I. Classification of skull base tumors Skull base tumors mostly originate from various types of tissues in the skull base and grow along the skull base, accumulating nerves and blood vessels. Tissue sources are often cranial nerves, blood vessels, meninges and other soft tissues or the skull itself, and they have the tendency to invade the structures of the skull base (e.g. paranasal sinuses, orbital cavities, pterygoid saddle, nasopharynx, slope, occipital foramen, craniocervical region or infratemporal fossa, etc.). The traditional classification of skull base tumors is based on the anatomical location of the tumor, which is divided into three major categories: anterior fossa, middle fossa and posterior fossa tumors, and sometimes tumors in the saddle area, the rocky slope area, and the occipital foramen magnum area are also listed as separate categories. Some people further classify tumors of the middle and posterior cranial fossa bases into pterygoid base, petro-temporal base and slope tumors. Lateral cranial base is below the skull base, along the infraorbital fissure and the rock-occipital fissure each make a prolongation line, the intersection angle of the two lines is about 90 °, inward intersection angle in the nasopharyngeal apex, lateral cranial base, that is, the area between the two lines. The lateral skull base can be divided into six subdivisions: (1) nasopharyngeal area; (2) pharyngeal tube area; (3) neurovascular area: the internal carotid artery, jugular foramen, hypoglossal nerve foramen, and penile mastoid foramen, a total of four foramina of the skull base. The external orifice of the internal carotid artery is passed by the internal carotid artery and the sympathetic fibers of the internal carotid artery from the superior cervical ganglion. The jugular foramen has the internal jugular vein and the IX, X, and Ⅺ cerebral nerves passing through it; the hypoglossal foramen passes through the nerve of the same name; and the facial nerve exits the foramen of the stem at the base of the temporal bone; (4) the auditory region; (5) the articular region; and (6) the area of the infratemporal fossa. Tumors of the lateral skull base mainly include jugular vein globe tumor, slope chordoma, meningioma, nerve sheath tumor, deep lobe tumor of parotid gland and nasopharyngeal fibrovascular tumor. Evolution of surgical approaches Over the past decades, doctors of neurosurgery, otorhinolaryngology, and head, neck and maxillofacial surgery have been exploring various surgical approaches and perfecting surgical methods through unremitting efforts, and generally developing along the two lines of the traditional open microsurgical approach and the transnasal endoscopic approach. Traditional open surgery involves the design of a large incision on the lateral or anterior aspect of the head and face, with direct visualization to reach and remove the skull base lesion. The access to the skull base from the lateral side has been proposed and perfected by the famous American otorhinolaryngologists, the House brothers, with the transcranial approach (including the labyrinthine approach and cochlear approach), on which Fisch of Canada expanded forward to design the classic infratemporal fossa approach, and Glasscock of the United States improved Fisch’s Y-shape incision into the large curved incision behind the ear, which is the more commonly used method nowadays. The lateral approach allows for the visualization and management of lesions in the infratemporal fossa, jugular foramen region, pontine cerebellar angle, acromion, and slope by resecting a portion of the acromion and, if necessary, dissecting the zygomatic arch and maxilla, and revealing and displacing the facial nerve. The subfrontal approach designed by Derome et al. is another classic skull base surgical approach, which opens the skull through a large coronal incision and bifrontal and orbital parietal flaps, and then dissects the posterior sieve, pterygoid plateau, and anterior saddle nodes to reach the slopes via the pterygoid sinus under direct visualization. This approach reveals the central region of the skull base up to the middle of the slope and is limited on both sides by the internal carotid artery. In the early 1990s, the transmaxillary approach proposed by Weilin in Hong Kong and the facial transposition approach proposed by Janecka in the United States belonged to the same category, which was to incise from the anterior face and turn over the maxilla or mandible to reach the skull base directly. This approach can reveal almost the entire skull base and can deal with a wide range of large malignant tumors and lesions under direct vision, with the disadvantage of leaving facial scarring. These traditional open surgical approaches have been improved over the past 30 years and have been able to deal with a wide range of lesions in all parts of the skull base and have been widely used in clinical practice. Most of the centers performing skull base surgery in China have become proficient in these means, and a large number of cases are reported each year. It is noteworthy that, whereas in the past a number of master neurosurgeons of the skull base demonstrated complex and “monumental” surgical accesses for removing bone from the base of the skull, there is a trend towards smaller and simpler accesses, as many specialists are becoming aware of the negative aspects of too much trauma in accesses. In other words, if you can use a simple approach, you don’t need to use a complicated one; if you can use a time-saving approach, you don’t need to use a time-consuming one; if you can use a familiar approach, you don’t need to use an unfamiliar one. The technique of transnasal endoscopic surgical access continues to mature and has been used to remove a variety of tumors and other lesions that encroach on the skull base, including chordoma and chordosarcoma, nasopharyngeal fibrous hemangioma, olfactoryoblastoma, trigeminal nerve sheath tumors, meningiomas of the saddle region, craniopharyngiomas, as well as anomalous bony fibrous hyperplasia, meningoencephalic brain bulge repairs, etc. Transendoscopic resection of cavernous blood vessel malformations in the cavernous sinus has even been reported. The endoscopic transnasal approach utilizes the naturally occurring nasopharyngeal cavity and sinuses as a conduit to reach the skull base, while avoiding facial dissection, which is less invasive than traditional craniomaxillofacial dissection. It should be emphasized that endoscopic skull base surgery is only less traumatic in terms of surgical access, and the extent and thoroughness of resection pursued in removing skull base lesions is not different from that of traditional open surgery, resulting in practically the same tissue and structural trauma, and therefore some senior specialists do not think it is appropriate to refer to it as a minimally invasive surgery in order to avoid being misleading. The enlarged endoscopic transnasal approach usually requires removal of the posterior nasal septum, part of the turbinate, and the paranasal sinus wall, creating the necessary surgical maneuvering space within the nasopharyngeal cavity, and these disruptions cannot be recovered. In the removal of intracranial lesions such as anterior skull base meningiomas, there is a high demand for repair of defects in the skull base, and there is still a lot of micromanagement with regard to the thoroughness of lesion management. Physicians accustomed to the use of surgical microscopes can absorb some of the concepts of the expanded endoscopic transnasal approach, but a cautious attitude toward the surgical approach and outcome is appropriate. Third, the development of related technology 1, microscopic operation technology: with the wide application of surgical microscope, a variety of microscopic operation technology gradually popularized, in the early 90’s Europe and the United States began to organize a variety of skullbase surgery training courses (skullbasehand-onworkshop), our country also in the late 90’s began to be held in Beijing, Shanghai and other cities to organize this kind of learning class, more than 20 years has been Over the past 20 years, a large number of excellent skull base surgical talents have been trained in large hospitals all over the country. In addition, many doctors pay attention to the anatomical study of the skull base and go abroad for further study, which also promotes the significant improvement of microscopic technology and the construction of the talent echelon, adding vitality to the development of microscopic neurosurgery in China. 2.Endoscopic technology: endoscopy has become an important tool in the field of skull base surgery, which can be used individually or jointly for the surgery of skull base tumors. At present, the application of endoscopic transnasal approach alone has been able to deal with pituitary adenomas, saddle node meningiomas, chordomas and other tumors of skull base in the midline area, but the results of some practices are still to be observed in the long term. Endoscopic resection of tumors extending laterally toward the skull base is in the stage of anatomical studies and preliminary clinical trials. In addition, endoscopy can be used to take biopsies through natural channels such as nasal cavity and sinuses to clarify the nature of the lesion and provide a basis for the development of treatment strategies and the selection of surgical access; it can also be used for follow-up, observation of postoperative healing of the skull base and the presence or absence of cerebrospinal fluid leakage, and early checking of the tumor for recurrence. With the progress of endoscopic equipment and technology, endoscopy will play an increasingly important role in the practice of skull base tumor surgery. 3, Endovascular intervention technology: Cerebral angiography can be used to understand the blood supply of the tumor and the traffic between the internal carotid artery system, vertebral artery system and external carotid artery system before operation. For the skull base tumor with rich blood supply, the main blood supply artery for the tumor can be embolized before operation, so as to reduce the intraoperative hemorrhage. Especially blocking those vessels with special location that can only be exposed at the late stage of tumor resection. Balloon occlusion test (BTO) plays an important role in skull base tumor surgery, which can assess the patient’s tolerance to occlusion of important blood vessels, and its sensitivity can be increased by combining with xenon CT, SPECT and other examinations. 4, neurophysiological monitoring technology: at present, brainstem auditory evoked potentials, somatosensory evoked potentials, nystagmography, intraoperative electroencephalogram, etc., Ⅱ to Ⅻ cranial nerves can be done intraoperative monitoring to improve the safety of the operation and the rate of total tumor resection. At present, many large hospitals in China have carried out intraoperative neurophysiological monitoring and achieved good results. 5. Skull base reconstruction technology: The development of materials and techniques for skull base reconstruction has greatly promoted the development of skull base surgery, making it possible to routinely resect inoperable skull base tumors decades ago. Especially since Jacobsen et al. pioneered microvascular surgery, free flap grafting with vascular tibia for skull base reconstruction has enriched the local myocutaneous flap, adjacent myocutaneous flap with tibia, artificial dura mater, titanium plate and other repair materials and methods. Reliable skull base reconstruction can establish a barrier between intracranial and extracranial structures to avoid cerebrospinal fluid leakage and intracranial infections; occlude the dead space left after tumor resection; cover or isolate the internal carotid artery to prevent vascular rupture caused by tumors or infectious erosion; and facilitate the restoration of the craniofacial shape. Cranial base reconstruction is constantly progressing and has become an important part of cranial base surgery. 6. Vascular reconstruction technique: Skull base tumors tend to involve the rocky bone segment of internal carotid artery and cavernous sinus segment, if there is a real need to sacrifice the infested blood vessels, we should try our best to carry out vascular reconstruction. As early as 1969, Yarsargil described superficial temporal artery-middle cerebral artery anastomosis, and then Lougheed, Spetzler, Story, Miller, Fisch, etc. described different vascular reconstruction techniques, which improved the surgical efficacy of cranial base lesions. At present, saphenous vein, radial artery and superficial temporal artery can be chosen to do vascular bypass according to the different diameters and flow rates of the required blood vessels, which is being gradually carried out in China. 7. Imaging technology: The application of CT in the 1970s and MRI in the 1980s have undoubtedly promoted the diagnosis and treatment of skull base tumors. With the combination and development of image processing technology and computer technology, virtual reality (virtualreality) technology can enable us to get three-dimensional, three-dimensional, interactive images, to understand in detail the relationship between the tumor and its surrounding important neurovascular structures, to formulate the surgical plan, to optimize the surgical access and to simulate the surgical operation in the virtual reality environment. Technologies such as neuronavigation and intraoperative open MRI can provide real-time intraoperative images to guide surgical operations. Intraoperative fluorescent imaging or digital subtraction angiography (DSA) can provide information such as vascular patency. The development of these imaging techniques has improved the early diagnosis rate of skull base tumors as well as the safety and effect of surgery. 8.Other technologies: Stereotactic radiosurgery technology: such as gamma knife, X-knife, Cyber knife and other precise radiotherapy technology, has become an important auxiliary means of treatment for surgery, for some cavernous sinus meningiomas and other lesions, radiosurgery can also be applied separately. In addition, advances in neuroanesthesia technology have made surgery safer and easier to operate.