Radiosurgery in trochlear and abducens nerve schwannomas: case series and systematic review.

Abstract

Schwannomas involving the occulomotor cranial nerves (CNs; III, IV and VI), can be disabling, due to the associated diplopia and decreased quality of life and are extremely rare. We evaluated the role of Gamma Knife surgery (GKS) in these cases. Five patients with CN IV and VI schwannomas (three and two, respectively) were treated in Lausanne University Hospital between 2010 and 2015. Four benefitted from upfront GKS and one from a combined approach (planned subtotal resection followed by GKS), due to a large preoperative tumour volume (size, 3×2×2.5cm; volume, 7.9ml), with symptomatic mass effect and oedema, as well as an entrapement cyst at the brainstem interface,in a young patient. Neuro-ophtalmological evaluation was performed at baseline and during each follow-up time-point. A systematic literature review is presented and compared to the present report. The mean follow-up was 44.4months (12-54). Initial clinical presentation was diplopia in four cases and cavernous sinus syndrome in one. The marginal dose was 12Gy in all cases. The mean target volume was 1.51cm3 (0.086-5.8). The mean prescription isodose volume (PIV) was 1.71cm3 (0.131-6.7). At last follow-up, all patients presented with disappearance of the baseline symptoms. Tumour control was achieved in 100%, with decrease in volume in all cases. The systematic review analysed 11 peer-reviewed studies, with a total of 35 patients. For uniformly reported CN VI, the mean marginal radiation dose ranged between 12 and 12.5Gy, with disappearance of symptoms in 12.5%, improvement in 31.25%, stabilisation in 6.25%, worsening in 12.5%. Tumour volume decreased in all cases. Our data suggest that first intention GKS is a safe and effective option for patients with small to medium sizeoculomotor schwannomas, providing a high rate of clinical alleviation and tumour control. When the initial tumour volume is too large for first intention GKS, a combined approach with plannedsubtotal resection followed by GKS can be performed, with favourable and comparable outcomes as in upfront GKS.