Faculty Bibliography

OBJECTIVE: One of the key limitations of gamma knife surgery arises from the radiation safety tolerance limit of the brainstem. The authors conducted an analysis of patients with intra-axial brainstem lesions and documented the incidence of adverse radiation imaging effects (ARIE) and new neurological deficits after gamma knife surgery. METHODS: Thirty-eight patients (39 lesions) with intra-axial brainstem astrocytomas or vascular malformations underwent gamma knife surgery during a 6-year interval. Brainstem exposure volume was calculated by subtracting the volume within the 12-Gy isodose line (12 Gray volume) from the prescription volume. ARIE was defined as a new parenchymal signal alteration on follow-up magnetic resonance imaging sequences. RESULTS: The average prescription volume was 1.46 cm, 12 Gy volume was 2.03 cm, and brainstem exposure volume was 0.57 cm. Seven (18.4%) patients developed ARIE. ARIE correlated only with the presence of new neurological deficits and age younger than 40 years. Three (7.9%) patients developed minor residual deficits without any ARIE. There was no mortality. CONCLUSION: Exposure of the brainstem to more than 12 Gy at volumes as low as 0.1 cm can produce ARIE and new neurological deficits. The tolerance of the brainstem to radiosurgery is related to patient age, lesion volume, and pathology. Analysis of the exposed volume of brainstem tissue may be useful in radiosurgical planning for individual patients.

OBJECTIVE: Tumor control, facial function preservation, and hearing preservation are important criteria for successful management of intracanalicular vestibular schwannomas, whether observation, microsurgery, or radiosurgery is chosen. We collected data prospectively to assess hearing preservation after intracanalicular vestibular schwannoma radiosurgery. METHODS: Between 1987 and 2003, 96 patients (65 men and 31 women) underwent gamma knife stereotactic radiosurgery (SRS) for intracanalicular tumors. The median patient age was 54 years (range, 22-80 years). Hearing was graded using the Gardner-Robertson (GR) and the American Academy of Otolaryngology-Head and Neck Surgery classifications. Dose planning was performed on intraoperative stereotactic images using multiple 4-mm isocenters. The median tumor volume was 0.112 mm3 (range, 0.05-0.447 mm3), and the median margin dose was 13 Gy (range, 10-18 Gy). RESULTS: The mean and median audiologic follow-up periods were 42 months and 28 months (range, 12-144 months), respectively. Serviceable hearing was preserved in 31 of 40 (77.5%) patients with initial American Academy of Otolaryngology-Head and Neck Surgery Class A hearing. Serviceable hearing was preserved in 40 of 79 (64.5%) patients with GR Grade I or II pre-SRS hearing. Ninety-two patients had GR Grade I, II, or III hearing before SRS, and GR Grade I, II, or III hearing was maintained in 78 patients (85%). Hearing grades improved in 7 patients. Facial and trigeminal nerve function was preserved in all patients. The tumor control rate (freedom from additional intervention) was 99.0% (95 of 96) at a median follow-up of 28 months (range, 12-144 months). One patient underwent tumor resection 18 months after radiosurgery. CONCLUSION: SRS is a minimally invasive first-line management option for patients with intracanalicular tumors and provides high rates of hearing preservation with minimal morbidity.

OBJECT: The recently introduced Leksell Gamma Knife (LGK) Perfexion is an entirely new system with a different beam geometry compared with the LGK 4C. The new Perfexion system has 192 cobalt-60 sources that are fixed on 8 sectors (each sector has 24 sources). Each sector can be moved independently of the others and can be set to 1 of 5 different positions: 3 positions defining collimator sizes of 4, 8, and 16 mm; an off position (sources are blocked); and a home position. The purpose of this study is to compare the dosimetric characteristics of the GK 4C and the Perfexion models. This comparison is important especially for the treatment of functional disorders when only a single shot with the 4- or 8-mm collimator is used. METHODS: A 160-mm-diameter spherical polystyrene phantom was used for all measurements and calculations. The irradiation geometry consisted of the placement of a single shot at the center of this phantom. Comparisons were made among different dosimetric parameters obtained from calculations performed using Leksell GammaPlan v. 8.0 and measurements performed using film dosimetry. The dosimetric parameters investigated were dose profiles for all collimators in all 3 stereotactic planes (x, y, and z) including the full width at half maximum and the penumbra for each profile, cumulative dose-volume histograms, the volume encompassed by the 50% isodose surface, the mean doses delivered to a defined matrix volume, and relative output factors for all collimator sizes. RESULTS: There was excellent agreement between the dosimetric parameters of GK 4C and Perfexion for the 4- and 8-mm collimators. CONCLUSIONS: The results of this study suggest that consistent treatments of functional disorders will be delivered using either GK 4C or Perfexion.

OBJECT: Schwannomas from the motor cranial nerves controlling eye movement are rare. The authors evaluated the role of Gamma Knife surgery (GKS) in the management of schwannomas originating from cranial nerves III, IV, and VI. METHODS: Over a 7-year period, 8 patients with schwannomas originating from the oculomotor (2 patients), trochlear (5 patients), or abducent (1) nerve underwent GKS. The mean patient age was 46.1 years (range 19-59 years). The presenting symptoms included diplopia in 5 patients, ptosis in 1 patient, ophthalmoplegia in 1 patient, and headache in 1 patient. Two patients had a history of neurofibromatosis Type 2. Gamma Knife surgery was performed as primary management in 7 patients and after prior resection in 1 patient. The median and mean tumor volumes were 0.22 and 1.32 cm(3) (range 0.03-7.4 cm(3)). A median margin dose of 12.5 Gy (range 11.0-13.0 Gy) was prescribed to the tumor margin. Clinical and imaging follow-up data were available for all 8 patients. RESULTS: Magnetic resonance imaging showed tumor regression in all patients. The progression-free period varied from 4 to 42 months, with a mean of 21 months. Over a mean of 23 months, 4 of the 5 patients with a trochlear schwannoma and symptoms of diplopia noted symptomatic improvement. No improvement was noted in the 2 patients with oculomotor nerve palsies. Headache was improved in the 1 patient with an abducent neuroma. CONCLUSIONS: Gamma Knife surgery is an effective and minimally invasive approach capable of inactivating schwannomas originating from the oculomotor, trochlear, and abducent nerves. Accompanying trochlear function may improve. Longer follow-up and larger patient samples are needed to confirm the authors' initial observations.