Labyrinthine Fluid Signal Intensity on T2-Weighted MR Imaging in Patients With Vestibular Schwannomas Undergoing Proton Radiotherapy: A Longitudinal Assessment.

OBJECTIVE: In vestibular schwannoma patients, a loss of signal intensity (SI) on T2-weighted magnetic resonance imaging (MRI) has been reported within the ipsilateral labyrinth. The purpose of this study was to quantitatively evaluate the occurrence and course of this intensity loss in relation to proton radiotherapy and its possible association with hearing loss. STUDY DESIGN: Retrospective chart review. SETTING: Tertiary referral center. PATIENTS: Patients who received proton therapy for a vestibular schwannoma and underwent at least two high-resolution T2-weighted cisternographic sequence (constructive interference in steady state/fast imaging employing steady-state acquisition/DRIVE) MRIs and audiometry assessments. MAIN OUTCOME MEASURES: Relative T2 SIs from the vestibules and basal/apical cochlear turns of the labyrinth, bilaterally. RESULTS: Ninety-five MRI scans from 34 patients were included. The apical turn of the ipsilateral cochlea showed a lower mean cochlear SI than on the contralateral side (±3.5 versus 5.0). The mean relative cochlear SI did not significantly change after proton radiotherapy. The ipsilateral vestibule showed a higher SI than the cochlea. The relative mean cochlear SI was not directly correlated to (the degree of) hearing loss before or after proton radiotherapy, nor did it predict future hearing loss. CONCLUSION: The relative mean cochlear SI on cisternographic T2-MRI in vestibular schwannoma patients is diminished on the treated side, when compared with the ipsilateral vestibule and the contralateral cochlea/vestibule. The SI of the ipsilateral cochlea does not further decrease after proton radiotherapy and seems to be related to the tumor rather than the therapy. The diminished cochlear SI does not correlate with subsequent loss of hearing.

On the effect of dose delivery temporal domain on the biological effectiveness of central nervous system CyberKnife radiosurgery applications: theoretical assessment using the concept of biologically effective dose.

OBJECTIVE: The diversity in technical configuration between clinically available radiosurgery systems, results in accordingly diverse treatment times for the same physical dose prescription, spanning from several min to more than 1 h. This, combined with evidence supporting the impact of dose delivery temporal pattern on the bio-effectiveness of low-LET radiation treatments, challenges the 'acute exposure' assumption adopted clinically to estimate the biological outcome of a given treatment scheme under the concept of biologically effective dose (BED).Approach: In this work, the treatment plans of 30 patients underwent CyberKnife radiosurgery for vestibular schwannoma (VS), prescribing a marginal dose of 13 Gy to the tumor, were retrospectively reviewed and the corresponding dose distributions were resolved in the temporal domain. For this purpose, the dose delivery timeline for each treatment was calculated based on relevant treatment plan data and technical specifications of the CyberKnife system, while dosimetry data were independently acquired on a CT-based digital model of each patient using an in-house developed dose calculation algorithm.Main results: Results showed that CyberKnife delivers highly inhomogeneous dose rate distributions in the temporo-spatial domain. This influences the delivered BED levels due to alterations in the sublethal damage repair (SLR) occurring within the treatment session. Using a BED framework involving SLR effects, it was shown that each physical dose iso-surface is associated with a BEDslrrange. For the patient cohort studied, a typical range of 2%, with respect to the mean BEDslrvalue was found at 1σ.Significance: The marginal BEDslrdelivered to the tumor by the prescription dose iso-surface deteriorates with treatment time, involving both beam-on time and beam-off gaps. For treatment time,T, between 21 and 50 min, this can be expressed byBEDslr(Gy2.47)=-0.35±2.8%∙Tmin+(76.74±0.4%).Compared to the acute exposure approach, a BED 'loss' of 21% is associated with the delivery of 13 Gy to the VS-tumor in 35 min.

Assessment of Tumor Volume Dynamics and Outcome After Radiosurgery for the Treatment of Vestibular Schwannoma: A Single-Center Experience.

OBJECTIVE: To assess the factors affecting early local and audiometric outcomes in vestibular schwannoma (VS) patients treated with stereotactic radiosurgery (SRS). STUDY DESIGN: A retrospective review of medical records. SETTING: Tertiary referral center. PATIENTS: Records of all adult patients who underwent SRS between 2010 and 2016 for the treatment of VS were retrospectively reviewed. Patients treated with microsurgery or multi-fractionation schemes, and those who had neurofibromatosis type 2, were excluded. INTERVENTION: SRS, tumor volume/size measurements. MAIN OUTCOME MEASURES: The impact of tumor volume dynamics on the early local and hearing-related outcomes, together with the factors that influence them following SRS, and comparison of different tumor size measurement methods. RESULTS: From 2010 to 2016, 53 patients underwent single fraction SRS of 12 Gy. Median follow-up time was 32 months (range, 6-79). At the last follow-up, only one patient had clinical progression. Age less than or equal to 65 years (p = 0.04; odds ratio [OR]: 0.17; 95% confidence interval [CI]: 0.03-0.93) and baseline pure-tone average (PTA) level less than or equal to 30 dB (p = 0.03; OR: 0.90; 95% CI: 0.84-0.96) were associated with maintenance of serviceable hearing. On multivariate analysis, PTA remained significant (p = 0.01; OR: 0.04; 95% CI: 0.003-0.45). In patients with a loss of serviceable hearing, the mean volume increase tended to be higher than in the patients whose hearing was maintained. The linear measurement method underestimated, and the A × B × C/2 equation overestimated, the radiological progression compared with 3D-volumetric delineations. CONCLUSION: During the median observation period of almost 3 years, we reported our early outcome results. Tumor volume increase may have an impact on serviceable hearing loss after SRS. Currently there is no widely accepted method for the evaluation of post-SRS response. Linear measurement and the A × B × C/2 equation produce less reliable estimates of radiological progression compared with 3D-volumetric delineations. Accurate volume measurements with 3D delineations should be considered as part of clinical routine for assessing progression and deciding on salvage therapies.

LINAC-based stereotactic radiosurgery versus hypofractionated stereotactic radiotherapy delivered in 3 or 5 fractions for vestibular schwannomas: comparative assessment from a single institution.

PURPOSE: The optimal treatment strategy for vestibular schwannoma (VS) is not known, and different radiation techniques and fractionation regimens are currently being used. This report aimed to assess outcomes after LINAC-based radiosurgery (SRS) and hypofractionated radiotherapy (hypo-FSRT) and identify possible differences in outcomes between hypo-FSRT delivered in 3 or 5 fractions. METHODS: From 2005 to 2017, 136 patients underwent treatment with radiotherapy for VS. Thirty-seven patients received SRS (12 Gy), and 99 received hypo-FSRT. Hypo-FSRT was delivered in 3 fractions (total 18-21 Gy, n = 39) and 5 fractions (total 25 Gy, n = 60). RESULTS: The median follow-up was 57 months. Eight patients had progression requiring surgery, corresponding to an overall local control rate of 93.4%, with no significant difference between the fractionation schedules. A correlation with borderline significance (p = 0.052) was detected between cystic tumors and local failure. A tendency toward a higher incidence of local failure was observed after 2015 when SRS treatment increased and included slightly larger tumors. Hearing preservation was observed in 35% of patients and 36% of patients experienced acute side effects, but persistent facial or trigeminal nerve toxicity was rare. CONCLUSION: SRS and hypo-FSRT with 3 or 5 fractions provided a high rate of local control with no significant differences between treatment schedules. SRS is a well-documented radiation technique for VS and is the recommendation for small- to medium-sized tumors. This report demonstrates excellent long-term outcomes after hypo-FSRT; this regimen can be delivered safely and is an alternative for selected patients.

A Retrospective Cohort Study of Longitudinal Audiologic Assessment in Single and Fractionated Stereotactic Radiosurgery for Vestibular Schwannoma.

BACKGROUND: Fractionated stereotactic radiosurgery (SRS) for vestibular schwannomas (VS) has been theorized to allow for tumor control with higher rates of hearing preservation in selected patients with useful hearing. However, there is a paucity of literature with formal audiologic measures of hearing preservation to support the standard use of fractionated SRS in VS. We hypothesized that fractionation would diminish the amount of hearing damage. OBJECTIVE: To evaluate the relationship between audiologic performance and SRS fractionation scheme. METHODS: We performed an IRB-approved retrospective review of patients treated with 1, 3, or 5 fraction SRS for VS at our institution from 1998 to 2016. Pre- and post-SRS audiograms with speech awareness threshold (SAT) in treated and contralateral ears were obtained. Contralateral ear measurements were used for hearing normalization to account for presbycusis. RESULTS: Fifty-six patients with median audiologic follow-up 2.0 yr (mean 2.66 yr, min-max 0.50-9.45 yr) were included. Patients treated with single fractionation had a significantly worsened SAT (dB) compared to patients treated with 5 fractions (P = .008) and compared to all multifraction patients (P = .009) at 12 to 24 mo follow-up. CONCLUSION: This retrospective analysis supports the use of fractionated SRS to preserve hearing in patients with VS. SAT can be used as an objective metric of hearing response to radiosurgery.

EAONO position statement on Vestibular Schwannoma: Imaging Assessment Question: How should growth of Vestibular Schwannoma be defined?

The relevance of defining the growth of vestibular schwannoma (VS) is that any significant VS growth may impact treatment strategy. A conservative treatment strategy is often proposed as a primary treatment option in the management of VS. Several authors have demonstrated that a significant proportion of VSs do not grow, and those that do, usually grow slowly. Surgical and/or radiosurgical treatment options may be offered to the patient according to the VS growth. Therefore, defining the VS growth is a determinant in managing treatment strategies. A comprehensive literature search was performed to examine the definition of tumor growth for VS. The literature review was conducted using PubMed and Embase databases dated back to 20 years (1995-2015) and was updated until February 2015. VS growth should be measured on contrast-enhanced T1-weighted images. Although there the overall quality of the present studies is low, all highlight a significant VS growth of > 2 mm, and/or 1.2 cm3, and/or 20% change in volume, and/or the square of the product of the 2 orthogonal diameters. We suggest that VS growth should instead change management strategies when a 3-mm increase in diameter on two consecutive MRI scans are performed 1 year apart.

Gamma Knife Radiosurgery for Vestibular Schwannomas and Quality of Life Evaluation.

BACKGROUND: Further investigation is needed to look at the impact of vestibular schwannoma (VS) on the health-related quality of life (QOL) of participants who undergo Gamma Knife® radiosurgery (GKRS). OBJECTIVES: Investigators compared the QOL for VS participants to reported US population norms in order to evaluate disease burden and long-term QOL several years after GKRS. METHODS: This cross-sectional study surveyed participants to assess hearing status, tinnitus, imbalance, vertigo, as well as the Short-Form 36-item Health Questionnaire (SF-36). The data were normalized, age adjusted, and functional status was correlated to determine clinically significant differences. RESULTS: A total of 353 participants who underwent GKRS between 1997 and 2007 were included in this study with a median postoperative period of 5 years. SF-36 scores were very similar to population norms, and age-adjusted scores for participants followed the US population curve. Frequent vertigo and balance problems had the largest statistically and clinically significant effect on physical and mental component summary scores followed by nonuseful hearing in the tumor ear. CONCLUSIONS: Participants reported a good long-term QOL that was very similar to the QOL of US population norms. Of the common VS symptoms, vertigo had the greatest impact on QOL followed by imbalance and then hearing loss.

Limitations of analytical dose calculations for small field proton radiosurgery.

The purpose of the work was to evaluate the dosimetric uncertainties of an analytical dose calculation engine and the impact on treatment plans using small fields in intracranial proton stereotactic radiosurgery (PSRS) for a gantry based double scattering system. 50 patients were evaluated including 10 patients for each of 5 diagnostic indications of: arteriovenous malformation (AVM), acoustic neuroma (AN), meningioma (MGM), metastasis (METS), and pituitary adenoma (PIT). Treatment plans followed standard prescription and optimization procedures for PSRS. We performed comparisons between delivered dose distributions, determined by Monte Carlo (MC) simulations, and those calculated with the analytical dose calculation algorithm (ADC) used in our current treatment planning system in terms of dose volume histogram parameters and beam range distributions. Results show that the difference in the dose to 95% of the target (D95) is within 6% when applying measured field size output corrections for AN, MGM, and PIT. However, for AVM and METS, the differences can be as great as 10% and 12%, respectively. Normalizing the MC dose to the ADC dose based on the dose of voxels in a central area of the target reduces the difference of the D95 to within 6% for all sites. The generally applied margin to cover uncertainties in range (3.5% of the prescribed range + 1 mm) is not sufficient to cover the range uncertainty for ADC in all cases, especially for patients with high tissue heterogeneity. The root mean square of the R90 difference, the difference in the position of distal falloff to 90% of the prescribed dose, is affected by several factors, especially the patient geometry heterogeneity, modulation and field diameter. In conclusion, implementation of Monte Carlo dose calculation techniques into the clinic can reduce the uncertainty of the target dose for proton stereotactic radiosurgery. If MC is not available for treatment planning, using MC dose distributions to adjust the delivered doses level can also reduce uncertainties below 3% for mean target dose and 6% for the D95.

Assessment of organs-at-risk contouring practices in radiosurgery institutions around the world - The first initiative of the OAR Standardization Working Group.

BACKGROUND AND PURPOSE: This study was an initiative of the Organs-at-Risk Standardization Working Group for evaluating the current degree of variability in the clinical practice of contouring organs-at-risk (OAR) for radiosurgery planning. MATERIALS AND METHODS: Imaging datasets for typical lesions (cavernous sinus meningioma, vestibular schwannoma, pituitary adenoma) treated with Leksell Gamma Knife Perfexion were circulated to 12 centers. Observers were asked to contour the target and OARs as per their standard clinical practice. The analyzed parameters were the intersection (AV100), union volumes (AV100/N) and the 50% agreement volume (AV50). The ratio of AV100 and AV100/N (the Agreement Volume Index, AVI) was used as a measure of agreement level together with a generalized conformity index (CIgen) and a pairwise averaged conformity index (CIpairs). The maximum doses were also determined. RESULTS: Results showed a wide variability in terminology, choice of structures contoured and in the size and shape of the contoured structures. The highest variability was observed for the left and right optic tract for cavernous sinus meningioma where the AV100 was zero. The highest consistency was observed for the right optic nerve in the cavernous sinus case followed by the cochlea for the vestibular schwannoma case for which the AVI was still only 0.13 and 0.054, respectively. Corresponding results for the CIgen and CIpairs also showed the highest variability for the right optic tract and the highest consistency in contours for the right optic nerve, both in the cavernous sinus meningioma case. CONCLUSION: The results quantify the large variability in OAR contouring in clinical practice across Gamma Knife radiosurgery centers with respect to the choice of OARs to be contoured, nomenclature and size and shape of OARs. This motivates future effort to standardize practices to enable more effective collaboration.

Assessment of the quality of very high-energy electron radiotherapy planning.

BACKGROUND AND PURPOSE: To assess the quality of very-high energy electron (VHEE) scanning pencil beam radiation therapy in relation to state-of-the-art volumetric modulated arc therapy (VMAT) and to determine the extent of its application. MATERIAL AND METHODS: We planned five clinical cases with VHEE scanning pencil beams of 100 and 120MeV, equally distributed in a coplanar arrangement around the patient. The clinical cases included acoustic neuroma, and liver, lung, esophagus, and anal cancer cases. We performed Monte Carlo (MC) dose calculations and we optimized the dose in a research version of RayStation. VHEE plan performance was compared against clinically delivered VMAT. RESULTS: With equal target coverage, mean doses to organs at risk (OARs) were on average 22% lower for the VHEE plans compared to the VMAT plans. Dose conformity was equal or superior compared to the VMAT plans and integral dose to the body was in average 14% (9-22%) lower for the VHEE plans. CONCLUSIONS: The dosimetric advantages of VHEE as demonstrated for a variety of clinical cases, combined with the theoretical ultra fast treatment delivery, afford VHEE scanning pencil beam radiotherapy a suitable and potentially superior alternative for cancer radiotherapy.

Vestibular schwannomas in the modern era: epidemiology, treatment trends, and disparities in management.

OBJECT: There are a variety of treatment options for the management of vestibular schwannomas (VSs), including microsurgical resection, radiotherapy, and observation. Although the choice of treatment is dependent on various patient factors, physician bias has been shown to significantly affect treatment choice for VS. In this study the authors describe the current epidemiology of VS and treatment trends in the US in the modern era. They also illustrate patient and tumor characteristics and elucidate their effect on tumor management. METHODS: Patients diagnosed with VS were identified through the Surveillance, Epidemiology, and End Results database, spanning the years 2004-2009. Age-adjusted incidence rates were calculated and adjusted using the 2000 US standard population. The chi-square and Student t-tests were used to evaluate differences between patient and tumor characteristics. Multivariate logistic regression was performed to determine the effects of various patient and tumor characteristics on the choice of tumor treatment. RESULTS: A total of 6225 patients with VSs treated between 2004 and 2009 were identified. The overall incidence rate was 1.2 per 100,000 population per year. The median age of patients with VS was 55 years, with the majority of patients being Caucasian (83.16%). Of all patients, 3053 (49.04%) received surgery only, with 1466 (23.55%) receiving radiotherapy alone. Both surgery and radiation were only used in 123 patients (1.98%), with 1504 patients not undergoing any treatment (24.16%). Increasing age correlated with decreased use of surgery (OR 0.95, 95% CI 0.95-0.96; p<0.0001), whereas increasing tumor size was associated with the increased use of surgery (OR 1.04, 95% CI 1.04-1.05; p<0.0001). Older age was associated with an increased likelihood of conservative management (OR 1.04, 95% CI 1.04-1.05; p<0.0001). Racial disparities were also seen, with African American patients being significantly less likely to receive surgical treatment compared with Caucasians (OR 0.50, 95% CI 0.35-0.70; p<0.0001), despite having larger tumors at diagnosis. CONCLUSIONS: The incidence of vestibular schwannomas in the US is 1.2 per 100,000 population per year. Although many studies have demonstrated improved outcomes with the use of radiotherapy for small- to medium-sized VSs, surgery is still the most commonly used treatment modality for these tumors. Racial disparities also exist in the treatment of VSs, with African American patients being half as likely to receive surgery and nearly twice as likely to have their VSs managed conservatively despite presenting with larger tumors. Further studies are needed to elucidate the reasons for treatment disparities and investigate the nationwide trend of resection for the treatment of small VSs.

Assessment of the alpha/beta ratios for arteriovenous malformations, meningiomas, acoustic neuromas, and the optic chiasma.

PURPOSE: To determine alpha/beta (alpha/beta) values of arteriovenous malformations (AVM), meningiomas, acoustic neuromas (AN), and the optic chiasma using clinical data. METHODS AND MATERIALS: Data of dose/fractionation schedules form the literature, iso-effective for a specific clinical outcome, were analysed using the Fraction Equivalent plot (FE) method and the Tucker method. Established safe dose/fractionation schedules for the optic chiasma were used to determine its alpha/beta value. RESULTS: With the FE plot method, an alpha/beta value of 3.76 Gray (Gy) (95% confidence level [CL]: 2.8-4.6 Gy) for meningiomas, 2.4 Gy (95% CL: 0.8-3.9 Gy) for acoustic neuroma, and 14.7 Gy (95% CL: 3.8-25.7 Gy) for arteriovenous malformations were determined. The respective alpha/beta values using the Tucker method were 3.3 Gy (95%CL: 2.2-6.8 Gy), 1.77 Gy (95%CL: 1.3-3.0 Gy) and -57 Gy (95%CL: -79.6 to -35.2 Gy). No meaningful alpha/beta values could be determined for the optic chiasma. CONCLUSION: Acoustic neuromas with a low alpha/beta value would show no lesion intrinsic benefit from fractionation. Meningiomas probably benefit from a hypofractionated schedule. The high alpha/beta value for AVM can be explained but needs further research. Fractionation versus radiosurgery can be considered when the primary objective is to avoid normal tissue damage.

Comparative study of treatment dose plans after the refinement of Leksell Gamma Knife single-beam dose profiles.

We investigated the amplification of discrepancy when using multiple shots of the same collimator size helmet, by comparing dose plans in the Leksell GammaPlan employing the default single-beam dose profiles and the Monte Carlo generated single-beam profiles. Four collimator helmets were studied. The results show that the largest amplification of discrepancy with multiple shots was found with the 8 mm collimator because of the largest discrepancy of its single-beam dose profile. The amplification of discrepancy is significant when tumor volumes increase but insignificant when the tumor volumes are in an elongated shape. Using close shot overlapping strategy (i.e., more shots close packed together) shows no observable increase in the amplification of discrepancy. For the best quality of Leksell Gamma Knife radiosurgery, it is suggested that the single-beam dose profiles should be refined, especially the 8 mm collimator, to prevent error amplification when using multiple collimator shots.

Tumour dose enhancement using modified megavoltage photon beams and contrast media.

This study examines the magnitude of tumour dose enhancement achieved by injection of gadolinium or iodine contrast media (CM) and treatment using modified x-ray photon spectra from linear accelerators. Monte Carlo modelling of the linear accelerator and patient geometry was used to explore the effect of removing the flattening filter for various beam qualities and the resultant effect on dose enhancement. In addition, ionization measurements were conducted to observe dose enhancement within a phantom containing CM. Simulation results indicate that for flattened 6-24 MV photon beams and realistic CM tumour concentrations, the dose enhancement remains below 5%. However, if the flattening filter is removed, dose enhancement is increased significantly. For a 30 mg ml(-1) gadolinium CM tumour concentration, for example, 8.4%, 10.8%, 13.7% and 23.1% dose enhancements are achieved for 18 MV, 6 MV, 4 MV and 2 MV unflattened beams, respectively. In contrast to the phototherapy technique, which uses the orthovoltage beam from a modified CT scanner to achieve dose enhancement, all unflattened spectra preserve the dose build-up at the surface, and thus the skin and bone are spared.

Audiological assessment before and after fractionated stereotactic irradiation for vestibular schwannoma.

PURPOSE: To find the audiological outcome after LINAC-based fractionated stereotactic irradiation (STI). MATERIALS AND METHODS: Twenty-four patients with vestibular schwannoma treated by fractionated STI between 1991 and 1997 had measurable hearing before STI and were followed audiologically for more than 6 months. The pure tone average (PTA) was measured by averaging the air-conduction threshold for five main frequencies (250-4000 Hz) before and periodically after STI in the 24 patients. Several possible prognostic factors for hearing preservation (defined as a PTA change at the last follow-up of less than 10 dB) were investigated. The median follow-up time was 22 months, ranging from 5 to 69 months. The irradiation schedule was 36 Gy in 20 fractions in 5 weeks to 44 Gy in 22 fractions in 6 weeks followed by 4 Gy/1 fraction boost. RESULTS: The pure tone average before STI was distributed from 7 to 73 dB. Fifty percent of patients showed a change in PTA of less than 10 dB, 79.2% of patients showed a change in PTA of less than 20 dB and 20.8% of patients showed a change in PTA of more than 21 dB at the last follow-up. Only one patient (4%) became deaf. Cases with a sudden loss of hearing were more likely to experience hearing preservation than those with gradual loss of hearing (P<0.05). The mean age was younger in patients whose hearing was preserved (P<0.05). Poor pretreatment PTA appeared to linearly correspond to the changes in PTA (regression coefficient 0.78). The size of the tumor was not related to the change in PTA. No relationship was observed between the maximum or peripheral dose and the PTA change. The real benefit of stereotactic boost after small-field fractionated irradiation was not certain. CONCLUSION: Fractionated STI produced a hearing preservation rate compatible with meticulously collimated multi-spots single fraction irradiation. Further follow-up is required to confirm the long-term benefits of fractionation.

Stereotactic radiosurgery of the brain using the first United States 201 cobalt-60 source gamma knife.

The first United States 201 cobalt-60 source gamma knife for stereotactic radiosurgery of brain tumors and arteriovenous malformations became operational at the University of Pittsburgh on August 14, 1987. Four and one-half years of intensive planning, regulatory agency review, and analysis of published results preceded the first radiosurgical procedure. Installation of this 18,000-kg device and loading of the 201 cobalt-60 sources posed major challenges in engineering, architecture, and radiophysics. In the first 4 months of operation, we treated 52 patients (29 with arteriovenous malformations, 19 with extra-axial neoplasms of the skull base, and 4 with intra-axial malignant tumors). Most patients either had lesions considered "inoperable" or had residual lesions after attempted surgical resection. Neither surgical mortality nor significant morbidity was associated with gamma knife radiosurgery. As compared with treatment by conventional intracranial surgery (craniotomy), the average length of stay for radiosurgery was reduced by 4 to 14 days, and hospital charges were reduced by as much as 65%. Based on both the previously published results of treatment of more than 2,000 patients worldwide and on our initial clinical experience, we believe that gamma knife stereotactic radiosurgery is a therapeutically effective and economically sound alternative to more conventional neurosurgical procedures, in selected cases.