Meningiomas in pineal region are rare, accounting for 0.3%-1.0% of intracranial meningiomas and 2%-8% of pineal region tumors[6, 7]. At present, the classification of pineal meningiomas into FT meningiomas and VI meningiomas is generally accepted. FT meningiomas originate from the arachnoid membrane attached to the FT junction and protrude into the pineal region. Therefore the tumors are directly related to the the dura of FT junction. However the VI meningioma originated from the arachnoid membrane covering the VI, located in the pineal region, and had no direct ralationship with the the dura of FT junction. Therefore, the main difference between FT meningiomas and VI meningiomas is whether the tumors are directly related to the dura of FT junction[4, 8]. Unfortunately, it is still difficult to distinguish between FT meningiomas and VI meningiomas even by modern imaging methods, so further confirmation is needed during surgical operation.
The VI meningiomas are rare clinically and only scattered case reports were found in literature review. According to the data provided by Champagne and Bojanowski, up to 2014, there were only 22 cases reported worldwide over a 70-year period. In 2014, Nowak et al. reported 6 cases of pineal meningiomas treated surgically during the last 20 years, of which 2 cases were VI meningiomas, accounting for 1/3. It can be seen that the proportion of VI meningiomas in pineal meningiomas is not low. These contradictory data indicate that the incidence of the VI meningioma is underestimated or overestimated. The author believes that the reason is that the accurate concept of the VI meningioma is still not clear enough.
The origin of meningiomas in pineal region was studied by imaging examination combined with intraoperative verification among this group of patients. Of the 21 cases of tumor, 12 cases were confirmed to be FT meningiomas, which was directly related to the the dura of FT junction; 9 cases should be classified as VI meningiomas，which had no direct relationship with the dura of FT junction according to the current commonly used classification of pineal meningiomas. However, only one case of the 9 patients originated from the posterior segment of the ICVs within the VI (Fig. 4), the other case from the posterior part of the pineal gland, and the other 7 cases from the arachnoid sleeve of the GV.
Since meningiomas originate from the cap cells of inner or outer arachnoid, those in the pineal region can originate from two sites. First, they can arise at the FT junction from the arachnoid layer, which tightly follows the dura. Although FT meningiomas are the most common, they are not considered to be “true” pineal region meningiomas because they do not originate from the region itself and just grow toward it[10, 11]. Second, they can derive from the arachnoid envelope over the pineal region (AEPG) and the arachnoid architecture within the VI[9, 12]. Therefore it is not appropriate to divide pineal meningiomas into FT meningiomas and VI meningiomas because the term "VI meningiomas" reflects only a part of meningiomas of the arachnoid of the pineal region. Although it is worth discussing, the term " VI meningiomas " is still used in this paper in order to be consistent with the literature. In fact, the term "VI meningiomas" appeared before the advent of CT and MRI the definition of VI meningiomas in the previous literature was vague. Lozier, A.P stated that tumors that arised from the ventral tela choroidea, the dorsal tela choroidea, or the posterior tenia fornicis (the site of attachment of the dorsal tela choroidea) in the third ventricle might be referred to as VI meningiomas. Nowak, A., et al. suggested that VI meningiomas, without dural attachment in the pineal region, arose from the posterior portion of the velum interpositum. Bojanowski found that VI meningiomas were more commonly found on the inferior leaflet of the VI because most VI meningiomas arose not from the VI itself, but from the cap cells presented in the choroid plexus, which was adjacent to the inferior leaflet of the VI. The reason why the definition of VI meningiomas was vague was that there existed controversy on the arachnoid architecture within the velum interpositum. In fact, there are two arachnoid layers within the VI. The dorsal layer of arachnoid membrane envelops the ICVs while the ventral layer of arachnoid membrane is a direct anterior extension of the APEG and covers the midline inferior layer of tela choroidea. So the meningiomas that really originate from these two parts of arachnoid cap cells can be called VI meningiomas. Meningiomas that actually originate from these two sites are very rare clinically. Most of the so-called “VI meningiomas” actually originate from the arachnoid sleeve of the GV ( the posterior part of APEG). As in our case group, only one case of the 9 patients, which had no direct relationship with dura of the FT junction, originated from the dorsal layer of arachnoid membrane within the VI, and the other 8 cases from the posterior part of APEG.
Efforts to differentiate FT meningiomas from VI meningiomas are of surgical significance. The arachnoid interface of FT meningiomas was clear, while most of the interface of VI meningiomas were damaged. In addition to their different relationship with the dura of FT junction, there are also fundamental differences in the blood supply of tumors. The blood supply of FT meningiomas may derive from the meningohypophyseal trunk, the meningeal branch of the external carotid artery, the small branch of the posterior cerebral artery, and the branches of the posterior medial and lateral choroidal arteries. The above arteries participate in blood supply alone or together and the blood supply of tumor is abundant. Cutting off the the dura of FT junction before tumor resection can reduce bleeding during tumor resection. The VI meningiomas is usually supplied only by the posterior choroidal artery, and the blood supply is generally not rich. However, if the supratentorial approach is used, the contralateral feeding artery is not easily blocked in the early stage of operation.
Before the removal of pineal meningiomas, more attention should be paid to the effects of tumors on the GV and the straight sinus, and the establishment of venous collateral circulation. It had been reported that there were two spatial relationships between tumors and veins, either in the anterior superior part of the ICVs and the GV, or in the posterior inferior part of these veins. However, Blasco proposed dividing FT meningiomas into 4 subtypes according to the Bassiouni classification and its relationship with the deep venous system: (A) FTM type I with inferior venous displacement, (B) FTM type II with superior venous displacement, (C) FTM type III with contralateral venous displacement, and (D) FTM type IV with growth over the straight sinus and superolateral venous displacement[7, 14]. In addition to the above spatial relationships the veins could be encapsulated totally by the tumor in the present group of patients (Fig. 5). The veins should be kept away in the selection of surgical approaches.
Compared with other pineal region tumors, meningiomas seem to have more obvious effects on veins and venous sinuses. The proportion of GV and straight sinuses that cannot be visualized on MRV is higher, which indicates that most meningiomas affect venous reflux. Intraoperative observation showed that the two types of meningiomas had different effects on veins. FT meningioma could invade venous sinuses and GV, and even wrap the GV completely in the tumor. If the veins were still functioning, it might be a wise choice to protect the veins from being electrocoagulated and perform a subtotal resection of the tumor with residue. The VI meningiomas only compress and displace the veins, and do not invade the veins. After the removal of tumor, the veins can be reopened. Therefore, even if the veins are not visualized before operation, the VI meningiomas should be treated according to the patency of the vein before operation, and it should not be cut off rashly unless the veins are completely occluded. The collateral circulation may be established to compensate for the obstruction of the straight sinus and the GV. The present study found that FT meningioma showed signs of venous vasodilation on the medial surface of the anterior occipital lobe and the posterior parietal lobe. However, similar imaging findings were not found in VI meningiomas, suggesting that there were differences in venous collateral circulation between the two types of meningiomas, suggesting that "VI meningiomas" had less influence on venous reflux. The POPPEN approach is more likely to destroy venous collateral circulation than subtentorial approach and therefore great attention should be paid to the protection of medial occipital vein and tentorial sinus during operation[1, 3, 13, 15]. In principle, the protection of the GV and collateral circulation is particularly important and if there are residual tumors, gamma knife can be used for the follow-up treatment.