**9.3 Oncological outcome**

The quality of surgical resection of meningiomas, assessed by Simpson's grading system, remains an important prognostic factor in the evolution of these tumors, regardless of the histological subtypes considered [40, 41]. Gross total resection,

defined as Simpson grade I to III, is achieved in 25–70% of SOM cases depending on the series [34, 42–44]. Given the complex anatomy of the spheno-orbital region, Simpson grade I or II resections are rarely feasible without risking the induction of cranial nerve deficits, especially at the level of the orbital apex, SOF, and CS [43]. In this context, the current trend is strongly in favor of symptom-oriented surgery rather than radical surgery, targeting optic nerve decompression to improve visual function, and intraorbital tumor resection to reduce proptosis [30, 32].

Histologically, SOMs are commonly World Health Organization (WHO) grade I tumors (77–100%, depending on the series), with the meningothelial subtype being the most frequent [11, 13, 28, 43]. Although much less frequent, WHO grade II (atypical) or III (anaplastic) meningiomas may be encountered, together representing 11% of the cases in the retrospective series presented by Belinsky *et al.* Moreover, the authors highlighted the strong correlation between WHO grading, Ki67 proliferation index, and clinical progression [45]. Thus, WHO grade II and III meningiomas, which are associated with an aggressive clinical course and high recurrence rate compared to WHO grade I tumors, have Ki67 proliferation indices that proportionally predict their behavior (14.9 and 58.3, respectively). When considering only WHO grade I SOM, a Ki67 index ≥3.3 is associated with a higher risk of recurrence. Comparing these different histological subgroups, Agi *et al.* reported a recurrence rate of 22% in the WHO grade I tumors and 50% in the WHO grade II tumors, with a mean follow-up of 57 months [46].

The highly variable recurrence rate of SOM in the scientific literature, ranging from 10 to 56%, is likely due to differences in the follow-up duration [34, 47–49]. Indeed, the risk of recurrence logically increases with the duration of follow-up (6% at 3 years and 46% at 6 years after the intervention) [28].

The role and timing of radiation therapy remain a matter of debate for meningiomas in general and SOM in particular. However, experts (Response Assessment in Neuro-Oncology Committee) agree on the importance of using adjuvant radiotherapy for WHO grade III meningiomas, regardless of the quality of surgical resection [40]. For WHO grade II meningiomas, the European Association of Neuro-Oncology guidelines recommends observation or fractioned radiotherapy in cases of gross total resection and fractioned radiotherapy in cases of subtotal resection [16]. For WHO grade I meningiomas, simple observation is indicated after gross total resection, and stereotactic radiosurgery or fractioned radiotherapy may be proposed after subtotal resection. We suggest radiological monitoring of the tumor residue for subtotal resections of WHO grade I meningiomas initially. Stereotactic radiosurgery is then justified in cases of objective progression. Cases of meningiomas of WHO grade II or III must be discussed in a collegial manner in a multidisciplinary consultation meeting.
