Posterior Fossa Tumors Workup

Lab studies include the following:

• 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 modalities include the following:

• Plain x-ray skull: May show signs of chronic intracranial hypertension; 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: 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 ^{[22, 23, 24, 25]}

  • 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

  • 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

  • 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

  • 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

  • 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

  • Fossa dermoid cyst:

    • Hyperintense on T1 images and hypointense on T2 images

MRI is a helpful postoperative tool to detect early recurrence before it is manifested clinically. The interval of MRI scanning depends on the tumor histology and postoperative course. Patients receiving chemotherapy for a malignant lesion also should be studied between chemotherapy cycles to assess response. The presence of residual or recurrent contrast enhancement 2 months after surgery suggests recurrence. Postirradiation changes may mimic recurrence, requiring follow-up after few weeks. 

Othman and Abdullah suggest that serial quantitative MRI may be able to predict patients at risk of developing neurocognitive impairment following chemoradiotherapy for posterior fossa tumors by identifying lower hippocampal volumes and altered hippocampal growth trajectory. ^[26]

Dynamic susceptibillity contrast perfusion MRI has been shown to  help identify posterior fossa tumors such as pilocytic astrocytoma, ependymoma, and medulloblastoma by identifying significant differences in relative corrected cerebral blood volume, relative quantifications of cerebral blood flow, and permeability indices. ^[22]

In one study, thallium-201-SPECT was combined with FDG-PET for differential diagnosis of posterior fossa brain tumors using multiple discriminant analysis. Hemangioblastoma, low-grade glioma, and malignant lymphoma were well discriminated, but anaplastic astrocytoma, glioblastoma, medullobastoma, and metastatic tumors could not be well distinguished. ^[27]

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.

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 histologic features of anaplasia.

Anaplastic astrocytoma (11%):

• More anaplastic features are present.

Medulloblastoma

See the list below:

• 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 histologic features)

See the list below:

• 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 (histologic features of malignancy)

See the list below:

• Cytoplasmic and nuclear pleomorphisms are present.

• Nuclear hyperchromatism occurs.

• Excessive mitosis occurs.

• Necrosis is present.

• Cytoarchitectural disorganization is present.

Choroid plexus papilloma

See the list below:

• 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

See the list below:

• Macroscopic picture: 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.