Letter: Cystic Vestibular Schwannomas Respond Best to Radiosurgery

摘要

To the Editor: We have read with great interest the recent articles published in Neurosurgery, related to the response after radiosurgery (RS) for cystic vestibular schwannomas (VSs).1,2 During the past 20 yr, a noteworthy reflection and dilemma have been ongoing whether surveillance, microsurgery, or RS should be performed in patients with newly diagnosed VSs.3 Particularly, tumor volume and cystic component have been considered related to tumor response and control after RS.1 VSs (small to medium size tumors) represent nowadays a common indication of RS and particularly of Gamma Knife surgery (GKS). The former is related to high rates of tumor control, ranging between 89% and 98%4-7 at 5 yr, with facial nerve preservation rates between 84% and 100%.3,4,6,8-12 The prescribed doses are currently low, ranging between 11 and 13 Gy in most of the centers, after a progressive de-escalation over the past 30 yr.13 There has been a technique refinement over time, and since the robotization of Leksell Gamma Knife® Perfexion™ (Elekta Instruments, AB, Stockholm, Sweden), the complications after GKS for VS have further declined, including less than 1% facial palsy in Marseille's practice.14 Acute clinical radiation effects during the first 6 mo after the procedure may be present but frequently disappear after short-period corticosteroid treatment.15 The recent papers published in Neurosurgery on cystic VSs prompted us to review our radiosurgical experience in Lausanne University Hospital, Switzerland, over a period of almost 7 yr (July 2010 to May 2017). Six cases of VSs with cystic component benefitted from GKS, with Leksell Gamma Knife® Perfexion™ (till June 2016) and ICON™ (since June 2016; Elekta Instruments, AB). The mean target volume was 1.9 mL (range 0.8-3.3 mL). The mean follow-up period for these 6 cases was 2.5 yr (range 0.5-5 yr). All patients underwent stereotactic computed tomography and magnetic resonance imaging (MRI), including a T2 constructive interference steady state (CISS)/Fiesta sequence, confirming a cystic part (see Figure). All VSs presented a decrease in size, including at the immediate 6-mo follow-up after the procedure. The mean residual tumor volume was 44.1% of the initial gross target volume (range of residual volumes 21.2%-70.4%) at the last outpatients visit. Each of the cases with their respective follow-up images coregistered to the stereotactic MRI is displayed in the Figure.FIGURE.: Example of patients with cystic VSs treated by GKS with a marginal dose of 12 Gy; from left to right, we show T1w and T2 CISS/Fiesta therapeutic images, with the dosimetry colored in yellow, and further the follow-up images at 2, 3, and 5 yr (with the superimposed dosimetry). TV, target volume.During the past 2 decades, it has been commonly considered that cystic VSs do not have a good response to RS. However, 2 issues made this analysis difficult: first was related to the paucity of the published data, second to the definition of a cystic tumor.6,16,17 In fact, there is currently no consensus of what a cystic tumor represents and how it should be defined. Furthermore, the therapeutic response related to radiation therapy might be discussed in function of the position of cyst inside the lesion itself, whether this cyst is peripheral or central, as 2 distinct pathological processes. An important aspect, underlined also by Frisch et al,2 is that intratumoral (central) cysts will receive significantly higher doses (close to the maximal one) as compared to the peripheral one. However, in our experience, all the treated tumors displayed peripheral cysts and, further, had an important decrease in size after GKS. Back in 2000, Shirato et al16 analyzed their experience with fractionated stereotactic radiotherapy in cystic VSs. The former exhibited a transient increase in size at less than 2 yr, with further greater reduction at 3 yr after the treatment, as compared to the noncystic group. In our experience, there was already a de