Current issues and future prospects of stereotactic radiosurgery for acoustic tumors

Abstract

Among the various kinds of stereotactic radiation therapy available, Gamma Knife surgery (GKS) is seen as the most accurate, and having been used widely for the treatment of acoustic tumors, its results have been well documented. According to its relative long term clinical results, tumor progression free rate was 91‒97％, hearing preservation rate was 49‒55％, and facial nerve preservation rate was 93‒ 100％；results not inferior to those of surgical resection. Based on such data GKS has been recognized as a commonsense option for general neurosurgeons. However, it was not yet mandatory to define critical indications for this data as we have only 25 years follow up with current unique dose planning using the prescribed marginal dose (12‒13 Gy). Therefore we do not have a definite treatment consensus for younger patients below the age of 50 years old with life reliability. Recently, dose planning based on highly developed MR imaging taking account precise knowledge of microanatomy has become possible, enabling critical separation of not only the facial nerve, but also the cochlea nerve from the radiation field (50％ isodose line). In this article, we propose the indications for stereotactic radiosurgery for acoustic tumors as fol-lows；for large size tumors (Koos stage 4), we strongly recommend surgical resection. For small‒middle size tumors (Koos stage 1‒3), follow‒up observation is fundamentally negative, and we recommend surgical resection for the patients with serviceable hearing who are younger than 50 years old, and then recommend stereotactic radiosurgery for other patients with serviceable hearing who rejected surgery and are older than 50 years old. For neurofibromatosis type 2 tumors we strongly recommend stereotactic radiosurgery as early as possible in order to retain auditory function.