Evolution of Radiosurgery

Lars Leksell coined the term stereotactic radiosurgery in 1951. He developed the technique for delivery of a single high dose of precisely focused radiation to achieve closed skull destruction of a small intracranial target. Acoustic tumor stereotactic radiosurgery using the Gamma knife unit was first performed by Leksell in 1969.21 In a review of his 20 years' experience with treatment of acoustic neuromas by radiosurgery, Norén et al.15 reported that more than 90% of patients had tumor growth control. The initial patient treated with the first North American 201-source Cobalt-60 Gamma knife unit at the University of Pittsburgh in 1987 had an acoustic neuroma.22 Radiosurgery technique has evolved steadily during the past decade.4,15,22-29 The results after radio-surgery of acoustic tumors have established it as an important minimally invasive alternative to microsurgery.24,26,27 Pollock et al.29 performed a retrospective matched cohort analysis of micro-surgical and radiosurgical outcomes at the University of Pittsburgh and found radiosurgery as an effective and less costly management strategy. Ogunrinde et al.3 described 10 patients with intracanalicular tumors managed by radiosurgery; no patient had a delayed facial or trigeminal neuropathy.

RADIOSURGICAL TECHNIQUE

Gamma knife radiosurgery begins with rigid fixation of an MRI-compatible Leksell stereotactic frame to the patient's head under local anesthetic infiltration and mild sedation. High-resolution-gadolinium enhanced T1-weighted sagittal scout MRIs are obtained next, in order to localize the area of interest. To define the radiosurgery target volume, a multi-planar volume acquisition contrast-enhanced MRI (divided into 28 axial slices of 1-mm thickness) that covers the entire lesion and surrounding critical structures is performed. Planning is performed on axial images supplemented by coronal

UNIVERSITY OF PITTSBURGH EXPERIENCE

Between August 1987 and December 1997, 29 patients with intracanalicular acoustic tumors underwent stereotactic radio-surgery at our center. All patients were evaluated with highresolution MRI or computed tomography (CT) scan and had clinical evaluation as well as audiology tests that included PTA and SDS. Hearing was graded using the Gardner-Robertson modification of the Silverstein and Norell classification, and facial nerve function was assessed according to the House-Brackmann grading system.30,31 Serviceable hearing (class I and II) was defined as a PTA or SRT lower than 50 dB and an speech discrimination score better than 50%.

After radiosurgery, all patients had serial follow-up gadolinium-enhanced MRI scans, which were requested at 6-month intervals for 2 years. If there was no appreciable change in tumor size, subsequent MRIs were requested at 2-year intervals. All patients who had some preserved hearing were advised to obtain audiologic evaluation including PTA and SDS near the time of their MRI follow-up. No patient morbidity or mortality was observed. Tumor control, as documented by serial imaging, was achieved in all patients. Six of 29 patients showed clear-cut reduction in tumor volume in follow-up MRI. Three patients had minimal temporary enlargement followed by stabilization; loss of central contrast uptake was noted in two of these patients. Stable appearance without any evidence of further tumor growth was noted in 20 patients in a follow-up period ranging from 12 to 108 months. No patient in this series required subsequent surgical intervention.

A pre-radiosurgery Gardner-Robertson class was preserved in 49%, whereas testable hearing was present in 68% of patients (Fig. 50-1). Seventeen of 29 patients had serviceable hearing at radiosurgery. Audiologic follow-up was available on 15 of these 17 patients. Serviceable hearing was preserved in 73% (11/15) of patients at a mean follow-up of 33 months. Long-term follow-up showed serviceable hearing preservation in all 10 of

PRESERVED HEARING 60

PRESERVED HEARING 60

TIME (months)

Figure 50-1 Graph showing preservation of serviceable, testable, and pre-radiosurgery hearing in the intracanalicular acoustic neuroma series at the University of Pittsburgh during the 10-year interval from 1987 to 1997.

10 patients (100%) treated with < 14 Gy and in 1 of 5 patients (20%) treated with > 14 Gy (Fig. 50-2). Five patients demonstrated significant improvement in hearing. No patient developed facial or trigeminal neuropathy. Seven of 13 patients with preoperative tinnitus continued to experience tinnitus in follow-up. Episodic vertigo continued in 3 of the 11 patients who presented with vertigo. No other perioperative complication occurred in any patient.

Our analysis suggests that the current technique (MRI-based conformal planning using exclusively 4-mm collimators and 13 to 14 Gy to the tumor margin) can achieve hearing preservation in almost all patients. The results of hearing preservation after stereotactic radiosurgery of intracanalicu-lar tumors appear distinctly superior to results achieved after microsurgical excision even at centers of surgical excellence. Our current 100% preservation rates of facial and trigeminal

SERVICEABLE 60 HEARING

100%

TIME (months)

Figure 50-2 Graph showing preservation of serviceable hearing in the intracanalicular acoustic neuroma patients treated with < 14 Gy versus > 14 Gy to the tumor margin using the Gamma knife at the University of Pittsburgh.

nerve function are also superior to those reported after microsurgical excision of intracanalicular tumors. Hearing preservation is now the rule for such patients.

0 0

Post a comment