Fig. 3.

Left thalamopeduncular pilocytic astrocytoma. A: Composite of 3 axial T1-weighted Gd-enhanced MR images demonstrates that the tumor arises from the lateral aspect of the peduncle underneath the thalamus, pushing the normal thalamus superiorly. The thalamic displacement made a transcallosal approach to the tumor a poor choice, because the surgeon would have violated the normal thalamus to reach the tumor. B: The optic tract, a structure that must be carefully avoided in removing the tumor, is deviated superior and lateral to the tumor. The arrow designates the optic tract. C: An axial DT image of the same tumor shows the CSTs deviated anteriorly and laterally (arrow; CSTs are in blue); this was the most common pattern of CST displacement in this series, noted in 7 of 10 patients. This pattern of tract displacement made a transsylvian approach unattractive; one would have to transect the tracts to reach the tumor. D: The authors chose an approach through the middle temporal gyrus by using frameless stereotactic navigation to approach the tumor just posterior to the CSTs.