The patient went on to have craniotomy with an interhemispheric transcallosal approach. 

Histology: 

Sections show anastomosing cords and sheets of squamous epithelium, which show peripheral palisading, surrounding loose fibrovascular cores containing foamy macrophages. No calcification or keratin are seen. No mitoses are identified. 

On immunohistochemistry, the atypical cells are positive for BRAF V600E and show membranous Beta-catenin staining. Nuclear Beta-Catenin staining is not seen. CK7 shows patchy staining, predominantly in the superficial layer of squamous cells. Ki-67 is 2%. 

FINAL DIAGNOSIS:  papillary craniopharyngioma (WHO grade I)

Discussion:

Although craniopharyngiomas are thought of as suprasellar tumors, they can be entirely within the third ventricle.  This is, admittedly rare. In such instances, they arise from the floor of the third ventricle and are usually of the papillary type in adults ¹.