Posterior Fossa Tumors in Adult Patients

In adults, the most common expansile “mass” lesion in the posterior fossa is a subacute stroke, whereas the most common neoplastic lesion in the posterior fossa is cerebellar metastasis (intra-axial) or vestibular schwannoma (extra-axial). Those diseases fall outside the scope of this article, which focuses on primary intra-axial tumors of the posterior fossa in adults. This category of tumors is uncommon and more frequently encountered in children. This article reviews tumors of the cerebellum, brainstem, and fourth ventricle that are seen in adult patients, following categories from the 2007 World Health Organization classification of central nervous system tumors.

  • In the adult posterior fossa, the most common “mass” is a subacute stroke, the most common tumor is a cerebellar metastasis, and the most common primary tumor is hemangioblastoma.

  • Medulloblastoma and pilocytic astrocytoma can be seen in adults as well as children; low-grade and high-grade diffuse gliomas can be seen in the brainstem as well as in the cerebral hemispheres.

  • Tumors of the fourth ventricle may originate from the wall (ependymoma, subependymoma, or rosette-forming glioneuronal tumor [RGNT]) or from the lumen (choroid plexus papilloma or carcinoma, meningioma, or metastasis).

  • In the adult posterior fossa, the most common “mass” is a subacute stroke, the most common tumor is a cerebellar metastasis, and the most common primary tumor is hemangioblastoma.

  • Medulloblastoma and pilocytic astrocytoma can be seen in adults as well as children; low-grade and high-grade diffuse gliomas can be seen in the brainstem as well as in the cerebral hemispheres.

  • Tumors of the fourth ventricle may originate from the wall (ependymoma, subependymoma, or rosette-forming glioneuronal tumor [RGNT]) or from the lumen (choroid plexus papilloma or carcinoma, meningioma, or metastasis).

Adult pilocytic astrocytoma is a rare diagnosis, with incidence of less than 0.1 case per 100,000 person-years in adults over age 45 years. Analysis of 3066 pilocytic astrocytomas from the National Cancer Institute Surveillance, Epidemiology, and End Results database found differences in location and prognosis between adult and pediatric patients. Although the cerebellum was the most common site in children (37%), it was slightly outnumbered by the cerebral lobes in adults (27% vs 30%). And although the cancer-specific 5-year survival rates were excellent in children (>95%), they dropped to 92%, 79%, and 64% within the 20 to 39, 40 to 59, and 60+ age groups, respectively. Molecular analysis in a large series of 127 adult pilocytic astrocytomas found the BRAF-KIAA1549 fusion gene in only 20%—whereas BRAF-KIAA1549 fusion is present in greater than 60% of pediatric cases and may confer a less aggressive clinical phenotype. Despite a higher recurrence rate in adults (30%), standard treatment is resection without adjuvant radiation.


There have also been a few case reports of supratentorial or extracerebellar liponeurocytomas, which are analogous to infratentorial or cerebellar liponeurocytomas on radiology (except for location), histopathology, and immunohistochemistry. Given the predilection for the lateral ventricles in younger adults, supratentorial liponeurocytomas are not currently part of the 2007 WHO classification and may be considered a rare lipidized variant of central neurocytoma. Other case reports have described familial (an underlying germline mutation has not been identified yet) and multifocal cerebellar liponeurocytomas. Treatment is surgical resection; however, up to 50% of patients have local recurrence on long-term follow-up. Adjuvant radiotherapy to the posterior fossa has been recommended for subtotal resections and for any liponeurocytomas with unusually aggressive histology (Ki-67 proliferative index >6%).

Neuronal tumors: rosette-forming glioneuronal tumor of the fourth ventricle (WHO grade I)

In 2002, Komori and colleagues reported “eleven cases of a distinctive tumor of the posterior fossa [with] relatively discrete, focally enhancing mass(es) primarily involving the aqueduct, fourth ventricle, and cerebellar vermis.” They were mixed neuronal-glial tumors, characterized by biphasic architecture with neurocytic and astrocytic components. Similar lesions had previously been classified as dysembryoplastic neuroepithelial tumors of the cerebellum; however, there were important histopathologic differences, for example, the formation of neurocytic rosettes and perivascular pseudorosettes as well as the frequent presence of a pilocytic astrocytoma-like component. These differences led to a new entity of RGNT of the fourth ventricle, officially introduced in the most recent 2007 WHO classification “as a rare, slowly growing tumor of the fourth ventricular region that predominantly affects young adults (mean age 33).”

In 1 meta-analysis of 41 patients with RGNT, the most common presenting symptoms were headaches (68%) and ataxia (39%), with frequent obstructive hydrocephalus (44%). Most are midline lesions of the posterior fossa; they are thought to arise from pluripotential cells in the subependymal plate at the ventricular wall ( Fig. 10 ). These tumors can be solid or mixed solid-cystic and often demonstrated focal contrast enhancement (82%). There is usually heterogeneous hypointensity on T1 and hyperintensity on T2 weighted images, with hypointense foci on T2* weighted images (gradient-recalled echo/susceptibility-weighted imaging) related to intratumoral calcification or hemorrhage. Both the occurrence of satellite lesions and drop metastases through CSF dissemination have been described. There have also been case reports of RGNT arising outside the fourth ventricle, most commonly in the pineal region, and around other CSF-containing spaces from third ventricle to central canal.

Fig. 10

A 22-year-old woman with possible seizure, found to have a lesion in the midline posterior fossa. ( A ) Sagittal T1 image shows a multicystic lesion involving the cerebellar vermis. ( B ) Axial T2 image shows a multicystic lesion at the posterior aspect of the fourth ventricle. ( C ) Axial postgadolinium T1 image shows no enhancement, in this particular RGNT. ( D ) Photomicrograph (H&E, original magnification ×20) shows a propensity to form neurocytic rosettes and perivascular pseudorosettes. (AIRP case contributor: Dr Jean Francois Mercier.)

Disclosures: None.


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