Abstract: Objective To investigate the anatomic characteristics of extended endoscopic endonasal transsphenoidal approach to the suprasellar region and the third ventricle, which include searching respective anatomic landmarks in different steps and finding different routes from the suprasellar region into the third ventricle. Methods Four formalin-fixed and one fresh adult cadaver heads were dissected using an extended endoscopic endonasal transsphenoidal approach to expose the suprasellar region, followed by entering into the third ventricle through infrachiasmatic and suprachasmatic region respectively. The observing was completed with 0-degree lens and 30-degree lens. Anatomic measurements were obtained. Results The anatomic landmarks were choana and sphenoid ostium in the nasal step, optic protuberance, medial and lateral opticocarotid recess, carotid protuberance, sellar floor, tuberculum sellae, planum sphenoidale, clival recess in the sphenoid sinus step, lamina terminalis and tuber cinereum in the intradural step. The numbers of the sphenoid septations were ranged from 1 to 4. There were 2 sphenoid septations inserted at the carotid protuberance in 4 cadavers. The distance between the choana and sphenoid ostium was (12.1±2.3)mm (9.9-15.0mm) in the left nasal cavity and (13.3±2.6) mm (9.6-16.8mm) in the right nasal cavity；The distance between the bilateral medial opticocarotid recess was (9.5±3.0)mm (5.5-13.8mm)；The distance between the bilateral lateral opticocarotid recess was (14.8±3.7)mm (9.2-19.2mm)；The distance between the carotid protuberance was (16.0±3.1)mm (11.3-18.8mm). Infrachiasmatic region and suprachiasmatic region were both fully exposed. After opening the tuber cinereum via the infrachiasmatic region, the roof, posterior wall and floor of the third ventricle were exposed. The third ventricle can also be exposed by opening the lamina terminalis via the suprachiasmatic region. The lamina terminalis approach was better in exposing the floor of the third ventricle, while the tuber cinereum approach was better in exposing the roof. The 30-degree lens supplemented the view of 0-degree lens. Conclusion The extended endoscopic endonasal transsphenoidal approach is feasible to expose the suprasellar region and enter into the third ventricle through two corridors, which is a safe route to remove lesions in this areas. The anatomic landmarks in differen

Key words: Endoscopy, Extended endonasal transsphenoidal approach, Suprasellar region, Third ventricle, Anatomy, Human