The optic chiasm lies just underneath the
HH. Doses higher than 10-12 Gy to this structure are very
likely to lead to visual loss. The other tissues that surround the
HH are much less sensitive to the toxicity of radiation and
tolerate doses of 17 to 20 Gy with little or no difficulty.
The work done by Dr. Rosenfeld in his large
series in Australia and our experience show that it is unnecessary
to remove the entire hamartoma. However, it is necessary
to remove the top of the lesion and to disconnect the side areas of
the mass. The implication is that if the center of the HH is the
target, the low dose that must be used will be inadequate while a
dose high enough to be effective risks damage to the optic chiasm
and visual loss.
Patients who unsuccessfully undergo Gamma
Knife treatment can have further treatment by a second Gamma Knife
treatment or by surgical resection. Undergoing radiosurgery does
not increase a patient's risk; an excellent outcome is still
possible.
The Barrow
Approach to Treatment:
Each patient is thoroughly reviewed by the
entire team, and treatment options are individualized to the needs
and desires of the patient and family. If Gamma Knife treatment is
selected, the treatment plan delineates the HH as the target and
the optic chiasm as the region of concern. The dose delivered to
the HH is at least 17 Gy to the roof and sides of the HH and no
more than 8 Gy to the optic chiasm.
Technique:
The patient has the stereotactic frame placed
on the skull under general anesthesia or under conscious
intravenous sedation (the patient is asleep for only 10 to 15
minutes) The choice depends on the needs of the patient. A
specialized MRI is obtained, reviewed, and transmitted to the Gamma
Knife Planning Computer.
When the plan for treatment is complete and
double-checked, the patient is brought to the Gamma Knife unit and
treated. During treatment 201 very thin beams of radiation
intersect only on the target (the hypothalamic hamartoma). The
treatment is painless. After treatment, the frame is removed
immediately and the patient is sent to recovery where he or she
remains until the effects of the anesthesia or medications used
during the procedure are no longer a concern.
Follow up is primarily by clinical
examination. Improvement in problems such as seizures is one
of the endpoints. MRI spectroscopy is recommended before and
6 months after treatment. This test best shows the effect of Gamma
Knife treatment on the HH.
