And still they gazed, and still the wonder grew,
That one small head could carry all he knew.
(Oliver Goldsmith, The Deserted Village, 1809)
Brain tumor, like any other tumor, is an abnormal growth of cells that proliferate in a dysregulated manner (Quinn, Gonçalves, Sehovic, Bowman, & Reed, 2015). They may originate from intrinsic brain tissue, or primary brain tumors, or represent metastatic disease from cancers involving other parts of the body. Primary brain tumors can be classified on the basis of location, types of tissue involved, benign versus malignant, and other types.
The calvarium is divided by a tent-like extension of dura mater called the “tentorium cerebelli.” This is an important landmark as brain tumors are divided on the basis of their location in relation to tentorium cerebelli into supratentorial and infratentorial tumors. About 54% to 70% of childhood tumors are in posterior cranial fossa (infratentorial) and, owing to their anatomical location and limited space, may present as critical brain lesions capable of causing hydrocephalus and with the risk of herniation (Rorke & Schut, 1989). Additionally, tumors are further classified on the basis of which lobe of the brain is involved.
Type of Tissue Involved
Glial cells are nonneuronal cells of the central nervous system and generally provide support to neuronal tissue. Functions include structural support, nutritional support, and even degradation of dead neurons and clearance of infection. Tumors originating from glial cells are called “gliomas” and, on the basis of the type of glial cell involved, are further divided into subtypes:
Astrocytomas: These include astrocytoma, anaplastic astrocytoma, and glioblastomas, depending on the histological characteristics and aggressiveness of the tumor. Glioblastomas and anaplastic astrocytomas are the most aggressive and malignant primary tumors.
Oligodendrogliomas: These involve oligodendrocytes and are usually less aggressive than their astrocytoma counterparts. Calcifications are frequently seen histologically, and these tumors are more commonly associated with seizures.
Schwannomas: Schwann cells are involved in myelin sheath formation, and therefore tumors involving Schwann cells are called “schwannomas.” They are generally low-grade, benign tumors, but can cause significant local damage. Acoustic schwannomas are associated with a genetic disease, neurofibromatosis type 2, and can cause hearing loss, vertigo, and ataxia.
Ependymomas: Ependymal cells compose the epithelial-like lining of the ventricular system of the brain and spinal cord. Tumors involving these cells are called “ependymomas.” In the pediatric age group, these tumors are usually intracranial, in contrast to the adult population in which they are usually spinal. They can also be found supratentorially in brain parenchyma and are hypothesized to occur because of abnormal cellular migration during fetal development.
Meningiomas: These tumors originate from meningeal tissue of the dura mater, the covering of the brain and spinal cord. Meningiomas are the most common primary brain tumors in the adult population and are relatively rare in children. The majority of these tumors are low-grade, benign lesions; infrequently, they are high grade and, if so, are frequently related to prior exposure to radiation. Because they irritate the surface of the brain, that is, the cortex, regardless of histological grade, they can be a focus for seizures.
Benign Versus Malignant
Brain tumors may be slowly growing tumors, with a long insidious course, and may be found as incidental findings on autopsy. This is a classic history of benign or low-grade lesions. On the other hand, they may grow aggressively over a short period of time and present as critical malignant brain tumors with quickly progressing neurological deficits and intractable headaches. This is more characteristic of malignant or high-grade tumors.
The cranium has other structures and glands that also can contribute to brain tumors and present in a variety of different ways. These include the following:
Irrespective of their origin, primary and metastatic tumors may have similar presenting features, depending on location and mass effect. Usually, the clinical course may be insidious but can present acutely depending on location, complication, and mass effect. The management of brain tumors involves a multidisciplinary approach and is challenging in terms of morbidity and mortality.
Early Greek physician Hippocrates mentioned “abnormal brain growths” as early as 400 BCE. In 1930, Harvey Cushing, a pioneer of neurosurgery, was probably the first to report a large series of posterior fossa tumors using a pneumoencephalogram or x-ray (Cushing, 1930). Symptom onset is frequently insidious. However, owing to complications such as hemorrhage or increased intracranial pressure, patients may present acutely. The clinical manifestations of brain tumors reflect the location of the tumor. As a result, different types of brain tumors may present similarly, if they are in the same location.
In children with brain tumors, there can be extreme variability in time from onset of symptoms to diagnoses of brain tumor, as migraines or gastrointestinal illness are much more common. According to one study from the United Kingdom, symptom onset to diagnosis for children with brain tumors was an average of 3.3 months. Headache was the most common symptom (40%) and, at the time of diagnosis, children had an average of six signs and/or symptoms (Cushing, 1930). Headache is not always consistent in characteristics and quality (McKinney, 2004).
Mental status changes, especially memory loss and decreased alertness, may be a clue of frontal lobe tumor or hydrocephalus. Likewise, emotional changes or behavioral disturbances leading to depersonalization could present temporal or frontal lobe tumors. Loss of visual fields indicates a tumor along the visual pathway. In children, posterior fossa tumors can cause obstructive hydrocephalus, leading to irritability, gait changes (i.e., ataxia), headaches, and vomiting.
Limited intracranial space due to fusion of bones and compression of neurological structures and pathways play an important role in the manifestation of brain tumors. The pathogenesis includes direct invasion, infiltration and/or supplantation of normal parenchymal tissue with neoplastic and results in disruption of normal neurological function. Another critical aspect involves increased intracranial pressure by mass effect or impedance of normal cerebrospinal fluid flow, resulting in herniation or shifting of brain tissue, which can be fatal. Furthermore, new blood vessel formation (i.e., neovascularization) from tumor-secreting growth factors can disrupt the normal blood–brain barrier and result in worsening edema.
The cause of primary tumor is not understood and is likely related to a cascade of genetic mutations. There are few factors known to increase the risk of primary brain tumors, and they include the following:
Radiation exposure. Radiation therapy or even repetitive head computed tomographies (CTs) or skull x-rays are known to induce primary brain tumors after 20 to 30 years (Tsang, Laperrierre, & Simpson, 2007).
There are few inherited conditions that are associated with brain tumors, including tuberous sclerosis, neurofibromatosis, Von Hippel–Lindau syndrome, Li–Fraumeni syndrome, and Turcot syndrome (Frantzen, Klasson, & Links, 2012; Gold & Cohen, 2003; Itoh, Hirata, & Ohsato, 1993).
There is some association of infectious agents and brain tumors, specifically lymphomas, like Epstein–Barr virus and HIV in immunocompromised hosts (Delecluse, Feederle, O’Sullivan, & Taniere, 2007).
To date, the data to support or refute cellular phone use and risk of brain tumor is lacking.
In the United States, annual incidence of primary brain tumors range from 7 to 19 cases per 100,000 populations. According to the American Cancer Society, 22,800 cases of tumors involving the nervous system will be diagnosed by end of 2015 and the number of metastatic tumors will be even higher. Meningiomas are the most common of all primary brain tumors and comprise 34% of all primary brain tumors. Pituitary adenomas are frequent incidental findings on autopsy (Siegel, 2015).
Brain tumors are the second most common cause of cancer in children. They comprise 15% to 25% of all childhood malignancies and, unfortunately, have very high morbidity and mortality. The incidence rate of primary brain tumors is 3.6 per 100,000 children each year, higher than the international incidence rate, likely due to better registration (McCormack, 2015). In children below the age of 20 years, brain tumors are the second leading cause of cancer-related deaths (American Brain Tumor Association, 2014).
There is an age-based difference in both location and types of brain tumors. Posterior fossa tumors are more common in children. The risk of supratentorial brain tumors increases from early adolescence to adulthood. Low-grade gliomas are more common in the younger age groups as compared with high-grade gliomas, which are typically seen in the fifth decade of life.