Long-term Outcomes of Gamma Knife Stereotactic Radiosurgery of Vestibular Schwannomas

The authors presented the gamma knife radiosurgical outcome of 59 vestibular schwannoma (VS) patients who had been treated with low radiation dose ranging 11-13 Gy during more than 5 years follow-up period. This literature is very impressive to me for long term follow-up and application of lower radiation dosage. The authors mainly discussed on the tumor control patterns after Gamma Knife radiosurgery (GKS) and hearing preservation. They showed very successful tumor control rate of 97% and as for the changes of tumor size, 29% of patients have characteristics of transient increase after GKS and then decrease. On GKS, 12 of 59 patients had serviceable hearing with Gardner-Robertson grade I, II. After long term follow-up, only 4 patients achieved their serviceable hearing. Recently the main matter of concern for the GKS for VS is to determine the optimal radiation dose for hearing preservation to improve the quality of life of patients. Effective tumor control rates even with low radiation dose ranging 12-14 Gy have been achieved by the helps of advanced diagnostic tools which enable to detect the accurate tumor margin and development of planning system. The object of irradiation with low radiosurgical dose is to improve the functional outcome like as hearing preservation rate. As the author's report, observation for more long term follow-up period is needed to estimate the accurate tumor control patterns and functional preservation. By published results, The hearing preservation rate after radiosurgery for VS has been reported to be in the range of 47 to 81%4). It is not easy to simply compare the various reports on global hearing preservation because of the lack of uniformity in reporting results. On the review of literatures according to the long-term outcome after GKS1,2,3), author's results for the hearing preservation rate showed relatively low in spite of low radiation dose of 12 Gy and this result was different from the published results. The possible causes of low hearing preservation rate in this article are very curious to me. There may be many possible causes like as tumor size, tumor location, radiosurgical planning as well as marginal dose. Especially the analysis of possible cause for 8 patients who had been decreased to non-serviceable hearing is needed requisitely. In my experience, the hearing preservation rate of GKS for small vestibular schwannomas below 5 cc can be approximately 70% for long term follow-up. In some cases, aggravation of hearing function occasionally occurred even after achievement of tumor control. Moreover the prospective study can give the precise information for relatonships between the hearing preservation and dose. GKS for VS have been increased for last 20 years and the propensities of radiosurgery have been increased. It is accepted that GKS is more advantageous treatment tools than microsurgery for the functional preservation As time goes by, it is expected that the functional preservation will be improved on the basis of the more long term follow-up results.