Pineal region tumours in the sitting position: how I do it

Background Pineal region tumours remain challenging neurosurgical pathologies. Methods Detailed anatomical knowledge of the posterior incisural space and its variations is critical. An opaque arachnoidal membrane seals the internal cerebral and basal veins, leading to thalamic, basal ganglia, mesencephalic/pontine infarctions if injured. Medium-size tumours can be removed en-bloc with all traction/manipulation applied on the tumour side, virtually without contact of ependymal surfaces of the pulvinars or third ventricle. Sacrifice of the cerebello-mesencephalic fissure vein may be required. Conclusions The sitting position offers superior anatomical orientation and remains safe with experienced teams. Meticulous microsurgical techniques and detailed anatomical knowledge are likely to secure safe outcomes. Supplementary Information The online version contains supplementary material available at 10.1007/s00701-021-04821-3.


Vascular anatomy
The second (P2) and third (P3) segments of the posterior cerebral artery (PCA) lie between the thalamic pulvinar and superior colliculi, while loops of the superior cerebellar artery (SCA) are often visible anterior to pineal gland. The surgical venous anatomy of the pineal region is complex and are covered by an arachnoid layer of varied thickness, ranging from thick and opaque to thin and translucent (Fig. 4b). The vein of Galen (VG) is the most dorsal part of the venous complex, joining the straight sinus (SS) while the paramedian, paired basal veins of Rosenthal (BvRo) exit the ambient cistern and converge to the VG. The internal cerebral veins (ICv) lie deeper, laterally and slightly underneath the BvRo exiting through the cistern of the velum interpositum, also converging to the VG [4].

Cisternal anatomy
The unpaired quadrigeminal cistern contains all three major venous systems, VG, the confluences of both BvRo and ICvs, This article is part of the Topical Collection on Brain Tumors Priya Sharma and Mohd Abdul are Co-first authors; these authors contributed equally to this work. P4 and the posterior medial choroidal artery. The paired ambient cisterns contain P2 and P3, the anterior choroidal artery, the CvRo and SCA segments. The cistern of the velum interpositum, extending from the foramen of Monroe anteriorly to the habenula encloses the ICvs and the posterior medial choroidal artery. The quadrigeminal and velum interpositum cisterns communicate with the posterior pericallosal cistern [4] (Fig. 1).

Preoperative work-up
The SC/IT approach in the sitting position (SP) offers optimal surgical field without cerebellar retraction but is fraught with the most feared systematic complication, venous air embolism [2,3] (Tables 1 and 2). Reported incidence of VAE varies significantly as accurate diagnostic criteria are not standardised. In addition, not all transoesophageal echodetected VAE are clinically significant (Table 1). A bubble echo can exclude patent foramen ovale, detected in 30% of population, a SP contraindication.

Positioning
The SP is assumed gradually aiming for midline head position with neck flexion ( Table 2 and Fig. 2). Neuronavigation can be employed but is of limited assistance, due to depth and proximity of small, key structures.

Opening
Following a midline muscle dissection from the external occipital protuberance to C2, a craniectomy is performed exposing completely the inferior half of the transverse sinus, initially thinning the bone with a diamond drill and completing the removal with 2-mm Kerrison's rongeurs. A craniotomy can, alternatively, be performed, provided that the transverse sinus exposure is performed in a controlled and gradual way to avoid inadvertent sinus injury. Adequate lateral and inferior bony removals are critical during exposure allowing wide angle approach and cerebellar fall under gravity, respectively (Fig. 3). The foramen magnum remains intact to avoid cerebellar slump.

Tumour exposure
The dura is opened in a standard "Y" fashion. Cortical veins suspending the superior cerebellar surface from the tentorium are diathermised at their tentorial attachment, allowing the cerebellum to fall freely, inferiorly. The vermian culmen is followed directly inferiorly until the tentorial edges are identified ( Fig. 4a) with an opaque arachnoid membrane stretching over the pineal region, thalami and tectal plate. The thickness of the arachnoid varies according to pathology, from significant to thin (Fig. 4b). Sharp dissection is avoided as the BvRos are directly vulnerable.

Tumour resection
Prior to any tumour resection, a clear understanding of the surgical field with the position of the vCMF, BvRo, ICv, VG, P3, P4, SCA, superior colliculi and pulvinars (Fig.  4b, c) is required; the pineal gland is not discernible. In addition, the consistency, vascularity, adherence to thalami and tectal plate of the tumour are assessed. The vCMF can be sacrificed in larger tumours.
Although a large tumour can be debulked, our technique is attempting en-bloc removal, as the safest way to distinguish tumoral f rom nor mal t issue. A microdissector Rhoton #8 (Integra, Plainsboro, NJ) is used to circumferentially develop a plane with all traction applied to the tumour rather than adjacent brain. The circumferential plane develops easier initially laterally from the pulvinar of the thalami (Fig. 5a), then inferiorly from the tectal plate and finally superiorly from the splenium of the corpus callosum (Fig. 5b).
No bipolar diathermy is used unless for rare selective vessel coagulation. Once the majority of the tumour is dissected, it can be manipulated as one block revealing its interface and final attachments which are again released with Rhoton #8. The posterior third ventricle is exposed last; the ependymal surfaces remain impeccable with no evidence of manipulation injury (Fig. 5c). A watertight closure with dural patch and dural sealant minimises the risk of CSF leak or pseudomeningocele. Postoperative MRI scan shows complete tumour removal (Fig. 6).

Limitations
Despite the uncommon frequency of these tumours, the IT/SC approach should be undertaken by teams on a relatively regular basis, as anaesthetic and surgical aspects can be challenging. Fig. 4 Stepwise exposure of the tumour. The culmen of the vermis is followed inferiorly leading to the tentorial incisura (a). The arachnoid membrane, A, is relatively thin in this case. The vein of the cerebellomesencephalic fissure, vCMF, the third segment of the posterior cerebral artery, P3 and a loop of the superior cerebellar artery, SCA, are seen displaced by the tumour, T (b). An initial, exploratory dissection, with Rhoton #8, from the pulvinar of thalamus, P, without sacrificing small arterial branches is performed. T, tumour