# Posterior Fossa Tumors in Adults
## Overview
Posterior fossa tumors in adults encompass a distinct spectrum of pathology compared with pediatric posterior fossa tumors. The most common adult posterior fossa tumors include hemangioblastomas, epidermoid cysts, metastases, meningiomas, vestibular schwannomas (covered separately), and fourth ventricular tumors such as ependymomas and choroid plexus papillomas. Surgical management is complicated by the confined space of the posterior fossa, proximity to the brainstem and cranial nerves, and the risk of obstructive hydrocephalus.
## Hemangioblastomas
### Epidemiology and Genetics
Hemangioblastomas account for 1-2.5% of intracranial tumors and 7-12% of posterior fossa tumors in adults, with peak incidence in the 3rd to 4th decade. Approximately 75% are sporadic and typically present as a single lesion. The remaining 25% occur in the setting of Von Hippel-Lindau (VHL) disease, an autosomal dominant condition caused by mutation of the VHL tumor suppressor gene on chromosome 3p25-26. VHL patients develop multiple and recurrent hemangioblastomas in the cerebellum, spine, brainstem, and retina. Associated findings include renal cell carcinoma, pheochromocytoma, pancreatic cysts and neuroendocrine tumors, endolymphatic sac tumors, and renal cysts. Screening for VHL involves annual ophthalmology, abdominal imaging, catecholamine levels, and audiology.
### Pathology
Hemangioblastomas are WHO grade 1, benign tumors composed of highly vascular tissue with stromal cells and an abundant capillary network. Approximately 60% present with the classic cystic-with-mural-nodule pattern, showing a large cyst with an enhancing mural nodule on the cyst wall. The remaining 40% are solid, densely enhancing, and may have prominent flow voids. These tumors do not undergo malignant transformation.
### Clinical Presentation
Patients present with headache, ataxia, and nausea or vomiting from cerebellar mass effect. Obstructive hydrocephalus develops from fourth ventricle compression. Polycythemia occurs in 10-20% of cases due to tumor production of erythropoietin. Subarachnoid hemorrhage is a rare presentation.
### Imaging
On MRI with gadolinium, hemangioblastomas show an intensely enhancing mural nodule with a large cystic component, or a homogeneously enhancing solid mass. On T2-weighted imaging, the cyst is hyperintense and flow voids may be visible in and around the nodule. DSA reveals a highly vascular tumor with prolonged blush, and preoperative embolization may be considered for large solid tumors. The entire neuraxis must be imaged in VHL patients to identify concurrent lesions.
### Surgical Management
Complete resection of the mural nodule is curative. The cyst wall is non-neoplastic and does not need to be resected. Preoperative embolization may reduce intraoperative blood loss for large solid tumors. Because the tumor is extremely vascular, circumferential dissection with progressive devascularization is the key technique. Piecemeal resection must be avoided, as internal debulking causes profuse hemorrhage. The goal is en bloc resection with coagulation of feeding vessels at the tumor surface. Cyst drainage decompresses the posterior fossa before tumor resection. Recurrence is rare for sporadic cases after gross total resection, though VHL patients develop new tumors at different sites over time.
Sagittal and axial MRI (T1 post-gadolinium) of a cerebellar hemangioblastoma showing the classic cystic tumor with an intensely enhancing mural nodule at the posterior aspect of the cyst. The cyst causes fourth ventricle compression and mild obstructive hydrocephalus. Flow voids are visible within the mural nodule. An inset DSA image shows the hypervascular tumor blush with arterial feeders from the PICA. Radiological teaching illustration with labeled structures.
## Epidermoid Cysts
### Pathology and Presentation
Epidermoid cysts are congenital inclusion cysts arising from ectodermal rests during neural tube closure. They are lined by stratified squamous epithelium and filled with desquamated keratin and cholesterol crystals, giving them a characteristic "pearly tumor" appearance. These lesions grow slowly by progressive desquamation and insinuate around neurovascular structures rather than displacing them. The most common location is the CPA cistern (40-50%), with other sites including parasellar, fourth ventricle, and diploe. Patients present with trigeminal neuralgia, facial spasm, hearing loss, or headache. Aseptic meningitis (Mollaret meningitis) may occur from leakage of cyst contents.
### Imaging
On CT, epidermoid cysts appear as hypodense, lobulated, non-enhancing masses with scalloped edges. On T1 they are hypointense (similar to CSF) and on T2 they are hyperintense. The key distinguishing feature is that they are bright on DWI (restricted diffusion), which differentiates them from arachnoid cysts that follow CSF signal on DWI. On FLAIR, epidermoid cysts do not suppress, unlike CSF. There is no enhancement; enhancement suggests dermoid or extremely rare malignant transformation.
### Surgical Management
The goal is maximal safe resection, with complete capsule removal being ideal but often not achievable. The capsule is frequently densely adherent to cranial nerves, vessels, and the brainstem. It is safer to leave adherent capsule on critical structures rather than risk permanent deficits. Recurrence occurs in 10-20% over 10-20 years if capsule remnant remains. Aseptic meningitis can occur postoperatively from keratin debris spillage, and copious irrigation during surgery helps reduce this risk.
## Fourth Ventricular Tumors in Adults
### Ependymoma
Ependymomas arise from the ependymal lining of the fourth ventricle and are classified as WHO grade 2 (classic) or grade 3 (anaplastic). They tend to extend through the foramina of Luschka and Magendie in a "plastic" growth pattern, wrapping around cranial nerves and vessels. On imaging they show heterogeneous enhancement with calcification and may fill and expand the fourth ventricle. The telovelar approach through the cerebellomedullary fissure is the preferred surgical corridor as it avoids vermian splitting. Complete resection is the strongest predictor of recurrence-free survival. The role of adjuvant radiation for grade 2 ependymoma after gross total resection remains debated, though RT is recommended for subtotal resection and all grade 3 tumors.
### Choroid Plexus Papilloma
These tumors arise from the choroid plexus of the fourth ventricle (less common than the lateral ventricle in adults). They are WHO grade 1 and benign. Overproduction of CSF can cause communicating hydrocephalus. On imaging they appear as intensely enhancing, lobulated, intraventricular masses. Complete surgical resection is curative.
### Subependymoma
Subependymomas are WHO grade 1, benign tumors arising from the floor or roof of the fourth ventricle. They are typically asymptomatic incidental findings unless they cause obstructive hydrocephalus. Observation is appropriate for asymptomatic lesions, with surgery reserved for symptomatic cases. The prognosis is excellent and recurrence is rare.
## Summary of Adult Posterior Fossa Tumors
| Tumor | WHO Grade | Key Imaging Feature | Surgical Goal | Special Considerations |
|-------|-----------|--------------------|--------------|-----------------------|
| Hemangioblastoma | 1 | Cystic + enhancing mural nodule; flow voids | En bloc nodule resection (cyst wall non-neoplastic) | Screen for VHL; avoid piecemeal resection |
| Epidermoid cyst | N/A (benign) | DWI bright, non-enhancing, no FLAIR suppression | Maximal capsule removal | Leave capsule adherent to CN/vessels |
| Ependymoma | 2-3 | Heterogeneous enhancement, fills 4th ventricle | GTR (strongest prognostic factor) | May extend through foramina of Luschka |
| Choroid plexus papilloma | 1 | Intensely enhancing, lobulated, intraventricular | Complete resection (curative) | May cause communicating hydrocephalus |
| Subependymoma | 1 | Non-enhancing 4th ventricle floor/roof mass | Surgery if symptomatic; observe if incidental | Excellent prognosis |
## Hydrocephalus Management in Posterior Fossa Tumors
Obstructive hydrocephalus occurs in 60-80% of fourth ventricular tumors. A preoperative external ventricular drain is indicated for acute hydrocephalus with clinical deterioration, though there is a risk of upward transtentorial herniation with aggressive CSF drainage. Endoscopic third ventriculostomy serves as an alternative that may avoid shunt dependence. Definitive tumor resection often resolves the hydrocephalus, but 10-30% of patients ultimately require permanent CSF diversion with a shunt.
## Surgical Approaches to the Posterior Fossa
### Suboccipital Midline (Posterior) Approach
This approach serves midline lesions such as fourth ventricle tumors and vermian lesions. The patient is positioned prone or in the concorde (three-quarter prone) position. A suboccipital craniotomy is performed with or without C1 laminectomy. The telovelar approach dissects the tela choroidea and inferior medullary velum to enter the fourth ventricle without splitting the vermis.
### Retrosigmoid Approach
This approach accesses CPA lesions including epidermoid cysts, schwannomas, and meningiomas. The patient is positioned in the lateral decubitus or park-bench position. The craniotomy is placed behind the sigmoid sinus.
### Far-Lateral Approach
This approach serves foramen magnum and anterolateral brainstem lesions. Condylar drilling provides anterior access without requiring brainstem retraction.
Diagram of the telovelar approach to the fourth ventricle showing a sagittal view with the cerebellum, tonsils, tela choroidea, and inferior medullary velum. The surgical corridor through the cerebellomedullary fissure is illustrated, opening the tela choroidea and inferior medullary velum to expose the floor of the fourth ventricle and an intraventricular tumor. The obex, facial colliculus, and hypoglossal triangle on the fourth ventricle floor are labeled as "no-go zones." Clean surgical anatomical illustration.
## Clinical Pearls
Every patient with a hemangioblastoma should be screened for VHL disease through genetic testing and systemic screening of renal, adrenal, retinal, and pancreatic organs. When resecting a hemangioblastoma, the mural nodule should be removed but not the cyst wall, as the cyst wall is non-neoplastic and will collapse after nodule removal. DWI is the key MRI sequence for distinguishing epidermoid cyst from arachnoid cyst, since epidermoid restricts diffusion while arachnoid follows CSF signal. The telovelar approach is preferred for accessing the fourth ventricle because it avoids splitting the vermis, thereby reducing the risk of cerebellar mutism and truncal ataxia. Fourth ventricular ependymomas can extend through the foramina of Luschka into the CPA, so preoperative imaging must assess for this "toothpaste" extension to plan the surgical approach. CSF should not be aggressively drained in the setting of a posterior fossa mass with obstructive hydrocephalus, because rapid supratentorial decompression can precipitate upward transtentorial herniation. Polycythemia in a patient with a posterior fossa mass should raise suspicion for hemangioblastoma due to erythropoietin production.
## References
- Lonser RR, et al. "Von Hippel-Lindau Disease." *Lancet*. 2003;361(9374):2059-2067.
- Rhoton AL Jr. "The Posterior Fossa Cisterns." *Neurosurgery*. 2000;47(Suppl 3):S287-S297.
- Mussi AC, Rhoton AL Jr. "Telovelar Approach to the Fourth Ventricle." *J Neurosurg*. 2000;92(5):812-823.
- Bertalanffy H, et al. "Surgical Management of Epidermoid Cysts of the Cerebellopontine Angle." *J Neurosurg*. 1992;77(6):913-920.
- Merchant TE, et al. "Ependymoma: Current Diagnosis, Risk Stratification, and Treatment." *CNS Oncol*. 2017;6(3):221-230.