Faculty Bibliography

To determine the accuracy of magnetic resonance (MR) imaging in comparison to cerebral angiography after radiosurgery for an arteriovenous malformation (AVM), the authors reviewed the records of patients who underwent radiosurgery at the University of Pittsburgh Medical Center before 1992. All patients in the analysis had AVMs in which the flow-void signal was visible on preradiosurgical MR imaging. One hundred sixty-four postradiosurgical angiograms were obtained in 140 patients at a median of 2 months after postradiosurgical MR imaging (median 24 months after radiosurgery). Magnetic resonance imaging correctly predicted patency in 64 of 80 patients in whom patent AVMs were seen on follow-up angiography (sensitivity 80%) and angiographic obliteration in 84 of 84 patients (specificity 100%). Overall, 84 of 100 AVMs in which evidence of obliteration was seen on MR images displayed angiographic obliteration (negative predictive value, 84%). Ten of the 16 patients with false-negative MR images underwent follow-up angiography: in seven the lesions progressed to complete angiographic obliteration without further treatment. Exclusion of these seven patients from the false-negative MR imaging group increases the predictive value of a negative postradiosurgical MR image from 84% to 91%. No AVM hemorrhage was observed in clinical follow up of 135 patients after evidence of obliteration on MR imaging (median follow-up interval 35 months; range 2-96 months; total follow up 382 patient-years). Magnetic resonance imaging proved to be an accurate, noninvasive method for evaluating the patency of AVMs that were identifiable on MR imaging after stereotactic radiosurgery. This imaging modality is less expensive, more acceptable to patients, and does not have the potential for neurological complications that may be associated with cerebral angiography. The risk associated with follow-up cerebral angiography may no longer justify its role in the assessment of radiosurgical results in the treatment of AVMs.

PURPOSE: Although radiosurgery is effective in obliterating the pathologic vessels of intracranial arteriovenous malformations (AVM), the relationships of both dose and volume to obliteration have not been well defined. METHODS AND MATERIALS: The results of radiosurgery in 197 AVM patients with 3-year angiographic follow-up were analyzed. Volume varied from 0.06-18 cc (median: 4.1 cc), and minimum target dose (Dmin) varied from 12.0-25.6 Gy (median: 20.0 Gy). RESULTS: Follow-up angiography revealed complete AVM obliteration in 142 out of 197 patients (72%). The targeted AVM nidus failed to obliterate in 20 patients (10%), but in-field obliteration was complete in the remaining 35 patients (18%) discovered to have residual untargeted AVM nidus. Multivariate logistic regression analysis of in-field obliteration revealed a significant independent correlation with Dmin (p = 0.04), but not with volume or maximum dose. A sigmoid dose-response curve for in-field obliteration was constructed that significantly differed from the dose-volume-response relationships that would have been expected from overall obliteration data. CONCLUSIONS: The success rate for in-field obliteration of AVM after radiosurgery depends on Dmin but does not appear to change appreciably with volume or maximum dose. Success rates for complete obliteration additionally are limited by problems defining the complete AVM nidus.

BACKGROUND: Several surgical approaches have proven effective in the management of colloid cysts. Cited disadvantages of the transventricular route are its difficulty in patients with small ventricles, and the risk of postoperative seizures; advantages include the avoidance of interhemispheric retraction and venous injury, and callosal section in the transcallosal route. METHODS: We retrospectively evaluated patient outcomes after minimally invasive transventricular microsurgical resection. Twenty patients underwent computed tomography-guided stereotactic resection of a colloid cyst based on trajectory planning through the middle frontal gyrus. The mean patient age was 38 years (range, 14-65 years). The colloid cyst was discovered incidentally in one patient; two patients presented in coma. Fourteen patients (70%) had preoperative hydrocephalus. RESULTS: Total or near-total (only a small remnant of cyst wall left attached to the fornix or thalamostriate vein) cyst removal was achieved in all patients. Mean follow-up after surgery was 4.3 years (range, 0.5-11 years). All patients had an excellent outcome (100%) with return to full function or employment status. Postoperative morbidity occurred in one patient (5%) who sustained a small caudate nucleus hemorrhagic contusion associated with temporary hemiparesis. A single postoperative seizure occurred in one patient 5 months after surgery; no patient developed a persistent seizure disorder. CONCLUSIONS: This technique relies on the use of a precisely placed limited craniotomy, a small cortical opening (10-20 mm), a precise trajectory to the foramen of Monro, and standard microsurgical instruments. Stereotactic transventricular microsurgical resection provided safe and effective management of patients with colloid cysts, even in the absence of hydrocephalus.

We describe a technique of stereotactic frame-based biopsy in young children who have open fontanels and a deformable skull. A 5-month-old girl with a growing lesion in the right thalamus and basal ganglia underwent stereotactic biopsy, which disclosed an anaplastic astrocytoma. To avoid insertion of the four stereotactic frame fixation pins through the infant's skin and into bone, the pins were advanced into the hollowed end of rubber tops obtained from Vacutainer blood sampling tubes. The pressure applied to the skin was diffused through the rubber onto a wide skin surface, obviating skin puncture or bone deformation. This technique provided firm head fixation, and target accuracy was confirmed on post-operative imaging. This technique is safe and should permit use of conventional stereotactic techniques in young infants.

PURPOSE: To define changes in treatment technique for vestibular schwannoma radiosurgery and to relate them to changes in outcome, a large single institution experience was reviewed. METHODS AND MATERIALS: Two hundred seventy-three patients with unilateral vestibular schwannomas underwent Gamma knife radiosurgery: 118 with computed tomography (CT) treatment planning during 1987-1991, and 155 with magnetic resonance imaging (MR) treatment planning in 1991-1994. Mean treatment parameters differed between the CT and MR groups: minimum tumor dose (D(min)) was 17 vs. 14 Gy, number of isocenters was 3.4 vs. 5.8, and volume was 3.5 vs 2.7 cc., respectively. RESULTS: The actuarial 7-year clinical tumor control rate (no requirement for surgical intervention) for the entire series was 96.4 +/- 2.3%, with a radiographic tumor control rate of 91.0 +/- 3.4%; these rates were similar for the CT and MR groups. Significantly lower rates of postradiosurgery facial, trigeminal, and auditory neuropathy were observed in the MR group compared to the CT group. Multivariate analyses found significant independent correlations of increasing rates of facial and trigeminal neuropathy with increasing transverse tumor diameter and D(min), as well as with CT treatment planning (compared to MR). Decreased hearing was similarly correlated with diameter and CT planning but not with D(min). CONCLUSIONS: Changes in radiosurgery technique and the use of lower doses improved the outcome after vestibular schwannoma radiosurgery by decreasing cranial neuropathy rates. MR-based treatment planning appears to have significantly contributed to this improvement. Despite decreases in radiation dose, no change in the high rate of tumor control has yet been observed.

To examine the role of stereotactic radiosurgery in the adjuvant management of children with growing and unresectable deep-seated pilocytic astrocytomas, we reviewed our experience in 9 patients. The tumors were located in the dorsolateral pons (n = 2), midbrain (n = 1, cerebellar peduncle (n = 2), thalamus (n = 1), temporal lobe (n = 1), hypothalamus (n = 1), and caudate nucleus (n = 1). The mean tumor diameter was 16 mm (range, 11-25 mm). Seven patients had prior partial tumor resection, and 2 had a stereotactic biopsy. Two patients had failed fractionated radiotherapy and 7 were considered at risk for adverse radiation effects because of their age. The mean dose to the tumor margin at radiosurgery was 15 Gy (range, 12-18). During mean follow-up of 19 months (range 13-41 months), there was a marked decrease in tumor size in 5 patients; 4 patients had no further growth. No early or delayed morbidity was associated with radiosurgery. Gamma knife radiosurgery proved a safe and effective therapeutic tool in the management of children with deep, small volume pilocytic astrocytomas. Because this tumor often appears well-delineated on contrast-enhanced neuroimaging, we believe that conformal radiosurgical targeting accurately irradiates tumor cells. For small tumor volumes it can be used in place of fractionated larger-field radiotherapy. The ability to treat the tumor yet spare surrounding brain may reduce the surgical morbidity associated with attempted radical resection and the potential cognitive and endocrine disabilities associated with fractionated radiation therapy.