Workup
Laboratory Studies
- Complete blood profile: Used for preoperative preparation, and as a diagnostic tool in some patients. In cases of hemangioblastoma, erythrocytosis may be found, presumably from erythropoietic factors secreted by the tumor.
- Coagulation profile
- Blood grouping
- Renal function tests
- Liver function tests
- Tumor markers
- A tumor marker is any substance that makes possible either a qualitative diagnosis of neoplasm or a quantitative estimate of tumor burden.
- They are used for screening, for staging, and for evaluation of response to therapy and detection of early tumor recurrence.
- Three examples of tumor markers include (1) carcinoembryonic antigen (CEA), (2) alpha-fetoprotein (AFP), and (3) placental proteins.
- The diagnostic role of tumor markers is less crucial in the posterior cranial fossa than in other regions of the body because of the ability of MRI to clearly visualize distinctive characteristics, such as cysts.
Imaging Studies
- Plain x-ray skull
- It may show signs of chronic intracranial hypertension.
- It may show calcification.
- In case of a dermoid cyst, a bone defect with sclerotic margins may be detected.
- CT scanning: CT scan of the posterior fossa is inferior to MRI in diagnostic value because of the artifact produced from the surrounding thick bone. However, CT scan is helpful for postoperative follow-up.
- Cerebral angiography
- Cerebral angiography is useful to assess the vascular supply of the tumor.
- With the wide availability of MRI, cerebral angiography is no longer used as the first option in brain tumor assessment.
- Magnetic resonance imaging
- MRI appearance of cerebellar astrocytoma
- Intra-axial, either midline or hemispheric, mass
- Prominent cystic component that is hypointense in T1-weighted images and slightly hyperintense in T2-weighted images (more protein content)
- Mural nodule, usually enhanced in T2 images
- Displacement of effacement of the fourth ventricle
- Hydrocephalic changes due to obstruction of the CSF pathway
- Nodule that usually is enhanced with contrast
- Cystic wall that may or may not enhance
- Heterogenicity and calcification, rarely found
- MRI appearance of PNET
- Intraventricular midline or paramedian mass that is isointense in T1 images
- Prominent enhancement with intravenous contrast
- May appear heterogeneous on MRI, with areas of cystic degeneration and central necrosis
- Infrequently, calcification, eccentric location, and lack of enhancement
- MRI appearance of ependymomas
- Commonly found intraventricularly
- Calcification found in 45% of cases
- Plastic ependymomas that can extend through the foramina of Magendie and Luschka to compress the craniocervical junction and/or the brain stem
- Extension to the cerebellopontine angle that is pathognomonic of ependymomas
- Enhance with intravenous contrast
- May be heterogeneous in character
- MRI appearance of posterior fossa hemangioblastoma
- Appears as a cystic mass with pial-based mural nodule that is slightly hyperintense and enhanced prominently with intravenous contrast
- Appearing as regions of signal void on spin-echo images, abnormal vessels that may be found adjacent or within the tumor
- Cyst that may be isointense or hyperintense on T1- and T2-weighted images
- Cyst wall that often is sharply delimited
- Variation of MRI appearance, including presence of hemorrhage in the cyst, solid rather than cystic mass, multiple lesions, and associated syrinx cavity
- MRI appearance of posterior fossa choroids plexus papilloma
- Intraventricular mass with lobulated margin
- Hypointense on T1 images
- Patchy regions of hypointensity that express calcification and hypervascularity
- Region of hyperintensity on T2 images (calcification and old hemorrhage)
- Prominent enhancement with intravenous contrast
- Tumor extension through foramina of Luschka or Magendie into the basal cisterns
- Commonly, hydrocephalic changes
- MRI appearance of posterior fossa dermoid cyst
- Hyperintense on T1 images and hypointense on T2 images
- Midline mass
- MRI appearance of cerebellar astrocytoma
Other Tests
- Brainstem auditory evoked response
- Brainstem auditory evoked response (BAER) specifically is helpful as an intraoperative monitor of brainstem function.
- Abnormal response alerts the surgeon about potential danger that, in most cases, can be reversed early.
Histologic Findings
- Cerebellar astrocytoma
- Pilocytic subtype (61%): Most astrocytes found in the tumor are fibrillary, with round-to-oval nuclei with some cytoplasmic pleomorphism and Rosenthal fibers. A honeycomb pattern (area with densely and areas with loosely cellular appearance) is present. Malignant transformation is uncommon.
- Infiltrative fibrillary type (28%): It usually occurs in adults. It displays the histological features of anaplasia.
- Anaplastic astrocytoma (11%): More anaplastic features are present.
- Medulloblastoma
- Grossly, it appears as a soft and friable mass. Area of necrosis and focal hemorrhage may be present.
- Calcification is uncommon.
- The tumor may infiltrate the brainstem and adjacent cerebellar structures in 30% of patients.
- Dissemination through the subarachnoid space is common. Metastatic lesions are present in 50% of patients at autopsy.
- The tumor appears highly cellular with round-to-oval cells.
- Cytoplasm is scant, and hyperchromic nuclei are present. With hematoxylin and eosin staining, it appears as a blue tumor.
- Homer-Wright rosettes and pseudorosettes are commonly present, suggesting neuronal differentiation.
- Ependymomas (3 classic histological features)
- Uniform-appearing cells are arranged around a vessel, forming perivascular pseudorosettes.
- A true ependymal rosette is present.
- Cells contain blepharoplasts (basal bodies of cilia).
- Ependymoblastoma (histological features of malignancy)
- Cytoplasmic and nuclear pleomorphisms are present.
- Nuclear hyperchromatism occurs.
- Excessive mitosis occurs.
- Necrosis is present.
- Cytoarchitectural disorganization is present.
- Choroid plexus papilloma
- Gross picture: It is a reddish mass with an irregular surface.
- Microscopic picture:
- Cuboidal epithelial cells similar to the normal choroids plexus are present.
- Hemangioblastoma
- Macroscopic picture: It is yellow to pink in color. The mural nodule is found touching the pial surface of the cerebellum. In 20% of cases, the dura is involved. The cystic wall and its contents may be yellow or white.
- Microscopic picture: Benign cellular proliferation, with formation of vascular channels, occurs. Reticulin-rich stromal network and (fat-laden) cells are characteristic.
More on Posterior Fossa Tumors |
| Overview: Posterior Fossa Tumors |
 Workup: Posterior Fossa Tumors |
| Treatment: Posterior Fossa Tumors |
| Follow-up: Posterior Fossa Tumors |
| References |
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Further Reading
Keywords
posterior fossa tumors, infratentorial tumors, brain stem tumors, cerebellar tumors, cerebellar medulloblastoma, medulloblastoma, pineoblastoma, ependymomas, primitive neuroectodermal tumors, PNETs, astrocytomas, glial tumors
