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Although cancer is one of the biggest killers of Americans today, its origins remain largely a mystery.   Brain tumors, a form of cancer, are no exception.   And yet, even though nobody knows for sure why we  get brain tumors, scientists have successfully classified all the different types of brain tumors, and in the process they have accumulated some basic knowledge about which groups are most susceptible to which kind.   

One form of tumor is  the eningioma,  which begins in the brain and seems to run in families.   Meningioma is more likely to occur in people who've had radiation treatment on their scalps already.    If the tumor is less than 2 cm long,  it may never have any symptoms and be discovered during an autopsy after the patient is dead.  Larger tumors, on the other hand,  do have symptoms which differ by location and size. 

Primary tumors, which begin in the brain and tend to stay there, are relatively uncommon in comparison to metastatic tumors, which began as a cancer elsewhere in the body and have spread to the brain.  In fact, the metastatic cancers, taken together, are four times as common as all primary tumors put together.     Primary and metastatic tumors are also likely to have different causes, although they may produce similar symptoms.    

Although primary tumors occur in people of all ages, they are statistically more frequent in children and older adults.    After leukemia, brain tumors are the leading cause of cancer-related deaths among children under age 20 and among young men under 39.  In 2009, approximately 4,000 children under 20 were diagnosed with primary brain tumors, of which 2,875 were expected to be under age 15.For girls between 20 and 39, brain tumors continue to hold their own in the macabre race, but they fall to fifth place in cancer-related deaths, behind killing machines such as breast and ovarian cancers. 

Metastatic brain tumors, on the other hand, are more common in adults than in children, which is also to be expected because most forms of cancer occur in late adulthood, and the four forms that tend to metastasize to the brain  -- namely melanoma (skin cancer), lung cancer, colon cancer and breast cancer -- are all forms of cancer that occur only in adulthood.    These cancers develop for different reasons than do the primary brain tumors, and they spread to the brain and spinal chord by travelling through the bloodstream and the cerebrospinal fluid. 

Although children may be somewhat more at risk for primary brain tumors than adults, they are only a small fraction of the US population, so it is not surprising that the median age of people who are diagnosed for primary brain tumors is 57 years old.    In addition to being an illness of the young and the seniors, the brain tumor is also slightly more likely to be diagnosed among Whites (18.4 per 100,000) than among Hispanics  (17.38 per 100,000) or African Americans (16.34 per 100,000).  

The most common form of primary tumors is meningioma, or cancer of the meninges, and researchers are studying several theories about the origins of this disease.   One of their most interesting discoveries so far is that 40% and 80% of meningiomas contain an abnormal form of chromosome 22, which is normally involved in suppressing tumor growth. Moreover, meningiomas frequently contain within their genetic code extra copies of the platelet-derived growth factor (PDGFR) and epidermal growth factor receptors (EGFR) both of which may contribute to the growth of these tumors.

Other statistically significant risk factors for meningioma include previous radiation to the head, a history of breast cancer, or neurofibromatosis type 2 Finally, a previous experience of meningioma may make a second episode more likely.  Between 5% and 15% of patients, particularly those with neurofibromatosis type 2, experience a recurrence of the tumor after the first was removed.  In addition, the progesterone receptors on some meningiomas can interact with sex hormones such as progesterone, androgen, and occasionally estrogen, and this may have a consequence.  It appears that occasionally meningiomas may grow faster during pregnancy. More recently, scientists have identified abnormalities on chromosomes 10 and 17 among meningioma patients.  . However, what causes those abnormalities is still uncertain. According to the  Mayo Clinic, risk factors for a meningioma include:

• A history of receiving radiation therapy that involves radiation to the head
• Female hormones, which may play a role in the higher prevalence of meningioma among men than among women. Meningiomas are more common in women, leading doctors to believe that female hormones may play a role.
• An inherited nervous system disorder called neurofibromatosis type 2, which is associated with tumors on the vestibulocochlear or eighth cranial nerve, and may cause headaches, balance problems, vertigo, hearing loss, facial paralysis, and eventually deafness.