Brain metastases are common intracranial malignant tumors that metastasize to brain tissue through blood and lymphatic systems from tumors originating outside the central nervous system ( about 10%-15% of intracranial tumors), and about 20%-40% of cancer patients develop intracranial metastases throughout the course of their disease (16). . The primary sites of brain metastases are lung and breast, followed by melanoma, choriocarcinoma, gastrointestinal tract tumors and kidney cancer (among which lung cancer brain metastases account for 30-40%). 50% of patients have multiple intracranial metastases. Most of the intracranial metastases are located in the bilateral cerebral hemispheres (85%), followed by the cerebellar hemispheres (10-15%), and rarely in the brainstem (1-3%) (16). The short clinical course and rapid progression of intracranial metastases, with significant peri-lesional edema and occupying effects, are the main causes of rapid failure of tumor patients until death. For patients with diagnosed tumors, if intracranial hypertension symptoms such as headache, nausea, vomiting and local occupying signs such as hemiplegia and aphasia appear within a short period of time, the possibility of intracranial metastases should be considered. Some patients first present with intracranial symptoms and localized signs, and the primary site is found elsewhere only after postoperative pathological diagnosis or diagnosis of brain metastases based on imaging. In some patients, the primary site is not even found during the entire course of the disease. The average survival of untreated intracranial metastases is about 1-2 months. There are several treatment methods for intracranial metastases as follows. 1.Total extracerebral radiation therapy: Considering that the occurrence of intracranial metastasis in tumor patients is already predictive of advanced stage, the standard treatment plan for most patients with intracranial metastases in the past decades has mostly been total extracerebral radiation, hormones and other palliative treatments, and the average survival after treatment is mostly reported to be 3~4 months (6). Whole brain external radiation treatment is long, with low local dose of lesion irradiation, poor local control, radiotherapy reactions such as local scalp edema, hair loss, vomiting, etc., and long-term complications such as dementia and under life treatment. At present, whole brain radiation therapy is mainly used as an adjunct to surgical resection and stereotactic radiosurgery in the treatment of intracranial metastases, and whole brain radiation therapy is rarely used as the preferred treatment method alone. Patchell RA reported in 1998 that after surgical resection of single intracranial lesions, combined with whole brain radiation therapy can significantly reduce the rate of local recurrence and recurrence in other parts of the skull compared with surgery alone (17). Surgical treatment: So far, surgical intervention is still the only effective means to relieve acute intracranial hypertension caused by intracranial metastases, and the main indications are: 1) large size of single intracranial metastases, obvious occupying effect, severe intracranial hypertension; or lesions leading to epilepsy that are difficult to be controlled by drugs. 2) lesions located in non-important functional areas and non-deep brain and brainstem sites, and the patient is generally in good physical condition and can tolerate surgery. The patient is in good health and can tolerate the surgery. 3. The cystic lesion is large in size. Surgical treatment combined with whole brain radiation therapy may reduce the recurrence rate, including in situ recurrence and recurrence in other parts of the skull, but it does not seem to have a significant impact on the average survival and survival treatment.4. In patients with multiple metastases, surgical resection is feasible for larger lesions to relieve significant intracranial hypertension, and the rest of the lesions receive whole brain radiation therapy or stereotactic radiosurgery. Stereotactic radiosurgery: In essence, stereotactic radiotherapy is a kind of radiotherapy, in which high-energy rays are focused on the location of intracranial lesions from different directions, and the radiation dose outside the lesion is sharply reduced. It is a more targeted and efficient radiotherapy than whole brain external radiation therapy. Since the application of stereotactic techniques in the treatment of intracranial metastases, a large number of retrospective clinical studies have demonstrated that stereotactic radiosurgery techniques or combined with whole brain radiotherapy can significantly improve the survival and achieve satisfactory local control of patients with intracranial metastases, especially in patients with less than 3 metastases, and that this method can target important functional areas and deep brain metastases. Andrews D.W et al. reported a retrospective study of patients with intracranial metastases treated with total external brain radiation alone and with total external brain radiation combined with stereotactic radiation therapy. The mean survival, local control rate, and postoperative quality of life of the two groups were compared, and it was noted that the combination of whole-brain external radiation therapy and stereotactic external radiation therapy significantly prolonged the survival of patients with single metastases, and that patients treated with stereotactic radiosurgery had higher and more stable KP scores and better local control rates. However, the authors’ study did not show a statistically significant difference in survival between the two groups in patients with multiple foci, but taking other factors into account, they also recommended stereotactic radiosurgery in combination with whole-brain external radiation therapy for patients with multiple metastases (3). The results of a randomized controlled clinical retrospective study of patients with multiple intracranial metastases treated with stereotactic radiosurgery showed that stereotactic combined with total extracerebral radiation therapy significantly improved survival (11 months versus 7.5 months) and local control rates in patients enrolled with two to four intracranial metastases, and the two groups were comparable in terms of age, gender, preoperative KP score, and distribution of number, size, and pathological type of lesions (2). The rationale for stereotactic treatment of intracranial metastases: most brain metastases are located in the middle cerebral artery blood supply area, and the lesions are mostly located in the frontal or parietal lobes, usually growing in relatively superficial areas, such as the gray-white matter junction with good blood supply; the shape of brain metastases is mostly round or oval, the imaging boundary of the lesions is clear, and most brain metastases are relatively small; within the brain tissue metastases often squeeze the brain tissue to the periphery to CDC dose distribution characteristics make the chance of normal brain tissue damage reduced; most metastases are less infiltrative, the irradiation field can completely include the lesion, and the scope is easy to conform. Indications: Since the application of stereotactic radiosurgery in the treatment of intracranial metastases, there is still no unified view on its indications. In principle, stereotactic radiosurgery is acceptable except for large size (over 3 cm in diameter), intracranial occupying effects requiring urgent surgical treatment, lesions with mainly cystic lesions, and extensive meningeal metastases. The lesions with mainly cystic lesions can be treated with stereotactic radiosurgery after stereotactic cystic fluid aspiration for their body parts. Since stereotactic radiation therapy can treat multiple lesions simultaneously and the injury and complications of the method itself are mild, the number of metastatic lesions is not the main limiting indicator for the indication, although most scholars believe that patients with less than three lesions have better outcomes after stereotactic radiation therapy. Serizawa 2000 reported a retrospective clinical study of stereotactic radiosurgery versus whole brain radiation alone for multiple intracranial metastases, and showed that stereotactic radiosurgery was superior to whole brain radiation alone in terms of survival and local control in the treatment of multiple intracranial metastases (12). The follow-up results showed that the mean survival of group B was lower than that of group A, but the mean survival of patients with RPA grade II in group B was 36 weeks, and finally the authors pointed out that for patients with multiple intracranial metastases with RPA grade I-II, the stereotactic radiosurgery approach is significantly better than that of whole brain external radiation therapy alone (14). For patients with a single lesion located in the deep brain or brainstem region or in important functional areas that cannot be surgically resected or whose general condition is too poor to tolerate general anesthesia, stereotactic radiosurgery can be received; however, for patients with a single intracranial metastasis that can be easily surgically resected and has a small lesion size that is not yet clearly intracranially suitable for stereotactic radiosurgery, whether to receive surgery or stereotactic radiosurgery is uncertain. Muacevic A 1999 reported a comparison of the efficacy of surgery combined with whole brain external radiation therapy and stereotactic radiosurgery for single intracranial metastases (11), the case selection criteria were: single, lesion diameter Q3.5 cm, stable systemic disease, no obvious intracranial occupancy The final conclusion was that the survival period of the stereotactic radiosurgery group was slightly shorter than that of the surgery group, but the difference was not statistically significant, and the one-year survival rate and the one-year local control rate of the two groups were basically the same. Stereotactic radiosurgery should be considered first for single intracranial metastases with a diameter of Q3.5 cm, while surgery with whole brain external radiation therapy is suitable for patients with a diameter >3.5 cm. Treatment implementation: head mounting, positioning scans, (where positioning scans are usually performed with thin layer enhanced MRI sequential scans, double dose of enhancer is recommended for clearer visualization of lesion margins and detection of small metastases) irradiation range outlining, treatment dose planning, and irradiation, etc.