas can range across all of the methods described below including surveillance, biopsy, surgical resection, and adjuvant therapy including various types of radiotherapy and chemotherapy.
2. Meningiomas
These are also common among the primary tumours, and although classified as brain tumours they in fact arise from cells in the linings of the brain rather than in the brain itself. There are many different subtypes based on the appearance of the cells under the microscope, but most important is the grade of tumour reflecting how quickly the individual cells are growing.
By far the majority of meningiomas are benign or grade I tumours. These can often either be observed or cured by resection. For those that cannot be fully removed, partial removal can help control them and reduce their risks in the future.
Grade II tumours known as atypical meningioma are less common. They may look just like a grade I tumour on an MRI scan and the distinction is made by the pathologist on the specimens. The main difference is that the grade II tumours have a higher chance of coming back after resection and so need to be watched more closely. Sometimes, they may need additional treatment including radiotherapy.
Rarely, a meningioma may be grade III or so-called malignant meningioma. Despite this name these tumours rarely if ever spread outside the skull. The main problem with grade III tumours is the high likelihood of recurrence and further re-growth after treatment. They will almost always need additional treatments such as radiotherapy.
Treatment options for meningioma involve a choice between surveillance, surgery, radiotherapy (often given by stereotactic techniques), or combinations of these.
3. Rarer lesions
Schwannoma are not uncommon usually benign tumours that arise from the cells that line nerves, and so can occur inside the head although generally outside the brain from one or other of the cranial nerves. The most common types include vestibular schwannoma (otherwise known as acoustic neuroma) and trigeminal schwannoma. Treatment is usually a choice between observation, stereotactic radiotherapy, surgery, or combinations of these.
Neuronal tumours are rare but do concentrate in neurosurgical practice. Ganglioglioma is perhaps the most common and fits within the grade I or benign subtype of primary brain tumours in most instances. Another is the so-called DNET (which stands for dysembryoblastic neuroepithelial tumour) may produce troublesome epilepsy and require removal in order to control the seizures.
Tumour like lesions (e.g. hamartoma, cortical heterotopia, cortical tubers, and a number of others) can sometime masquerade as a more typical brain tumour. Many of these are completely benign and require either no treatment, observation, or treatment of symptoms only.
Secondary tumours
This term is used to refer to those tumours that have developed somewhere else in the body and then spread to the brain.
- Metastases (or metastatic tumours) refer to those tumours which have spread from their original site often via the blood stream to lodge in a separate part of the body. Many common cancers or so-called solid tumours can spread in this way and therefore involve the brain as part of the overall problem that the patient is facing.
Any cancer may spread but for various reasons some tend to more commonly travel to the brain than others. The more common types include cancers that begin in the lung, breast, skin (especially melanoma), kidney, and thyroid.
Sometimes the cancer is already known about and subsequently spreads to the brain, but in others the very first symptoms of the underlying disease may be from a metastatic deposit in the brain.
Depending on the individual case treatment of the brain involvement may be a crucial step in the overall treatment of the cancer in order to reduce symptoms, improve prognosis or longer term outcomes, and in some cases obtain tissue for a definitive diagnosis to be made.
Treatment may involve surgery to remove the individual metastases, especially if there is only one or few in number. Often other treatment such as radiotherapy will also be needed and increasingly given using specialised targeted techniques such as stereotaxy. In some situations, single dose one off stereotactic radiotherapy may eliminate the metastasis without the need for it to be physically removed.
- Lymphoma is a tumour of the blood that can involve any part of the body including the brain. If the brain is the sole site of disease the condition is often referred to as primary cerebral lymphoma. Treatment for this condition usually require chemotherapy and sometimes radiotherapy, but surgery for biopsy may be critical in establishing the diagnosis with the certainty that’s needed to start these sorts of treatment.
