♦ Preoperative
Operative Planning
- Review imaging (magnetic resonance [MR] imaging)
Equipment
- See Chapter 194, Gamma Knife Radiosurgery for Arteriovenous Malformations
Anesthetic Issues
- See Chapter 194, Gamma Knife Radiosurgery for Arteriovenous Malformations
♦ Intraoperative
Frame Fixation
- Because of the anatomic distortions that are more or less inherent to all types of MR units, the target should be placed as near to the center of the frame as possible.
- With TN, ideal frame placement usually dictates that the base of the frame be at the level of the mouth in a neutral position; this will ensure that cranial nerve V’s root-entry zone is centered in the Z plane (approximately 100).
Stereotactic Magnetic Resonance Imaging Scan
- Magnetic resonance indicator box is placed
- Volumetric MR (1 mm, skip 0) (1-mm thick slides with no interstice interval) is performed (gadolinium may be useful to delineate vessel versus nerve) along with fast imaging employing steady state acquisition (optional) and sent via computer transfer to the gamma knife suite.
Gamma Planning—Image Definition and Treatment Planning
- Dose planning for TN requires that a single 4-mm collimator shot be used to place an oval of radiation on the nerve near the root-entry zone.
- Effort should be made to orient this oval in line with the nerve.
- The 50% isodose curve should just abut the brain stem (or be outside of it).
Gamma Planning—Dosimetry
- Seventy-five Gy (some surgeons prefer other doses, between 70 and 90 Gy) should be given to the 100% isodose line.
Closure
- Adhesive bandages are placed over the pin sites after cleaning with hydrogen peroxide and bacitracin or Betadine gel.
♦ Postoperative
- May discharge same day
- Continue medications for 1 month (average time to result), then wean off medications
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