Skull Base Surgery of the Posterior Fossa

124

Fig. 9.6 Illustration demonstrating the type of tumor by

location. A tumor located over the vein of Galen and com-

pressing it downward is classified as the superior type (a),

and one situated under the vein of Galen and dislocating it

upward is the inferior type (b)

technique are available in the literature [ 1 , 3 ].

Because of the lesion’s depth from the surface

and its anatomical proximity to critical neural

and vascular structures, surgical access and

technique are complex issues. A variety of fac-

tors, including the tumor location and the patency

of the vein of Galen and the straight sinus, influ-

ence surgery and the outcomes. Falcotentorial

meningiomas are difficult to treat, but they can be

well controlled by meticulous strategy.

Surgical Planning

Preoperative neuroimaging investigations include

MR imaging, MR venography, CT venography,

and angiography. Apart from evaluating the physi-

cal characteristics of the tumor, the relationship of

the tumor to the great vein of Galen, the patency of

the vein of Galen and the straight sinus are evalu-

ated, and pre-existing occlusion of the Galenic

system and the subsequent development of

collateral venous circulation are important factors

when considering surgery on pineal region tumors,

including falcotentorial junction meningiomas [ 3 ].

Depending on the relationship of the tumor to

the great vein of Galen, tumors are classified into

two types: tumors located superior to the vein

and compressing it downward are labeled the

superior type, whereas those displacing it superi-

orly are labeled the inferior type (Fig. 9.6).

The superior type of falcotentorial meningi-

oma growing inside the posterior pericallosal cis-

tern might compress deep veins over the

arachnoid membrane. In this situation, a thick

arachnoid membrane septum between the poste-

rior pericallosal cistern and the quadrigeminal

cistern protects the deep veins from direct tumor

invasion, which enables the surgeon to dissect

the lesion from the deep veins. Therefore, in the

case of superior type tumor, even when the vein

of Galen is patent, careful surgical technique

enables the surgeon to separate the tumor from

the vein of Galen. Thus, complete surgical

removal can be relatively safely performed in the

case of superior type tumors.

The inferior type of falcotentorial meningi-

oma growing in the quadrigeminal cistern might

compress the deep veins and dorsal midbrain in

direct contact with it. Accordingly, in many

cases of inferior type, the tumor adheres tightly

to the vein of Galen, basal vein, collateral veins,

and midbrain. In cases of inferior type tumor

with an occluded Galenic system, dissecting the

H. Morisako et al.