The thalamus accounts for less than 2% of the volume of the neuraxis, so tumors of this site origin are relatively few, accounting for 1%-5% of intracranial tumors. Glioma is the most common tumor in the thalamus, and it is mainly of low grade. Because of the deep location and adjacent to important functional areas, its treatment has been a difficult problem in neurosurgery. 1.Pathophysiology Thalamic glioma is a disease with unique anatomical structure and clinical process, which is mostly swelling growth. It is surrounded by brain parenchymal nuclei, so the boundaries of the tumor are relatively clear. The prevalent sites include the anterior superior thalamus (anterior thalamic nuclei, ventral nuclei and central nuclei) and the posterior nodes. The tumor may travel along the conduction tract, subventricular tract, or spread intraventricularly. The growth pattern of thalamic glioma can be divided into three types: (1) tumors are confined to the thalamus with destruction of important surrounding structures such as the internal capsule and neuropil; (2) tumors extend beyond the thalamus and reach upward or outward to the subcortical white matter of the adjacent lobes or gyrus; (3) tumors grow toward the ventricles but do not penetrate the ventricular wall. Depending on the growth site and scope, the tumor may involve the ventricular system and destroy the thalamic nuclei, nerve fiber conduction bundle or visual radiation, causing different symptoms and signs in patients. Clinical manifestations Thalamic glioma can occur in all age groups. The onset of the disease is mainly in adolescents. The rate of male and female is approximately equal. Some literature reports a slightly higher rate in males. Age has an important influence on the prognosis of thalamic glioma, and the prognosis of patients under 40 years of age is significantly better than that of patients of advanced age. The time interval from onset to clinical diagnosis is short, and early diagnosis is often possible, especially in children. Common clinical manifestations include: intracranial hypertension, motor deficits, epilepsy, involuntary movements, sensory deficits and thalamic syndrome. Involuntary movements are rare in patients with unilateral thalamic glioma and are more common in patients with bilateral thalamic involvement. Some patients may also present with visual field changes, oculomotor disorders, and psychobehavioral abnormalities. A small number of patients also have psychodermal syndrome. 3.Imaging performance Thalamic glioma is usually large, spherical or round-like, with relatively clear edges. CT shows that the lesion is well-defined, peritumoral edema is rare, and low or mixed density is predominant, which may be accompanied by enhancement. T2-WI shows a homogeneous or heterogeneous slightly high signal shadow with no edema band around the tumor. In a few cases, there may be no enhancement of the tumor, and astrocytoma and glioblastoma are similar in appearance, so it is not easy to distinguish them. Adult thalamic glioma can also infiltrate into the midbrain and spread to the contralateral thalamus through the middle block. Differential diagnosis (1) germ cell tumor is one of the few curable intracranial malignant tumors. Germ cell tumors of the thalamus account for about l3% of all germ cell tumors and 25% of tumors of the basal ganglia and thalamus. Thalamic germ cell tumors are almost always seen in adolescent males. Experimental radiotherapy can help diagnose the tumor, and radiation therapy is preferred. On imaging, tumors are morphologically heterogeneous with heterogeneous signal, cystic degeneration, necrosis and hemorrhage are common, and paraneoplastic edema is relatively mild. In addition, germ cell tumors in the basal nucleus may be accompanied by ipsilateral cortical atrophy. (2) Malignant teratoma is rarely seen in the thalamus, and the tumor rarely contains fat and calcification; it has irregular low signal shadow on T1-WI and non-uniform high signal shadow on T2-WI. (3) Ganglioneuroblastoma is rare in the thalamus, but it has unique imaging performance. T1-WI shows that the lesion is poorly demarcated from the surrounding brain tissue, and small focal hyposignal shadow can be seen inside; T2-WI shows that the lesion is cortical signal shadow, and small focal lesion inside is high signal; there is no paraneoplastic edema, and the tumor does not enhance after contrast injection, but small focal lesion inside is obviously enhanced. It may be accompanied by ipsilateral cerebral hemisphere malformation and ventricular enlargement, etc. 5.Treatment Thalamic glioma is located deep in the midline of the brain. It is adjacent to the internal capsule, hypothalamus, third ventricle and other important structures, so it is difficult to operate and has a high rate of surgical death and disability. Most scholars have a conservative attitude toward surgery and advocate radiotherapy after biopsy to prolong the survival time of patients. In recent years, due to the improvement of neurosurgical techniques and the improvement of thalamic surgical access, the surgical disability and mortality rates have been greatly reduced, and the surgical mortality rate in some hospitals has been reduced from 40% to less than 5%. The effective radiotherapy dose and the size of the residual tumor are directly related, and too high a radiation dose can lead to radiation brain injury, so gliomas with large residual tumors. Postoperative conventional radiotherapy is often ineffective. In order to prolong the survival time of patients, more and more scholars advocate surgical treatment. Although the thalamus is deep, except for its ventral lateral surface adjacent to the basal nucleus and internal capsule. The above anatomical features determine the feasibility of surgery. The existing surgical routes include transcortical (frontal, parietal, temporal, parieto-occipital) transcallosal thalamotomy, transcallosal (anterior corpus callosum, posterior corpus callosum) transcallosal thalamotomy, transcallosal intervallosal thalamotomy. Trans-lateral fissure thalamotomy and transcallosal supracerebellar thalamotomy. The basic surgical principles are: to remove the tumor to the maximum extent while preserving the normal physiological function, to ensure the smooth circulation of cerebrospinal fluid, to relieve the cranial hypertension, and to create conditions for radiotherapy and chemotherapy. (1) The choice of surgical approach is to adopt transcortical approach in the early stage: if the tumor is located in the anterior part of the thalamus, transcortical approach is adopted. If the tumor is located in the anterior part of the thalamus, transfrontal cortical approach is adopted; if the tumor is located in the posterior part of the thalamus (such as thalamus occipital), trans parieto-occipital cortical approach is adopted. In order to avoid complications arising from the disruption of the cerebral cortex by this approach, Prakasht used a transparietal hemispheric gap approach for thalamic gliomas located in the dominant hemisphere. For tumors located on the ventral posterior aspect of the thalamus, a transtemporal sulcus approach can be used to reduce damage to important vessels. The disadvantage of the transcortical approach is that it requires incision of the cerebral cortex, especially when the patient’s ventricles are small and the strain on the cerebral cortex and white matter is large, which can cause epilepsy or important neurological deficits (e.g., hemiparesis, visual field impairment, aphasia) after surgery. The advantage is that it is simple to perform and easy to reveal important reflux veins with a low probability of injury. To reduce the incidence of postoperative epilepsy and neurological deficits. Nowadays, more and more scholars advocate the use of a trans-callosal anterior lateral ventricular approach to remove tumors that protrude from the main body into one of the lateral ventricles or a trans-callosal posterior lateral ventricular approach to remove tumors located in the thalamus-occipital region, which takes advantage of the natural gaps in brain tissue and is not affected by the size of the ventricles. A small number of thalamic gliomas originate from the medial thalamus, grow in the midline and protrude into the third ventricle, which are mostly in the early stage when they are found and have low malignancy. As the main body of the tumor protrudes into the third ventricle, surgery should be performed via the third ventricle. The surgery should be performed via the third ventricle approach. Some scholars in China have adopted the trans-callosal interforaminal approach to resect medial thalamic glioma and achieved initial satisfactory results. The surgical features are as follows: ① Utilize the potential cavity in the developing brain tissue for surgical operation to minimize the surgical trauma. ② Short path to reach the third ventricle, adequate exposure, and separation of the internal cerebral vein or large cerebral vein adhering to the tumor under direct vision. ③Explore the interventricular foramen and superior port of the conduit under direct vision. Intraoperative fistula and hyaline septal fistula were performed according to the intraoperative situation. The hydrocephalus is released to the maximum extent possible. ④ It is not necessary to damage the cerebral cortex, fornix column, and thalamus vein to reduce postoperative complications. In addition, for thalamic glioma that grows ventrally and posteriorly to the thalamus and is closely connected to the insula, trans-lateral fissure thalamotomy can be chosen. The full length of the lateral fissure needs to be opened during the operation. The posterior part of the insula cortex is clearly identified (because this area is adjacent to the posterior border of the posterior limb of the internal capsule), and an incision is made in the middle of the central posterior sulcus of the insula. The subcortical tumor can then be found and removed. In principle, this approach is less traumatic to the brain tissue than the parieto-occipital cortical approach or the superior temporal sulcus approach, and is especially suitable for thalamic gliomas that are closely connected to the insular cortex. However, familiarity with the anatomy of this area is required. (2) Intraoperative points The following points should be noted during surgery of thalamic glioma: ① Before opening the dura mater. Intravenous drip of mannitol and dexamethasone, and patient aspiration of cerebrospinal fluid after opening the lateral ventricles. ② When entering through the anterior corpus callosum. Separate the longitudinal fissure 2 cm anterior to the coronal suture in a vertical direction toward the line of the two ears to reach the corpus callosum. The corpus callosum incision is controlled to within 2 cm. ③ When separating the fornix to enter the third ventricle, enter along the midline and separate the fornix suture above the interventricular foramen. If necessary, a hyaline septal fistula on one side is feasible, and the location of the interventricular foramen is determined via the lateral ventricle. ④ Fully expose the tumor and its surrounding important structures to facilitate tumor resection and hemostasis. ⑤ When resecting the tumor, first separate the ventricular system from the tumor operative area with cotton sheets to prevent intraoperative blood diffusion to the distant area, and then open the cerebrospinal fluid pathway after near total resection. (6) The thalamic vein is used as the anatomical marker of the ventral lateral border of the thalamus to protect its lateral important structures, and the tumor is first resected in intra-tumoral blocks, and the margins of the tumor are separated after the tumor volume is reduced. (7) The choroid plexus in the operative field can be electrocoagulated. If the interventricular foramen cannot be opened, a transparent diaphragmatic fistula should be performed. (8) The layers should be tightly sutured, and the lateral ventricles should be routinely drained with tubes. If hydrocephalus exists after surgery, shunt should be performed as early as possible. (3) Postoperative complications Postoperative complications include hydrocephalus, intracranial infection, intraventricular hemorrhage, hemiplegia, hemianesthesia, visual field loss, aphasia, coma, etc. Good knowledge of anatomy. The selection of the correct surgical approach and skilled micro-neurosurgical operation can help the operator to reduce the occurrence of the above complications.