As of December 2001, 180,222 patients have been treated with 156 Leksell Gamma Knives worldwide. Among them, 22,529 cases of meningioma, accounting for 34.8% of benign brain tumors treated with Gamma Knife, ranked first among benign brain tumors treated with Gamma Knife, and ranked third after brain metastases and cerebral arteriovenous vascular malformations among the single disease types treated with Gamma Knife. This shows that stereotactic gamma knife treatment in the treatment of meningioma has a fairly important position. 1, the rationale for gamma knife treatment of meningioma The following factors determine the applicability of stereotactic radiosurgery (gamma knife) to the treatment of meningioma. First, most meningiomas have benign biological behavior. Enhanced CT or MR images can clearly reveal small meningiomas, and the image changes are specific, especially for meningiomas that have not been surgically removed. By accurately identifying the tumor boundaries during gamma knife treatment, the dose plan can completely cover the tumor tissue. Moreover, the dura mater, arachnoid and supplying blood vessels attached to the tumor can also be included in the treatment area. Second, meningiomas grow slowly. Meningiomas are generally rich in blood supply, and higher radiation dose irradiation produces delayed vascular occlusion, resulting in ischemia and necrosis within the meningioma. According to radiobiological classification, meningioma, as a benign brain tumor, is a late-responding tissue, and its surrounding brain tissue is also a late-responding tissue. Therefore, the effects of radiobiological effects such as tumor cell damage and meningioma supply vessel occlusion will be fully expressed. Thirdly, although radiosurgery uses radiation as external radiotherapy, the chance of secondary brain tumors after treatment is less than external radiotherapy because gamma knife is locally focused irradiation. In addition, the chance of secondary hypopituitarism after gamma knife treatment of meningioma in the saddle area is also lower than that of external radiotherapy. Fourth, although radiosurgery is a high-dose radiation, the incidence of cranial nerve function paralysis adjacent to the tumor after gamma knife treatment is still low because the irradiated dose to normal structures around the tumor decreases sharply in a gradient with increasing distance, suggesting the safety of radiosurgery treatment in protecting cranial nerve function. Finally, the treatment time of gamma knife is greatly shortened compared with ordinary radiotherapy, and the risk of surgical anesthesia, bleeding or infection is avoided, which is easily accepted by patients. 2, the indications for gamma knife treatment of meningioma Comprehensive literature [1,6], the indications for gamma knife treatment of meningioma include: ① meningioma growing at the base of the skull or deep in the brain; ② the average diameter of the tumor is less than 30 mm; ③ the tumor edge must be more than 5 mm from the optic nerve, optic cross and optic bundle; ④ multiple meningiomas, residual or recurrent meningiomas after surgery. ⑤ Patients with advanced age (>70 years old) and imaging data confirming persistent tumor growth. ⑥Patients with heart, lung, kidney, hematological system diseases or diabetes, etc., who are contraindicated to surgery or cannot tolerate surgery. 3, gamma knife treatment dose selection Until 1989 there was a large group of detailed reports on gamma knife treatment of meningioma. In the early days, the marginal dose of gamma knife for meningioma treatment was mostly chosen to be 15-18 Gy, or even up to 32 Gy [2]. Ganz (1993) proposed that the marginal dose of gamma knife for meningioma treatment should be no less than 12 ~15 Gy, and believed that the higher the dose, the better the treatment effect. However, after comparing the efficacy of two groups of cases with different marginal doses, Ganz (1995) emphasized that the marginal dose should not be less than 12 Gy. In 1997, after following the complications of high marginal doses for meningiomas, Ganz concluded that it is better to have a marginal dose of no more than 15 Gy and definitely not more than 18 Gy [4]. Pan et al. (1998) summarized the results of 80 cases and concluded that the choice of marginal dose was related to the volume of the tumor under illumination: for small meningiomas (≤5 ml) 15-16 Gy should be used; for larger meningiomas (≥10 ml) 12-14 Gy should be used. for meningiomas above 5 ml, marginal doses above 17 Gy are prone to significant complications [5]. In the group of cases where high marginal doses were used, Stafford (2001) correlated the choice of marginal dose with tumor volume, which was divided into three groups according to 14.1 cm3, with marginal doses corresponding to 20 Gy, 18 Gy and 16 Gy. Statistical analysis showed that there was no statistically significant difference between excessive marginal doses and the growth control rate of meningiomas [2]. Wang Binjiang et al. (1996) reported gamma knife treatment of 38 cases of postoperative residual or recurrent meningiomas. The mean marginal dose was 12.6Gy (10-20Gy). With a mean follow-up of 20 months, 14 cases had tumor shrinkage, including 5 cases with a marginal dose of 12 Gy and 9 cases greater than 12 Gy. A marginal dose greater than or equal to 12 Gy has shown good control of tumor growth [6]. Good results have also been reported with low-dose irradiation, and Nakaya et al. (1999) reported gamma knife treatment of 11 meningiomas with lesions adjacent to the optic cross or compressing the brainstem, with a marginal dose of less than 10 Gy. Patients were followed up for a mean of 35.7 months without recurrence of enlargement [7]. However, Shin (2001) reported that in 40 cases of cavernous sinus meningioma treated with gamma knife, 22 cases had a marginal dose greater than 14 Gy, with a mean follow-up of 37 months and a recurrence rate of 0%. 15 cases had a marginal dose of 10-12 Gy and a recurrence rate of 20%-100% [12]. At present, the marginal dose of gamma knife for meningioma treatment is mostly recommended to choose 12~15 Gy. 4. Tumor control rate The control of tumor volume change after gamma knife treatment was evaluated by regular imaging follow-up. Tumor shrinkage or growth control is a sign of effectiveness after gamma knife treatment of meningioma, and the tumor control rate of the treatment group can be derived from this. Stafford reported a tumor control rate of 89% at 5 years after gamma knife treatment, with 56% of meningiomas shrinking after treatment. The tumor control rate was significantly correlated with the histological performance of the tumor (P10ml), and in cases where side effects were also observed 6 months after treatment, the tumor area enhancement on MR gradually increased in volume and then decreased, but the abnormal enhancement area on follow-up images was always larger than the volume before treatment. 5.Multiple meningiomas The development of the GammaPlan system has made it possible to treat multiple lesions at once. The plan system can accurately calculate the dose of normal tissue irradiated during the one-time treatment of intracranial multiple lesions, providing the possibility of safe treatment. The emphasis on one-time treatment of multifocal lesions is aimed at reducing patient pain and treatment costs. Moreover, through proper dose planning and accurate expression of dose distribution, complications such as radioactive brain damage caused by excessive radiation dose can be reduced and satisfactory treatment results can be obtained. However, whether to adopt the one-time treatment method for multiple-issue lesions accompanied by the following conditions should be carefully considered When the lesions happen to be distributed on both sides of the brainstem, the brainstem is located in the high-dose area in the one-time treatment plan if treated simultaneously, which is more dangerous, so staged treatment is recommended. Patients with multiple tumors of different pathological nature in the brain, such as cavernous sinus meningioma combined with pituitary tumor, should also be treated with Gamma Knife in stages because of the significant difference in the treatment dose required for each of the lesions. 6. malignant meningioma Stafford, based on case follow-up [2], found that atypical meningioma or malignant meningioma, even with high-dose gamma knife treatment, the 5-year survival rate was only 76% or 0%, respectively, and the 5-year tumor growth control rate was 68% or 0%, respectively. It is believed that the recurrence rate of malignant meningioma after treatment is significantly higher compared to benign meningioma even after combined surgery, external radiation therapy and gamma knife treatment, and the 5-year survival rate after treatment is extremely low. This is supported by studies on pathological changes and immunohistology after gamma knife treatment [15-16].Ojemann treated 22 malignant meningiomas with gamma knife and the 5-year survival rate was 40%. Based on statistical analysis, it was found that malignant meningiomas less than 8 cm3 in size, where the patient is also younger, have a relatively good prognosis for gamma knife treatment [18]. 7. staged treatment For meningiomas that are too large to undergo surgery, staged treatment has been suggested. iwai reported staged gamma knife treatment of 7 cases of large rocky diagonal meningioma and cavernous sinus meningioma. The average volume of the lesion was 53.5 cm3 and the average volume of the target area treated was 18.6 cm3. 6 months between treatments, the average marginal dose was 9 Gy. The average follow-up was 39 months, and tumor growth was controlled in 6 cases [17]. pendl staged 12 large meningiomas with a tumor volume of 19 ~ 90 cm3, marginal dose of 10 ~ 25 Gy, 1-8 months between treatments, and a follow-up of 5 to 89 months. The follow-up was 5 to 89 months with good results [19]. For larger meningiomas, staged gamma knife treatment can reduce the incidence of side effects and improve the tumor control rate. 8. Complications 8.1 Early symptoms Transient headache, nausea or vomiting may occur within 24 to 48 hours after gamma knife treatment, especially in patients with tumors in the saddle area, rock slope or pontocerebellar horn. The reason for the appearance of symptoms is related to the acute reaction caused by radiation stimulation of the vomiting center at the base of the four ventricles, which can be relieved by symptomatic treatment with antiemetic and hormonal therapy [6]. Symptomatic antiepileptic treatment is still required for those with symptoms such as headache and epilepsy before treatment. 8.2 Cranial nerve dysfunction Stafford reported [2] that 24 (13%) of patients developed treatment-related complications. Of these, 15 cases (8%) developed cranial nerve palsy, including optic nerve, actinic nerve, trigeminal nerve, abducens nerve, facial nerve, and auditory nerve damage. There was no posterior group of cranial nerve damage. The time of appearance of cranial nerve damage ranged from 1 to 98 months (mean 6 months) after treatment. By the time of reporting, 2 cases had worsened, 8 cases had remained unchanged, and 4 cases had improved significantly. Statistical analysis did not reveal any association between tumor volume, marginal dose, and history of previous radiotherapy with radiation injury complications.Morita recommended [21] that the irradiated dose to the optic nerve should be less than 10 Gy and to the trigeminal semilunar ganglion less than 19 Gy. The irradiated dose to the lateral wall of the cavernous sinus should be less than 20 Gy. Because of long-term tumor erosion or compression of cranial nerves, it decreases the radiation tolerance of the nerve tissue. The tumor compression itself can cause nerve degeneration. Therefore, the treatment plan should be considered in a comprehensive manner. 8.3 Cerebral edema The most common complication after gamma knife treatment for meningioma is cerebral edema. Factors affecting the exacerbation of perineurial edema include: perineurial edema before treatment, treatment margin dose, tumor volume, and site, etc. Ganz et al [4] concluded that supratentorial meningiomas are adjacent to cortical veins and lack collateral circulation. When gamma knife treatment affects the drainage of deep veins, it will cause or exacerbate peritumoral edema. 8.4 Internal carotid artery injury Stafford [2] reported 2 cases of ischemic symptoms due to internal carotid artery injury after treatment of cavernous sinus meningioma. in a commentary following the article, Kondziolka denied that gamma knife causes late injury to the internal carotid artery, suggesting that such a rare complication may be related to prolonged compression of the internal carotid artery by the tumor. 9. Conclusion Surgical resection of the tumor remains the treatment of choice for meningiomas. For convex meningiomas, surgical resection is still recommended if there is no contraindication to surgery, even if the lesion is small. For deep, multiple or skull base meningiomas, especially those in the cavernous sinus, ventral side of the brainstem, and rock slope, if the tumor volume is within a certain range, gamma knife is preferred to effectively control the tumor and avoid surgical damage to the cranial nerves. Gamma knife treatment is an important treatment for post-surgical residual as well as recurrent meningiomas.