| |
Brain
tumors are among the most difficult of cancers to treat. Hidden within the
skull and protected by the blood brain barrier that prevents toxins, including
chemotherapy, from reaching delicate brain tissues, the brain is less accessible
and less amenable to treatment than other organs.
Improvements in imaging techniques that allow us literally to look inside
the brain are allowing earlier detection and diagnosis of tumors. Radical
new approaches to treatment as well as refinements in more traditional therapies
are lengthening survival time for some brain tumor patients and offer hope
for more effective treatment options in the future.
Not all brain tumors are alike. Some are essentially benign and if removed
may never return. Others are aggressive and highly malignant and usually
return in the months following surgery. Tumors are normally classified according
to cell type; rather than referring to them as benign or malignant, physicians
generally prefer to talk about them as secondary tumors, which often have
a very good prognosis, and malignant primary tumors that tend to have a
poor outcome.
Tumors of the brain and spinal cord can occur at any age, and no specific
cause has been identified. They are most common in adults over age 50 and
in children. Brain and spinal cord tumors are the second most common form
of cancer in children, accounting for about one fifth of all childhood cancers.
Generally, children with brain tumors fare better than adults.
According to the American Brain Tumor Association, 180,000 people were diagnosed
with brain tumors in 2001. Of that number, more than 16,000 were diagnosed
with a malignant primary brain tumor, the most serious kind. The most common
malignant brain tumors, glioblastoma multiforme and anaplastic astrocytoma
have a poor prognosis because they tend to recur despite treatment. A great
deal of research is focused on these tumor types in an effort to improve
outcomes.
High-Tech Helpers
Sophisticated noninvasive imaging techniques that allow doctors to look
inside the brain include PET scans and magnetic resonance imaging (MRI).
An MRI can detect and pinpoint the position of a tumor, providing a three
dimensional image of the mass. An accurate picture of the tumor makes
it possible to target surgery and follow-up radiation to highly specific
areas.
Surgery to remove a brain tumor is the initial standard treatment. Often
the surgery can be performed through small holes drilled in the skull.
Unfortunately it’s never possible to remove the entire tumor. Surgery
to remove a breast lump takes out the lump plus a margin of healthy tissue
surrounding the lump to ensure that all the cancerous cells are removed.
Taking a margin of healthy brain tissue would destroy healthy brain cells
with unacceptable results for the patient. The challenge for the surgeon
is to remove as much of the lump as possible while leaving healthy tissue
intact. Surgery is then followed by radiation or sometimes chemotherapy
to try to eradicate the remaining tumor cells.
A new form of radiation delivery has shown promise in extending the life
of some brain tumor patients. The new technique, called GliaSite is currently
being tested in clinical trials at Stanford University Medical Center.
Rather than delivering radiation through the scalp and healthy brain tissue,
this technique uses a balloon attached to a catheter that is threaded
to the site to deliver radiation. It allows more controlled delivery of
higher doses of radiation directly to the area from which the tumor was
removed.
Traditional surgery and radiation therapy extends the life of patients
with invasive tumors by about a year. GliaSite has extended survival to
about 80 weeks. A number of other targeted approaches to radiation delivery
are being investigated at other medical centers.
Photodynamic Therapy (PDT) offers a different approach to targeting cancerous
cells left behind after surgery. Currently in Phase III clinical trials
at multiple sites, PDT uses red light at a specific wavelength to destroy
tumor cells that have been treated with a photosensitizing agent.
Patients are given a photosensitizing drug (porfimer sodium is the drug
most often used) which accumulates in tumor cells. Then a catheter carrying
optical fibers is introduced to the site from which the tumor was removed
during surgery. A red light generated by a laser is transmitted to the
optical fiber, destroying malignant cells in the proximity of the fiber.
The advantage of PDT is that it is relatively selective, killing tumor
cells but sparing healthy tissue, and it works regardless of tumor cell
type. There is no cumulative tissue damage, and the procedure can be repeated
if necessary.
Disadvantages of this approach include swelling in the brain that accompanies
the death of tumor cells. The therapy is usually done after surgery, using
the cavity left by the tumor removal to allow space for some of the swelling.
Another complication is the light sensitivity patients suffer after receiving
the photosensitizing drug. Eyes and skin must be vigilantly protected
from natural light for about 30 days. Patients need to wear protective
clothing and glasses and to stay out of sunlight for that period.
A number of other photosensitizing drugs are also being tested. More information
about clinical trials of PDT can be found at the National Cancer Institute’s
Cancernet Website (http:/cnetdb.nci.nih.gov/trialsrch.shtml)
A different solution in being pursued by researchers at Duke University
using genetic engineering to modify a poliovirus. The virus has shown
early success in treating brain tumors in mice.
Researchers chose the poliovirus because it selectively targets brain
cells, making it an excellent delivery system for a virus aimed at brain
tumor cells. To avoid the risk of causing polio, the poliovirus has been
combined with the rhinovirus, the virus responsible for the common cold.
The engineered virus was tested in mice with experimental brain tumors.
The tumors were destroyed within days after a single dose of the virus
replicated in the cancerous cells. Researchers now need to test both the
safety and effectiveness of this approach in humans, usually a lengthy
process.
Research into better ways to treat brain tumors continues on multiple
fronts. Better delivery systems for chemotherapy, more targeted approaches
to radiation therapy and recent breakthroughs in genetics raise hope for
better survival rates and cures. For brain tumor patients, the National
Cancer Institute is a valuable source of information about ongoing clinical
trials and the most recent treatment options.
REFERENCES:
“Better Treatments for an Enigmatic Disease,”
Harvard Health Letter, April 1998.
Edward E. Conway, Jr. et al, “Diagnosing and Managing Brain Tumors:
The Pediatrician’s Role,” Contemporary Pediatrics, November
1999.
Charles Eberhart, “Decreasing Incidence of Sudden Death Due to Undiagnosed
Primary Central Nervous System Tumors,” JAMA, December 26, 2001.
“Genetically Engineered Poliovirus Fights Brain Tumors,” Cancer
Weekly, June 5, 2001.
Edward Laws, Jr. “Central Nervous System Tumors: What Have We Learned
and Where Are We Heading?” Ca, November 1998.
Jeannette K. Lowry, “Brain Tumors in the Elderly: Recent Trends
in a Minnesota Cohort Study,” JAMA, December 23, 1998.
Troy May, “New Form of Brain-Tumor Radiation Promises to Extend
Life,” Silicon Valley/San Jose Business Journal, December 2001.
J. Netting, “Poliovirus Slaughters Brain Tumors in Mice,”
Science News, May 26, 2001.
Teresa Tarnowski et al , “Photodynamic Therapy: A Novel Treatment
for Primary Brain Malignancy,” Journal of Neuroscience Nursing,
December 2001.
“Technique Delivers Brain Tumor Chemotherapy Across Blood-Brain
Barrier,” Cancer Weekly, February 22, 2000.
|