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Posterior Fossa Tumors in the Pediatric Population: Multidisciplinary Management
Introduction
Brain tumors are the most common solid pediatric cancer and constitute the most frequent cause of cancer-related death in children. Pediatric brain tumors are most often located in the posterior fossa (PF). They include medulloblastoma, ependymoma, and cerebellar astrocytoma, but also less common tumors such as atypical teratoid rhabdoid tumor (ATRT), (epi)dermoid cysts, and choroid plexus papilloma.
In this chapter we will discuss the clinical and radiological diagnosis of the most common PF tumors. We will describe their multidisciplinary treatment, with a specific focus on the surgical treatment, including surgical routes, complications, and the treatment of hydrocephalus.
Symptoms and Signs
Children with PF tumors usually present with signs and symptoms related to increased intracranial pressure (ICP). This is caused by obstructive hydrocephalus due to the blockage of outflow of cerebrospinal fluid (CSF) by the tumor. Symptoms associated with increased ICP include headache, which is often present upon awaking, nausea, vomiting, and lethargy. Papilledema is found to be present in 30% to 40% of the patients with PF tumors. In infants the unfused cranial sutures can still accommodate rising ICP. Therefore, symptoms in infants are often non-specific and can mimic the symptoms of more common pediatric illnesses such as poor feeding, failure to thrive, irritability, and increased head circumference.
In the absence of hydrocephalus patients may report symptoms related to the direct mass effect of the tumor on the cerebellum and brainstem, such as ataxia, nystagmus, and gait disturbances. Ataxia may be more difficult to diagnose in young children and may manifest itself as a regression in walking or clumsiness during the development of walking. Cranial nerve palsies may result from brainstem involvement. A facial nerve palsy for example may point to a lesion in the cerebellopontine angle. If the tumor is located in the area postrema or the obex, the tumor itself may lead to vomiting, even in the absence of raised ICP. Syringomyelia and related neurological deficits can develop secondary to space-occupying lesions in the PF. Downward migration of the tumor into the foramen magnum may result in torticollis and opisthotonic posturing.
The time between the first appearance of symptoms and the establishment of the diagnosis is strongly related to tumor histology, because the aggressive growth of high-grade tumors will rapidly lead to clinical symptoms. For instance, in patients with medulloblastoma the interval between the first symptoms and diagnosis is 4 weeks, while the average pre-diagnostic interval in patients with low-grade cerebellar astrocytoma is 41 weeks. Acute obstructive hydrocephalus or intratumoral hemorrhage may lead to a sudden deterioration of the clinical status.
Diagnostic Evaluation
The radiological evaluation of PF tumors depends on MR imaging, although in the emergency setting a CT scan is often acquired first due to its more rapid availability. MR imaging must include pre- and postcontrast images of the full craniospinal axis to evaluate the presence of metastatic dissemination, as part of the staging process that determines both treatment strategy and prognosis. Leptomeningeal dissemination can appear as diffuse enhancement of the meninges, which is most frequently located along the suprasellar subarachnoid space, PF cisterns, the Sylvian fissures, or over the cerebral hemispheric convexities. At the thoracic and lumbosacral levels drop metastases present as diffuse enhancing nodules along the spine and cauda equina. Imaging should be performed before surgery, because manipulation and subarachnoid hemorrhage may cause leptomeningeal irritation and inflammation that mimics intradural dissemination. The imaging should be repeated to evaluate the extent of resection within 48 hours after the surgery, while at a later stage it will be more difficult to differentiate residual lesion from postsurgical changes.
In addition to this, CSF cytology should be performed to evaluate leptomeningeal dissemination. CSF should be obtained more than two weeks after the surgery to prevent a false positive result deriving from surgery.
The imaging characteristics of specific tumors on CT and MRI are described in the following section.
Medulloblastoma
The majority of medulloblastomas are well-defined midline lesions, arising from the cerebellar vermis or inferior medullary velum, while a minority is located in the cerebellar hemispheres. On CT medulloblastomas appear as a hyperdense, homogeneously enhancing mass, which distorts the fourth ventricle ( Fig. 68.1A and B ). Calcification is present in approximately 20% of medulloblastomas. The various medulloblastoma subgroups (see further) have distinctive MR imaging characteristics. In general, however, medulloblastomas show a hypo- to isointense signal on T1-weighted images and a slightly hyperintense signal on T2 (see Fig. 68.1C ). The high cellularity of medulloblastoma causes diffusion restriction, which differentiates medulloblastoma from low-grade lesions. Almost all medulloblastomas demonstrate diffuse contrast enhancement and mild peritumoral edema (see Fig. 68.1D ). Medulloblastoma can be disseminated at diagnosis with a predilection for the subependymal areas of the ventricles. For this reason, children should have imaging of the entire neuraxis prior to surgical intervention.
Ependymoma
Ependymomas extend from the ventricular lining, most often from the floor of the fourth ventricle, pushing the brainstem anteriorly, or from the lateral recess, pushing the brainstem to the contralateral side. Contrary to medulloblastoma they often enlarge the fourth ventricle without distorting it. Ependymomas may protrude through the foramina of Luschka and Magendie. CT shows an isodense mass with heterogeneous enhancement ( Fig. 68.2A ). Calcification is more frequently observed in ependymoma than in other PF tumors. In addition, intratumoral hemorrhage may be present. The MRI characteristics are specific, with low T1 and high T2 signal (see Fig. 68.2B and D ). Contrast enhancement (see Fig. 68.2C ) and edema are variable, but diffusion restriction is mostly present. Leptomeningeal dissemination at presentation is possible but rare and should therefore prompt the consideration of another diagnosis.
Cerebellar Astrocytoma
Approximately 90% of cerebellar astrocytomas are pilocytic astrocytoma (PA), with other tumors mainly being diffuse astrocytoma (DA). High-grade cerebellar lesions such as anaplastic astrocytoma and glioblastoma are uncommon in the pediatric population and will therefore not be further discussed in this chapter.
Two-thirds of PAs have a classical appearance on imaging that consists of a large cyst with an eccentric solid component known as the mural nodule ( Fig. 68.3A and B ). A hemangioblastoma may look similar on imaging as it also appears as a cystic mass with an enhancing mural nodule. However, hemangioblastomas are rare in children and they can easily be distinguished from PA because they are more vascular and typically abut a pial surface. The minority of PA appears as a solid mass with little or no cystic components. The cystic PAs are frequently localized in the cerebellar hemispheres, while the solid lesions mostly originate from the midline. The solid components of PA are hypodense on CT (see Fig. 68.3C ) and hypointense on T1- and hyperintense on T2-weighted images on MRI. The cyst wall and in particular the mural nodule enhance vividly (see Fig. 68.3B and D ). PA do not show diffusion restriction, but the majority is surrounded by mild edema.
DAs are characterized by ill-defined borders. They lack the macrocystic appearance and the marked contrast enhancement that is seen in PA. Both PA and DA can invade the cerebellar peduncles.
Low-grade glioma (LGG) metastasis is possible but very uncommon.
Atypical Teratoid Rhabdoid Tumor
ATRT is more often than other PF tumors located off midline and frequently extends into the cerebellar pontine angle. The CT appearance is dominated by hyperdensity due to high cell density and heterogeneous enhancement ( Fig. 68.4A ). Additionally, ATRTs frequently display calcification and intratumoral hemorrhage. On MRI, lesions can appear heterogeneous because of the variable signal intensity on T1 and T2 sequences and the presence of cysts, blood products, and necrosis (see Fig. 68.4B and D ). , Most, but not all ATRTs display heterogeneous enhancement of varying intensity (see Fig. 68.4C ). , ATRT typically shows diffusion restriction. Metastasis at presentation is present in 10% to 40% of patients. ,
Surgery
The aims of surgery are to maximally resect the tumor while preserving neurological function, to acquire accurate histological diagnosis, and to restore normal CSF flow. Tumors located laterally in the cerebellopontine angle can be approached using a retrosigmoid approach. If tumors extend superiorly to the tentorial surface, a supracerebellar infratentorial route may be used. A transcortical approach is performed for tumors laterally in the cerebellar hemispheres, using a transverse cortectomy in the cerebellar folia that overly the tumor. Most PF tumors, however, will be accessed using a telovelar or transvermian approach. These latter two approaches will be discussed in more detail below (see Fig 68.5A–D ).
Patient Positioning
In children PF surgery is most frequently performed in the prone position. This position provides a good overview of the surgical field and a comfortable working position for both surgeon and assistant. The disadvantage of this position is venous congestion that may result in blood pooling in the operative field and facial, conjunctival, or airway edema. In the prone position, the abdominal wall must be able to move freely to prevent increased intra-abdominal pressure that can impair ventilation and increase epidural venous pressure and bleeding. Therefore, the torso is supported by chest rolls and if necessary, a pelvic roll. The head is stabilized using pin fixation in older children. Children aged 3 or younger are placed on a padded horseshoe, as pin fixation carries the risk of skull fracture and epidural hematoma. Care must be taken to ensure that there is no direct pressure on the eyes to prevent postoperative vision loss. The neck should be slightly flexed to open up the craniocervical junction. For lesions extending superiorly, the prone position can be modified into the Concorde position by lifting the upper thorax and maximally flexing the neck.
Alternative positions include the lateral decubitus position, that is used for laterally located tumors, and the sitting position. Both these positions have the advantage of better drainage of blood and CSF and minimal need for retraction. The sitting position, however, carries a higher risk of air embolism, hypotension, and brain collapse, that can lead to subdural hematoma. Moreover, it is difficult to place younger children in the sitting position.
Opening and Closure
A midline skin incision is made extending from just above the inion to the level of the spinous process of C2 ( Fig. 68.5A ). Next, the fascia and muscles are split along the avascular midline and the occipital bone and the posterior arch of C1 are exposed. The exposition of the superolateral surface of C1 has to be done carefully to prevent damage to the vertebral artery. The posterior arch of C1 can be removed if the tumor is extending into the spinal canal or if the tonsils are pushed below the level of the foramen magnum. Bony removal should be limited because post-laminectomy kyphosis and cervical instability can develop if a laminectomy is performed over multiple levels, particularly if C2 is involved and if the laminectomy extends laterally.
One burr hole is placed on each site of the midline just below the transverse sinus (see Fig. 68.5B ). In addition, two burr holes can be placed above the foramen magnum. Starting from the upper burr holes and running down caudally, the craniotomy is performed incorporating the posterior rim of the foramen magnum.
A Y-shaped dural opening is made exposing the inferior cerebellum and brainstem ( Fig. 68.5C ). Attention has to be paid to the possible presence of persisting occipital sinuses, particularly in younger children (Rollins 2005). Bleeding from the occipital and marginal sinuses can be controlled using silver clips or suture ligatures. CSF can be drained from the cisterna magna to relieve ICP. Alternatively, cystic lesions can be punctured with a brain needle.
After resection and hemostasis, the dura is closed in a watertight fashion. If necessary, a dural patch can be interposed. The bone flap is usually placed back as this is associated with a decreased risk of pseudomeningocele and CSF leak compared to a craniectomy. Finally, the muscle, fascia, and superficial layers need to be approximated meticulously to prevent the development of a pseudomeningocele.
Tumor Removal
Telovelar Approach
In this approach the fourth ventricle is entered through the cerebellomedullary fissure which is the virtual space between the posterior aspect of the medulla oblongata and the anterior surface of the tonsils. This natural corridor provides access to the fourth ventricle without the necessity of splitting the cerebellar vermis. A second advantage of this approach is the good view of the lateral recess of the ventricle and the exposure of the foramen of Luschka.
After dural opening, the arachnoid around the inferior part of the tonsils and the adjacent surface of the uvula is sharply dissected. The tonsils are gently retracted supralaterally. This exposes the inferior part of the roof of the fourth ventricle consisting of the tela choroidea and the inferior medullary velum. In case of a large tumor the roof is often stretched out by the tumor, making the lower part of the tumor visible at this point.
Subsequently, the posterior inferior cerebellar arteries have to be visualized and care has to be taken to protect them and their perforators on each side. The roof of the ventricle is opened by incising the tela choroidea from the midline to the foramina of Luschka laterally. This incision is continued superiorly dividing the inferior medullary velum.
The tumor can then be dissected from surrounding cerebellar and brainstem structures using microsurgical techniques and the ultrasonic aspirator. Early during the resection, the floor of the fourth ventricle must be identified in order to prevent accidental entry into the brainstem. Additionally, the extent of invasion of the fourth ventricle floor should be explored. In case of extensive invasion manipulation of the tumor during resection has to be limited to prevent damage to the brainstem. If possible, a plane should be developed between the tumor and the surrounding structures. A cottonoid can be placed on the floor of the fourth ventricle for protection if the tumor is not adherent to the floor. Tumor invading the floor of the fourth ventricle should be carefully thinned leaving a sheet of tumor left on the floor without manipulating the floor of the ventricle. Cauterization of small vessels in the floor of the fourth ventricle has to be avoided to prevent neurological deficits.
Transvermian Approach
In the transvermian route, the fourth ventricle is entered through splitting of the cerebellar vermis ( Fig. 68.5D ). This route may be useful for lesions that originate from the vermis. Compared to the telovelar approach, it might offer a slightly better angle to reach the rostral part of the fourth ventricle. However, the transvermian approach is possibly associated with a higher rate of neurological complications.
Once the dura is opened the arachnoid in the uvulotonsillar and medullotonsillar space is dissected. The cerebellar tonsils are retracted laterally. As the dentate nucleus is located just above to superior pole of the tonsil, care should be taken not to exert too much traction. The vermis is incised longitudinally over the smallest possible length necessary to remove the tumor. The incision should not extend beyond the inferior border of the superior medullary velum because the decussating fibers of the superior cerebellar peduncle run underneath this surface. The two halves of the vermis are retracted in opposite direction, after which the tumor can be dissected in a similar fashion as described above.
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