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To date, morphometrical data providing a systematic quantification of accessibility and operability parameters to the boundaries of the posterior portion of the third ventricle (PTV) when applying an endoscopic-assisted paramedian supracerebellar infratentorial approach (EPSIA) are lacking. We performed an anatomical study and complemented our findings with surgical cases.Eight EPSIAs towards the PTV were performed in cadaveric specimens. Optimal approach angles (OA), surgical freedom (SF) and operability indexes (Oi) to the PTV boundaries were assessed. Additionally, a 54-year-old man and 33-year-old woman were operated on PTV tumors applying the EPSIA.Sagittal OA to ventricle's roof and floor was 36±1.4° and 25.5±3.5° respectively, axial OA to the ipsilateral and contralateral ventricle's wall were 9.5±1.3° and 28.5±1.6°. SF was maximal on the contralateral wall (121.2±19.3mm2), followed by the roof (112.7±18.8mm2), floor (106.6±19.2mm2) and ipsilateral wall (94.1±15.7mm2). SF was significantly lower along the ipsilateral compared the contralateral wall (p0.01) and roof (p0.05). Facilitated surgical maneuvers with multiangled exposure were possible up to 8.5±1.07mm anterior to ventricle's entrance, whereas surgical maneuvers were possible but difficult up to 15.25±3.7mm. Visualization of more anterior was possible up to a distance of 27±2.9mm, but surgical maneuvers were barely feasible. EPSIA enabled successful resection of both PTV tumors and postoperative course was uneventful.EPSIA can be effective for approaching the PTV, enabling surgery along all boundaries, but especially on its roof and contralateral wall. In the not-enlarged ventricle, surgical maneuvers are feasible up to the level of the Monro foramen, becoming more limited anteriorly.