Chapter Neoplasms of the Posterior Fossa 1.4 Kelly K. Koeller Contents The opinions and assertions contained herein are the private views of the author and are not to be construed Introduction . 69 as official or representing the views of the Departments Pediatric Neoplasms . 69 of the Navy or Defense. Medulloblastoma . 69 Incidence and Clinical Presentation . 69 Pathologic Findings . 70 Introduction Imaging Findings . 70 Pilocytic Astrocytoma . 71 Incidence and Clinical Presentation . 71 Neoplasms of the posterior fossa form a heterogeneous Pathologic Findings . 72 group of tumors that manifest either within the cerebel- Imaging Findings . 72 lum, the fourth ventricle, or the cisternal spaces. Using Ependymoma . 73 the predominant age group in which each occurs is a Incidence and Clinical Presentation . 73 convenient method to analyze this group. Pathologic Findings . 73 Within the pediatric population, medulloblastoma Imaging Findings . 73 and pilocytic astrocytoma arise within the cerebellum Choroid Plexus Papilloma and Carcinoma . 74 Incidence and Clinical Presentation . 74 while ependymoma and choroid plexus papilloma oc- Pathologic Findings . 74 cur within the ventricular system. In the adult popula- Imaging Findings . 74 tion, hemangioblastoma is the most common cerebellar Atypical Teratoid / Rhabdoid Tumor . 75 tumor, followed by dysplastic cerebellar gangliocytoma Adult Tumors . 76 (Lhermitte-Duclos disease) and cerebellar liponeuro- Hemangioblastoma . 76 cytoma while the fourth ventricle is the most common Incidence and Clinical Presentation . 76 location for the subependymoma. In the extra-axial Pathologic Findings . 76 cerebellopontine angle, the vestibular schwannoma is Imaging Findings . 77 the most common tumor, followed by meningioma. Dysplastic Cerebellar Gangliocytoma (Lhermitte-Duclos Disease) . 77 Incidence and Clinical Presentation . 77 Pathologic Findings . 78 Pediatric Neoplasms Imaging Findings . 78 Cerebellar Liponeurocytoma . 78 Medulloblastoma Subependymoma . 78 Incidence and Clinical Presentation . 78 Incidence and Clinical Presentation Pathologic Findings . 79 Imaging Findings . 79 Medulloblastoma is the most common pediatric central Cerebellopontine Angle Tumors . 79 nervous system malignancy and the most common Vestibular schwannoma . 79 (38%) primary tumor of the posterior fossa in children. Incidence and Clinical Presentation . 79 Pathologic Findings . 80 Following only astrocytoma, it is the second most com- Imaging Findings . 80 mon pediatric brain neoplasm overall (6–8%) with an Meningioma . 81 incidence of about 1 in 200,000 [1–7]. Males (60%) are Incidence and Clinical Presentation . 81 slightly more commonly affected [5]. Among children, Pathologic Findings . 81 the mean age is about 7 years with small peaks at 3 years Imaging Findings . 81 and 7 years [3]. Other cases may manifest between the Other Tumors . 81 ages of 20 and 40 years [8, 9]. Conclusion . 81 Nearly all cases occur in the cerebellum (94.4%) and References . 81 most (>75%) of these arise in the midline cerebellar 70 Kelly K. Koeller vermis [3, 10]. A more lateral location within the cere- moplastic subtype, characterized by nodular reticulin- bellar hemisphere is typical when these tumors mani- free “pale islands” that are surrounded by reticulin- fest in older children, adolescents, and adults, and is staining collagen fibers, the “medulloblastoma with likely related to the lateral migration of undifferentiated extensive nodularity and advanced neuronal differen- cells, the putative cell of origin, away from the midline tiation” subtype, occurring primarily in children less in older children and adolescents [3, 6, 11]. Brain stem than 3 years of age and associated with a “grape-like” infiltration is a common (33%) manifestation [12]. Less nodularity seen on imaging studies, and the large cell common locations include the fourth ventricle (3%), medulloblastoma subtype [10]. The loss of genetic other areas of the brain (2.1%), and spinal cord (0.6%) material from chromosomal arm 17p, site of a suppres- [3]. sor gene, is the most common (35–40%) of many chro- The clinical presentation is usually less than 3 months mosomal abnormalities associated with this disease in duration, reflecting the tumor’s aggressive biologic [24]. Medulloblastoma, a WHO grade IV tumor, is behavior [12, 13]. Headache (generalized or localized to regarded as a distinct entity from primitive neuro- the suboccipital region) and persistent vomiting (with- ectodermal tumor (PNET) in the WHO classification out or with nausea) are common symptoms [12,13].Sei- scheme [10]. zure activity is uncommon and may herald metastatic spread [13]. Truncal ataxia, secondary to destruction of the cerebellar vermis, is the most common objective Imaging Findings clinical sign. and is frequently accompanied by spastic- ity [12, 13]. Other common clinical signs include papil- The classic computed tomography (CT) appearance of a ledema (related to hydrocephalus), nystagmus, limb medulloblastoma is a hyperattenuated well-defined ver- ataxia, and dysdiadochokinesis, the last two findings re- mian cerebellar mass with surrounding vasogenic ede- flecting a more laterally located mass within the cere- ma, evidence of hydrocephalus, and fairly uniform con- bellar hemisphere [12,13].Abducens nerve palsy,result- trast enhancement on post-contrast images in a child ing from compression of the relatively exposed sixth less than 10 years of age [13, 25, 26] (Fig. 1). Most nerve nucleus along the anterior margin of the fourth (89–95%) of all medulloblastomas demonstrate at least ventricle, may indicate extraventricular tumor exten- some hyperattenuation compared to normal cerebellar sion [13]. attenuation on nonenhanced CT and have marginal vas- Most patients with medulloblastoma have 5-year ogenic edema [13, 25, 27] Heterogeneity, with cyst for- survival rates between 50% and 80% [13–19]. Patients mation (59%) and calcification (22%), are commonly with gross total resection of the tumor have improved survival rates compared to those with subtotal resection [12, 18, 20]. Recurrence of medulloblastoma is very common with most occurring within 2 years of initial treatment [21, 22]. Long-term recurrences may also oc- cur in children but are less common with the imple- mentation of chemotherapy in combination with radia- tion therapy [21–23]. Pathologic Findings The gross appearance of a medulloblastoma is variable. Some are firm and discrete masses while others may be soft and less well-defined [10]. Four major subtypes of the tumor are recognized in the World Health Organiza- tion (WHO) classification of central nervous system (CNS) neoplasms [10]. The most common subtype is the classic subtype, defined by dense, sheet-like growth of cells with hyperchromatic round-to-oval nuclei ac- companied by increased mitotic activity and conspicu- ous apoptosis [10]. Neuroblastic or Homer-Wright ro- settes, consisting of neoplastic cell nuclei disposed in a radial arrangement around fibrillary processes, are Fig. 1. Medulloblastoma, CT appearance. Axial nonenhanced CT common features while areas of necrosis are less com- image demonstrates heterogeneous hyperattenuation of a vermian mon [10]. Other less common subtypes include the des- cerebellar mass with surrounding vasogenic edema Chapter 1.4 Neoplasms of the Posterior Fossa 71 seen [27]. Atypical features include ill-defined margins, results in the first 2 post-operative weeks, either from absence of vasogenic edema or hydrocephalus, hypoat- the presence of methemoglobin or from leptomeningeal tenuation, hemorrhage, absence of enhancement on irritation caused by subarachnoid blood [34] Corrobo- post-contrast images, and the appearance of “primary” ration with clinical and cytopathologic CSF findings is leptomeningeal dissemination [13, 25, 27–29]. The pres- crucial to substantiate the diagnosis of CSF dissemina- ence of falcine calcification in children with medullo- tion [35]. In contrast to ependymoma and choroid plex- blastoma may be a marker for nevoid basal cell carcino- us tumors, foraminal extension by a medulloblastoma ma [30]. from the fourth ventricle to involve the cerebellopon- On magnetic resonance (MR) imaging, an even tine angle (CPA), cisterna magna, and other cisternal greater degree of heterogeneity among these lesions is compartments is not common [27, 36]. noted on MR imaging than on CT [31]. Iso- to hypoin- Recurrence of medulloblastoma most commonly tensity compared to white matter on T1-weighted imag- manifests as leptomeningeal enhancement or focal pa- es and variable signal intensity compared to white mat- renchymal nodular enhancement within the brain [22]. ter on T2-weighted images is typical [31]. Virtually all Extraneural metastasis is uncommon (7.1%) with the (97%) show at least some enhancement on post-con- skeletal system as the most common (77%) site of in- trast imaging studies [27] (Fig. 2). MR spectroscopy volvement [37]. typically shows a characteristic,albeit not specific,spec- Medulloblastomas occurring in the adult population trographic signature for a neuroectodermal tumor with tend to manifest as hyperattenuated poorly defined elevated choline,reduced N-acetyl aspartate (NAA),and masses located in the cerebellar hemisphere. Cyst-like reduced creatine peaks, and occasionally elevated lipid regions, from either cystic degeneration or necrosis, are and lactic acid peaks [32]. Striking grape-like nodular- more commonly noted (82%) than in those that occur ity characterizes the CT and MR imaging appearance