Cancer – A Definition
Cancer is a collection of many related diseases that are all characterized by one common feature—the inappropriate and chaotic growth of cells. Under normal conditions, cells grow only when needed; for example, during the healing of a wound. However, when cells become cancerous, they lose the ability to be controlled by the body’s needs, and continue to grow. The mass of cells that results from this uncontrolled growth is called a “tumor.” While some of these tumors do not spread beyond their point of origin, others are capable of spreading large distances, often to other organs and tissues. This process is called “metastasis,” and tumors that metastasize are called “cancers”.
Brain tumors can either originate from within the brain (“primary brain tumors”) or from cancer cells that have metastasized from other organs or tissues (“secondary brain tumors”). Primary brain tumors can be derived from the over twenty different cell types that make up the brain. They are named and categorized by the type of cells that produce them. The most common forms of cancer that produce secondary brain tumors include lung, breast, kidney, and skin (melanoma) cancer. While primary brain tumors rarely metastasize outside of the central nervous system (“CNS”), they all have the propensity to spread within the normal brain, and are all, therefore, cancerous. However, primary brain tumors do vary significantly in terms of how rapidly they grow and spread. Thus, most primary brain tumors are described in terms of “grade.” Low-grade tumors tend to grow slowly and frequently remain dormant for long periods of time, while high-grade tumors grow and spread rapidly. The ability of all of these tumors to invade normal brain tissue means that surgery is rarely able to cure them, since it is generally not possible for the surgeon to remove all of the cancerous cells without removing an unacceptably large amount of normal brain tissue as well.
By contrast, metastatic brain tumors tend not to invade normal brain tissue, but rather push it aside as they continue to grow. This means that surgery and focused forms of radiation therapy, such as Gamma Knife, proton beam, and Stereotactic Radiosurgery, can often completely remove or destroy the metastatic tumor.
Brain Tumor Statistics
Each year, more than 190,000 people in the United States and 10,000 people in Canada will be diagnosed with a brain tumor. Over 40,000 will be primary brain tumors.
In the United States, the overall incidence of all primary brain tumors is 14.1 per 100,000 people per year. Primary brain tumors are the leading cause of solid tumor cancer deaths in children under the age of 20, now surpassing acute lymphoblastic leukemia (“ALL”), and are the third leading cause of cancer death in young adults ages 20 to 39. Although as many as 70% of children diagnosed with primary brain tumors will survive, they are often left with long-term side effects.
Over 150,000 secondary (“metastatic”) brain tumors occur yearly in the United States. Secondary brain tumors occur at some point in 10%-15% of people with cancer.
Primary Brain Tumor Types in Adults
As noted above, brain tumors are characterized by the presumed cell of origin. While there are over 120 different types of brain tumors, most occur from a very limited set of cell types. Listed below are the most common primary brain tumor types in adults.
Glial cells are the “connective tissue” of the brain, providing crucial support for the nerve cells that do the actual work of the brain. Glial cells come in several different types. The most common of these include astrocytes, oligodendrocytes, and ependymal cells. Each of these glial cell types can produce brain tumors, referred to as “gliomas.” Gliomas are the most common of primary brain tumors, and they are named for their cell of origin as follows:
Tumors derived from astrocytes:
AAstrocytomas represent the most common type of glioma. They develop from glial cells called astrocytes. They can arise anywhere in the brain and are most common in areas that have the largest number of astrocytes, namely the frontal, temporal, and parietal lobes of the brain. These tumors can be very low-grade (“pilocytic astrocytomas,” also called “grade I astrocytic tumors”), low-grade (“astrocytomas,” also called “grade II astrocytic tumors”), intermediate-grade (“anaplastic astrocytomas,” also called “grade III astrocytic tumors”) or high-grade (“glioblastoma multiforme,” also called “grade IV astrocytic tumors”). The risk of tumor growth, spread, and patient death increases as the tumor grade increases. Furthermore, a significant number of low-grade astrocytomas become a higher grade over time. Hence, even low-grade astrocytomas have the potential to have serious, malignant consequences, and these tumors should never be called or considered “benign.”
Pilocytic astrocytomas (“grade I”) are relatively uncommon in adults. A high percentage of these tumors can be effectively treated with surgery alone. However, a subset of these tumors (10%-20%) can spread and become aggressive. If you are diagnosed with such a tumor, it is important to have regular examinations by your physician to detect any possible recurrence.
Astrocytomas (“grade II”) tend to grow relatively slowly, and depending on their location, can initially be treated with surgical removal (“resection”) alone. However, while surgery can often remove much or all of the visible tumor, it should be remembered that small nests of tumor cells are almost always left behind, since these tumors can and often do invade normal brain tissue. Thus, it is sometimes necessary as well to treat these tumors with additional therapies, including radiation therapy and/or chemotherapy. The median survival rate with this tumor is in the range of five to seven years, which means that approximately 50% of patients with this disease are alive at least this long or longer. If you have a low-grade astrocytoma, you should be seeing your treating physician on a regular basis (every three to four months) with MRI scans, since early detection of tumor regrowth or progression to a higher tumor grade can often be effectively treated with surgery, radiation therapy, and chemotherapy.
Anaplastic astrocytoma (“grade III”) and glioblastoma multiforme (“grade IV,” also called “GBM”) are the more aggressive versions of astrocytomas. In addition, GBMs are the most common of primary brain tumors. Median survival for these tumors is in the range of two years for the former and one year for the latter. However, it should be kept in mind that these numbers reflect the results of studies that are rapidly becoming out of date, as significant advances in treating these diseases are regularly being made. Nevertheless, either type of tumor still carries with it a serious risk of death or disability, and patients with either tumor need to be closely followed by physicians who specialize in the treatment of these diseases. Treatment of either type of tumor generally involves surgery, radiation therapy, and chemotherapy. The advances in the treatment of these tumors that have occurred have resulted from studies of novel therapies in patients who have enrolled in clinical trials. If you have been diagnosed with either of these tumor grades, you should ask your physician if you qualify for a clinical trial in your area and if so, should seriously consider enrolling in one.
In children, astrocytomas can occur in some places that rarely harbor tumors in adults. These include the brainstem and the hypothalamus. These are structures near the base of the brain that control appetite, heart rate, breathing, sleep/wake cycles, eye movements, speech, and swallowing. Surgery is most likely not an option for these tumors, since they occur in portions of the brain that are essential for life. Treatment generally involves radiation therapy, although chemotherapy can be used as well.
Tumors derived from oligodendrocytes:
Oligodendrocytes are the cells that produce the “insulation” that surrounds nerve cells. When oligodendrocytes become cancerous, they can produce tumors called oligodendrogliomas. When these tumors take on more aggressive features, they are called anaplastic oligodendrogliomas.
Oligodendrogliomas tend to be slow growing, indolent tumors that constitute about 8% of all primary brain tumors. Because they grow so slowly, these tumors are also associated with a fairly long median survival of about 11 years. Depending on their size and behavior, these tumors can be treated by close monitoring, surgery, or radiation therapy. Anaplastic oligodendrogliomas have a shorter median survival (approximately three years)—a reflection of their more rapid growth. About 85% of these tumors have specific deletions of parts of their chromosomes (called “1p/19q deletions”), and when these deletions occur, the tumor is quite sensitive to chemotherapy. Hence, anaplastic oligodendrogliomas can be treated with surgery, radiation therapy, and/or chemotherapy. If you have an anaplastic oligodendroglioma, you should ask your doctor if you are eligible for any chemotherapy protocols in your community.
Tumors derived from ependymal cells:
Ependymal cells form the lining of fluid-filled spaces in the brain called the “ventricles.” When these cells become cancerous, they are called ependymomas, and when ependymomas become aggressive, they are called anaplastic ependymomas.
Ependymomas are relatively uncommon tumors in adults, constituting about 5% of primary brain tumors. They can occur in the brain or the spinal cord. They are generally slow growing. Treatment can consist of surgical removal of all visible parts of the tumor, followed by a close follow-up with brain or spinal cord MRI scans. Radiation therapy is sometimes added to reduce the chance of tumor recurrence. When the tumor cannot be completely removed, radiation therapy is generally prescribed to reduce the chance of tumor growth.
Anaplastic oligodendrogliomas constitute a minority of ependymomas (about 15%). These are more aggressive tumors, and generally require surgery, radiation therapy, and chemotherapy. If you are diagnosed with an anaplastic ependymoma, you should ask your doctor if you qualify for any clinical investigations in your community.
The meninges form a set of linings around the brain that protect it from mechanical injury. When tumors develop from the meninges, they are called meningiomas.
Meningiomas are the second most common form of primary brain tumor (after GBM), and increase in frequency with age. They typically grow very slowly, and can stop growing for periods of time altogether. It is estimated that approximately 20% of people above the age of 80 have meningiomas, and for the vast majority of these individuals, these tumors are of no concern or consequence. However, when meningiomas occur near important neurological structures, such as the optic nerves, brainstem, hypothalamus, or important blood vessels, they can compress and damage these structures and cause significant neurological disability or even death. In these cases,the treatment typically consists of complete surgical removal, when possible. If complete surgical removal is not possible, radiation therapy is often added to prevent or slow the growth of the remaining tumor. In some cases, it may be possible to treat problematic meningiomas without surgery, using forms of focused radiation therapy such as Gamma Knife, proton beam, or Stereotactic Radiosurgery. These treatments are generally given over several hours, and do not require extensive hospitalization.
Meningiomas can become aggressive, and when they do, they often invade normal brain tissue. These tumors are called “atypical meningiomas” and generally are treated with surgery and radiation therapy. The role of chemotherapy in atypical meningiomas is unclear, and is being investigated in several clinical studies.
For more information on primary brain tumors in adults, feel free to explore the following web sites:
American Brain Tumor Association
The National Brain Tumor Society
The Brain Tumor Funders’ Collaborative
A Federal government-sponsored clinical investigative consortia for brain tumor research
Primary Brain Tumors in Children
Several features distinguish brain tumors in children from those that occur in adults. First, some of the most common tumors seen in adults are relatively uncommon in children, and vice versa. Second, most primary brain tumors in adults occur in the cerebral hemispheres, which make up the majority of the mass of the brain. However, primary brain tumors in children tend to occur in the base of the brain, in the cerebellum, brain stem, and hypothalamus. An extensive discussion of childhood brain tumors is beyond the scope of this chapter, and if you are interested, you should explore several user-friendly websites, including The Pediatric Brain Tumor Foundation, and The Pediatric Brain Tumor Consortium.