Evaluation of tumor response after gamma knife radiosurgery for residual vestibular schwannomas based on MRI morphological features

¹ Department of Neurosurgery, DongGuk University International Hospital, Korea, Republic of
² Department of Neurosurgery, Seoul National University College of Medicine, Korea, Republic of
³ Department of Neurosurgery, Seoul National University Hospital, Korea, Republic of
⁴ Department of Pathology, Seoul National University College of Medicine, Korea, Republic of

^* To whom correspondence should be addressed. E-mail: gknife{at}plaza.snu.ac.kr.

Accepted 7 July 2007

	Abstract

Objective:To evaluate tumor response after gamma knife (GK) radiosurgery for residual vestibular schwannoma (VS) based on magnetic resonance imaging (MRI) morphological features.

Methods: Sixty-one patients with histopathologically confirmed VS underwent GK radiosurgery with marginal tumor doses of 9.0–14.0 Gy (mean, 12.5). Mean tumor volume at GK radiosurgery was 3.65ml (range, 0.52–15.50). GK radiosurgery was performed 0.3–95.7 months (median, 5.8) after microsurgery. Tumor volumes and half-reduction time were calculated using serial MRI. The morphological features of VSs were documented by pre-microsurgical MRI. Histopathological investigation included Antoni type dominance, the proliferation marker Ki-67, and tumor vascularity.

Results: Median duration of radiological follow-up was 53.7 months (range, 24.1–102.2) and the 8-year actuarial tumor control rate was 93.5%. No factor was associated with tumor control, although a cystic VS had borderline significance (p = 0.089). Mean tumor half-reduction time was 8.70 years (range, 0.57–79.89) and tumor half-reduction time in cystic VSs proved to be significantly shorter than those in solid VS (p = 0.006). Thrombotic vessels (p = 0.015) and abnormal vessel proliferation (p = 0.003) were significantly more prominent in cystic VSs than those in solid VSs.

Conclusions: GK radiosurgery appeared to be an effective treatment modality for residual tumor control after microsurgery. Owing to having relatively abundant tumor vascularity, residual solid portions of cystic VSs resulted in efficient shrinkage after GK radiosurgery. Therefore, GK radiosurgery was found to be a rewarding therapeutic approach to the residual solid portions of cystic VSs.

Keywords: gamma knife radiosurgery, half-reduction time, tumor, vestibular schwannoma