From all treated solitary, primary VS, 6% showed MR-graphic cystic character in this study. The majority of cystic VS was treated with SURGERY. Premorbid status was worse in cystic compared to solid VS. Generally, cystic VS were larger compared to solid VS and more often required hydrocephalus-related treatment (i.e. shunt surgery). The incidence of pre-operative trigeminal and vertigo-related symptoms are significantly more frequent in cystic VS compared to solid tumors – but not higher compared to same-size solid VS. SRS was inferior in tumor control (i.e. RFS) in cystic VS compared to solid VS. The highest rate of tumor control was ensured, when treated with GTR (compared to SRS and STR). The rate of GTR, however, is significantly lower in cystic VS compared to surgically treated solid VS. In the general cohort, poor postoperative facial outcome was significantly more prevalent in cystic VS, but not worse compared to same-size solid VS. Cystic morphology was not associated with a higher rate of therapy-related side-effects.

The incidence of cystic morphology in the VS is reported with considerable variability: from 11.3 to 48% [9, 22, 23]. This cohort shows an overall incidence of 6% of the treated primary, solitary VS similar to Fundova et al. in 2000, when 773 VS patients were retrospectively reviewed [24]. However, the incidence of cyst formation rises with tumor size with the lowest rate in Koos I at 2% and the highest incidence in Koos IV VS at 15.43%, this distribution could attribute to the reported incidence discrepancies [25].

The CCI was significantly higher in patients with cystic VS compared to solid, suggesting that there was a significantly worse premorbid status in these patients, even though age is indifferent in either group. Even though the exact pathophysiology of cyst formation is not entirely clear, it has been related to micro-hemorrhages and inflammation process and one could imagine, this being a possible effect of the patients’ overall health condition or relevant comorbidities [8, 26].

The incidence of trigeminal symptoms and vertigo was significantly raised in cystic VS compared to solid VS. This has also been shown and discussed by Constanzo et al. in 2019, who showed that patients with cystic VS faired objectively worse in video head impulse testing and therefore was associated with worse vestibular dysfunction [26]. Constanzo et al. attribute this difference to a local inflammatory reaction caused by hemorrhages, which would induce a ‘‘neuritis’’ of the vestibular nerves, therefore altering vestibulo-ocular reflex gain beyond what its expected [26]. However, in our matched subgroup-analysis, where tumor size was taken out of the equation as a potential bias, there was no significant difference in vertigo, trigeminal affection, hearing status and even facial nerve outcome. This suggests that the pronounced clinical features of a cystic VS is most likely caused by the sheer space demanding tumor volume in the CPA. The incidence of Shunt-Dependency was also significantly higher in the cystic VS cohort, which has been described in the past [25, 27].

Patients treated by SRS were significantly older compared to cystic VS treated with SURGERY, which is a phenomenon often described in the past [14, 28,29,30]. Interestingly, women more often received SRS than men. However, this study design does not allow any investigation to any sex-related difference in provided VS care. However, differences in provided surgical care have been described in the past – although it remains unclear, whether this phenomenon is a result of medical care provider bias or gender-related decision-making by the patient [31, 32].

It has been repeatedly shown in the past that SRS is safe in cystic VS with a high rate of functional preservation [1, 4, 12, 33]. Within the SRS-group, however, it was not able to ensure the same RFS in cystic VS as in solid VS (75% versus 90%). When treated with SRS, the incidence of post-SRS tumor volume change (increase, decrease or pseudoprogression) was significantly higher in cystic VS in the matched subgroup analysis. This phenomenon has been described by previous studies focusing on SRS in cystic VS, who describe a significant tumor volume decrease in macrocystic compared to solid VS [1, 12, 34, 35].

Bowden et al. observed a very high tumor control rate of > 95% (compared to 75% in this study) with a mean follow-up of 4 years. However, our data shows a mean time to recurrence/progression in the SRS-treated cystic VS at > 5 years. It is possible that past studies have not been able to report the true incidence of recurrence, when cystic was treated with SRS as a monotherapy [4, 12, 13]. In 2016, Frisch et al. reported a tumor control of 80% in cystic VS with a mean follow-up of 5.25 years, which more reflects to the results presented in our study [1]. These differences point out the importance of long-term FU when discussing RFS in a benign tumor, such as VS.

To our knowledge, this is the first study to directly compare SRS and SURGERY of cystic VS in one study design. Noticeably, if tumor recurrence/progression appeared, mean time to recurrence/progression after SRS-treatment was significantly shorter compared to STR or GTR – even though pseudoprogression was systematically ruled out in this study. Therefore, the tumor volume increase in cystic VS is most likely due to the fluid uptake in the intratumoral cyst and not due to increase in contrast-enhancing tumor tissue. Concordant, cystic schwannomas are histologically associated with a 36-fold decrease in nuclear proliferation as measured with Ki-67 staining when compared with solid tumors. This suggests that the rapid clinical growth seen in cystic schwannomas is related to the accumulation of fluid during cyst formation and not by an actual increase in the growth rate of tumor cells [3, 36].

The prognosis after SURGERY is reported to be worse than for other solid VS because of the difficulty in preserving the arachnoid plane, the presence of hypervascular solid portions of the tumor, unusual cranial nerve displacement, and a greater tendency for postoperative bleeding [5, 6, 8, 37, 38]. Therefore, cystic VS presents a therapeutic dilemma and should preferably be treated in specialized centers routinely treating solid VS.

Safe maximal resection—if achievable—would to date be the best long-term tumor control in cystic VS according to our results. However, cystic morphology is associated with a lower rate of EOR (GTR 87% vs. 96%), when treated with SURGERY in this study cohort. A comparative study in 2005 achieved a high GTR-rate in cystic tumors of 92% (solid tumors 93%) by retrosigmoid approach, but they also report that 42% of patients with cystic VS showed unfavorable facial nerve function one year post-operatively [9]. Notably, their classification for good facial outcome included HB grade 3, which was classified as poor facial outcome in our study. In 2000, Fundova et al. reported a substantially higher rate of complete facial nerve loss in cystic compared to solid VS (41% versus 27% respectively) with similar GTR rates in both groups (89% and 82%) [24].

The rate of poor facial outcome (HB3-6 after one year) was rather low with 16% in cystic VS and comparable in same sized solid VS in our study compared to the literature. Jian et al. presented a cohort with significantly different achieved EOR with 52% of GTR in cystic and 70% in solid VS, this constellation yielded in insignificantly different facial function outcome in either group [39]. In summary, studies on cystic VS yielded in comparable facial nerve outcome, when the rate of STR was higher compared to solid VS suggesting a direct correlation between EOR and facial nerve outcome [7, 11, 39,40,41].

A trend towards a more conservative surgical approach, which has also been reported in a longitudinal analysis by Piccirillo et al. in 2009, may conceal the worse facial function resulted by the cystic morphology [42]. Facial function has been shown to be a major predictor of VS patients’ quality of life [28]. Along with the unfavorable prognosis after SRS, it is enticing to conclude that a combination approach, e.g. STR with cyst resection followed by STR in cystic VS might ensure best facial nerve outcome and tumor control [2, 43]. Still, before putting these kinds of recommendations forward, further investigation must be done, on how the solid residual tumor reacts to SRS. It has been described that SRS may cause the micro-hemorrhages and therefore, could promote new cyst formation in tumor tissue already prone to cyst formation [6, 44, 45]. Yang et al. have shown retrospectively that adjuvant SRS after complete cyst removal ensures higher tumor control rates then in solid VS, however, their mean time of follow up was 4.5 years, which for a benign tumor such as VS may be too short of a follow-up [43]. A prospective study on a combination approach in cystic VS has yet to be done. As STR significantly prolongs mean time to recurrence to 8.92 years, STR can be a valid option for Elderly patients with cystic VS.

This study is limited by its nature of retrospective design, even though it involves more than one study center. This study involved a large group of patients with cystic tumor characteristics, however, the patient number and its value has to be put into its statistical context. An even larger patient cohort could even allow subgroup analyses within cystic VS (e.g. micro- and macrocystic morphology).