Miscellaneous Infratentorial Tumors
Although medulloblastoma, ependymoma, and astrocytoma constitute the majority of tumors in the posterior fossa in children (cerebellar, fourth ventricular, cerebellopontine angle [CPA], or intrinsic brainstem in location), there are other less common, but certainly challenging tumors that should be acknowledged ( Table 38.1 ). The variety of histopathologies is immense, but this chapter specifically addresses the following: choroid plexus papilloma, schwannoma, meningioma, hemangioblastoma, oligodendroglioma, and epidermoid tumors.
General Clinical Presentations
Commonly presenting with obstructive hydrocephalus, posterior fossa tumors, depending on size, location, and rapidity of growth, may come to light because of focal or generalized neurologic deficit. Fourth ventricular tumors, such as papillomas, and large intraparenchymal or CPA tumors, can either directly or indirectly (from shear mass effect) obstruct the fourth ventricle, cerebral aqueduct, or fourth ventricular outlets. Accelerated head growth, a bulging anterior fontanelle, splayed cranial sutures, developmental delay, loss of developmental milestones, lethargy and vomiting, and failure to thrive may represent hydrocephalus in noncommunicative infants with pliable skulls and patent sutures ( Table 38.2 ). Young children with hydrocephalus may be lethargic with unexplained vomiting, and have abducens nerve palsies or papilledema on exam. Older children are much more articulate regarding symptoms of nausea, headache, and visual disturbances.
Cerebellopontine angle lesions, such as schwannomas, meningiomas, and epidermoid tumors, because of their locations among the cranial nerves, may not be obvious in young children unless the facial nerve is involved. Hearing loss, facial sensory deficits, and slowly acquired lower cranial neuropathies may go unnoticed. The older child is more cognizant of and communicative about even subtle symptoms, such as hearing loss and diplopia. Large CPA tumors, without hydrocephalus, may also present with headache, appendicular, or truncal ataxia.
Cerebellar parenchymal tumors, such as oligodendrogliomas and hemangioblastomas, may present with obstructive hydrocephalus, ataxia, dysmetria, or headache. Focal cranial neuropathies are rare.
General Surgical Principles
Posterior fossa brain tumor surgery in children can present obstacles not encountered in older children or adults, and thus requires special considerations. The standard position of the child for posterior fossa surgery is prone. This is preferable for midline tumors and some parasagittal approaches, particularly if the neck can be flexed safely. Rigid fixation facilitates head flexion, but young children are typically positioned in a padded horseshoe. Injury to the eyes or other pressure issues may be avoided by rigid fixation, but fear of a fracture, epidural hematoma, or poor fixation because of unfused sutures precludes pin fixation in infants or in children or adolescents with a thin calvaria from long-standing hydrocephalus. When rigid fixation is an option, positioning choices also increase, allowing a park-bench or lateral decubitus position to facilitate lateral access, (particularly for retrosigmoid approaches to the CPA). Alternatively, if rigid fixation is deemed unsafe, the CPA and far lateral cerebellum may be approached with the patient supine and the head rotated.
The dural opening, particularly in very young children, can be challenging and treacherous as there are often large intradural venous lakes and persistent venous sinuses. Profuse bleeding should always be expected and addressed promptly with nonferromagnetic hemostatic clips and bipolar electrocautery. Lateral durotomies to access the CPA via a retrosigmoid approach can also be difficult in young children with full brains or large tumors. Removing cerebrospinal fluid (CSF) from the cisterna magna or ventricular catheter (placed pre-craniotomy for hydrocephalus) will allow for brain relaxation and decrease the need for significant cerebellar retraction.
Retraction should also be used sparingly for intraparenchymal, vermian, and fourth ventricular tumors. Although often unpredictable, and felt to be due to a variety of injuries, including to the vermis and dentatothalamocortical tracts, cerebellar mutism and pseudobulbar palsies can be devastating.1,2 The telovelar surgical approach, believed to offer the potential for less injury to the underlying brain, is not immune from association with similar deficits.3 Although intraoperative neuromonitoring cannot guard against the posterior fossa syndrome, it still serves a valuable function in posterior fossa surgery, particularly with regard to cranial nerve monitoring, as well as motor and somatosensory evoked potentials.
Imaging is helpful in the preoperative evaluation, and there is potential benefit in image guidance. Intraoperative ultrasound, computed tomography (CT), magnetic resonance imaging (MRI), or a combination of real-time imaging may be helpful, particularly for parenchymal lesions whose borders may not be well defined. Postoperative imaging is important to assess surgical success, to search for potential complications, and to document baseline images for future comparison.
Choroid plexus papilloma
Accelerated head growth
Full-fontanelle/splayed sutures/frontal bossing
Developmental delay/loss of milestones
Failure to thrive
Sixth nerve palsy
Choroid Plexus Papilloma
Choroid plexus tumors (CPTs) in children include both choroid plexus papillomas (CPPs) and choroid plexus carcinomas (CPCs). Isolated posterior fossa CPCs are not readily described. They constitute 1% of all brain tumors in adults, whereas 1.5 to 4% of intracranial tumors in children are CPPs.4 CPTs in younger children are typically in the lateral ventricles, and older children and adults are more likely to harbor fourth ventricular tumors.4,5 In children, 19% of CPTs are in the fourth ventricle, and another 14% in the third ventricle.6 These tumors have also been found in the CPA and along nerve roots.7
Infratentorial CPPs on neuroimaging are classically well-delineated irregular lobulated masses in the fourth ventricle. On CT they are iso- or hyperintense relative to normal brain and enhance strongly with contrast. On MRI, T1-weighted images reveal an isointense tumor, and T2-weighted images are iso- to hyperintense ( Fig. 38.1a ). Contrast enhancement is intense ( Fig. 38.1b ).
Choroid plexus papillomas are slow-growing, histologically benign masses corresponding to World Health Organization (WHO) grade I8 ( Table 38.3 ). Macroscopically they have a cauliflower appearance, and although they may be adherent to the cerebellum or brainstem, they typically are not. A single layer of cuboidal to columnar epithelial cells covers the fibrovascular connective tissue fronds, which resemble normal villi of choroid plexus.8 Mean proliferative indices, however, can be as high as 4.5%, in one way differentiating CPPs from normal choroid plexus.9
These are vascular tumors, and the arterial feeders are typically from the posterior inferior cerebellar artery.6 Interventional neuroradiological embolization of the feeders may prove helpful, particularly in the very young child whose blood volume may not tolerate significant blood loss. CPPs have been successfully treated with embolization alone.10 Alternatively, initial and intentional disruption of the blood supply at surgery renders the rest of the procedure easier, as these tumors rarely invade normal cerebellum or brainstem.
Well-delineated, lobulated tumor in the fourth ventricle
Strongly enhancing on CT and MRI
T2: iso- to hyperintense
Benign, cauliflower-appearing tumor
WHO grade I derived from choroid plexus epithelium
Proliferative index up to 4.5%
As CPPs are benign lesions, complete resection is curative, and long-term prognosis is excellent.11 There is no evidence that chemotherapy or radiation plays any role in the primary management of these tumors.6 Unresectable lesions may respond to radiosurgery, which has been show to control third-ventricle CPPs.12 Fourth-ventricle CPPs typically cause an obstruction of CSF flow such that resection typically leads to reestablishment of flow.13 In very young children, however, an absorptive deficit may lead to a communicating hydrocephalus requiring diversion.
Cranial Nerve Schwannoma
Intracranial schwannomas in children are rare, despite being the most common CPA tumor in adults. In the pediatric population, as in the adult, vestibular schwannomas are the most common, and typically associated with neurofibromatosis type 2 (NF-2), particularly when there are multiple lesions.14 Schwannomas, however, can grow from any of the cranial nerves including nerves III, V, VII, VIII, IX, X, XI, and XII.15,16 Non-NF patients may also have these tumors, typically isolated to a single lesion.15 Ostensibly benign, they are usually large, and in the CPA they present a surgical challenge, particularly in young children.
Neuroimaging reveals a well-circumscribed tumor, typically in the CPA. On CT, schwannomas are isointense relative to normal brain and they homogeneously enhance with contrast. In addition, evaluation of the bone may reveal expansion of the porous acusticus or the jugular foramen. T1-weighted MRI shows the tumor to be isointense relative to gray matter, and on T2-weighted MRI with fluid-attenuated inversion recovery (FLAIR) the tumor is hyperintense ( Fig. 38.2a ). Postcontrast MRI shows homogeneous enhancement ( Fig. 38.2b ). MR angiography, particularly for large tumors, may show significant vascularity, and subsequent catheter angiography may be indicated for tumor embolization.15
Schwannomas are benign nerve sheath tumors corresponding to WHO grade I ( Table 38.4 ). They consist of well-differentiated Schwann cells and are smooth, globoid masses macroscopically.17 The classic microscopic appearance is that of two distinct cellular regions: Antoni A pattern and Antoni B pattern.18 Antoni A regions are characterized by compact, polarized, elongated cells, whereas Antoni B regions have a distinct, loose, microcystic appears. Cellular schwannomas can have MIB-1 indices as high as 6%.19
Symptomatic lesions should be resected, if possible. Complete resection is curative, although not always possible. Although many approaches have been described for accessing the various cranial nerves, most of these lesions can be accessed via a retrosigmoid approach to the CPA. This non–skull-based approach is particularly appealing in infants and young children. Trigeminal schwannomas, however, because of their propensity to occupy both infra- and supratentorial compartments, typically require a combined procedure or some variation of a middle fossa approach.20 Preoperative embolization, particularly for large tumors in young children, may be helpful if vascularity is of concern14 ( Fig. 38.2c ). Although the tumor will arise from an isolated nerve, the surrounding nerves may be involved and should be carefully monitored electrophysiologically. Residual, recurrent, or unresectable tumors may be amenable to stereotactic radiosurgery. Based on excellent control rates of vestibular schwannomas with radiosurgery, nonvestibular schwannomas should respond concordantly, with potentially lower treatment-related morbidity.21
In general, meningiomas in children are rare, representing 0.4 to 4.1% of pediatric brain tumors.22 Pediatric meningiomas may have a higher incidence in the posterior fossa than adults, and are associated with NF-2.23,24 They may present along any dural surface, including the tentorium, CPA, and foramen magnum.
Although some meningiomas on CT may be isointense compared with normal brain, the majority are hyperintense, often with significant peritumoral edema, and strong, homogeneous enhancement. Meningiomas are hypo- to isointense on T1-weighted MRI, and, unlike similarly appearing schwannomas, T2-weighted images can show an isointense tumor relative to the cerebellum, often with surrounding edema ( Fig. 38.3a ). There is strong and homogeneous enhancement on postcontrast imaging ( Fig. 38.3b ). Because of meningiomas’ dural vascular supply and attachment, MRI is additionally valuable in evaluating the blood supply, dural enhancement and potential “tail,” and dural venous sinus involvement preoperatively. Formal catheter angiography may prove helpful in preoperative planning.
The macroscopic appearance of an infratentorial meningioma is that of a firm, rubbery, sometimes lobulated mass with a broad dural attachment. Microscopically, the WHO grading scheme includes typical, atypical, and anaplastic meningiomas, corresponding to grades I, II, and III, respectively25 ( Table 38.5 ). Children, however, have a higher incidence of atypical and aggressive tumors.26,27 There are many histological subtypes typically representing meningiomas, the most common being meningothelial and fibroblastic, both grade I lesions. The classic round configurations of calcium, the psammoma bodies, and whorls are rarely seen in these two tumors. Meningothelial meningiomas are characterized by uniformed cells with oval nuclei, resembling normal arachnoid cap cells, and fibroblastic variants consist of spindle cells with wide fascicles and striking collagen deposition.25
Well-circumscribed, heterogeneously enhancing mass in the cerebellopontine angle
May be vascular, with extension into and widening of the corresponding foramen
WHO grade I, composed entirely of Schwann cells
Antoni A: compact, elongated, palisading
Antoni B: distinct, loose, microcystic