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Medulloblastoma

 

 

Pathology

Medulloblastomas are Primitive NeuroEctodermal Tumors (PNETs) - a form of embryonal neoplasm that is grouped along with ependymoblastomas, pineoblastomas, and cerebral neuroblastomas. 

These tumors are all composed of small round blue cells.

Medulloblastoma is a PNET of the posterior fossa, arising from the cerebellar midline.

Cellular Origin:

  • The cellular origin of medulloblastoma remains controversial.
  • There are currently two dominant hypotheses as to the origin.

 

Cytogenetics:

The commonest specific abnormality in medulloblastoma (present in about 50 %), is isochromosome 17q [i(17q)].

See Atlas of Genetics and Cytogenetics in Oncology and Haematology

 

Table :   Postulated cellular origins of medulloblastoma

Proposed cellular origin

Evidence

External granular layer of the cerebellum
  • medulloblastoma cells are cytologically similar to those of the external granular layer
  • this layer persists for the first year of life, before involuting

Posterior medullary velum
  • undifferentiated cells migrate to the external granular layer and persist for a short time after birth

 

Tumor Histology 

Location

Cerebellar vermis or cerebellar hemispheres

Cell density

Densely cellular

Nuclear characteristics

Round, oval or angulated (‘carrot-shaped’)
Hyperchromatic

Vascular features

Low vascular density

Cellular patterns

       -     sheets

  1. rows
  2. lobules
  3. intertwined fascicles of cells
  4. Homer-Wright rosette

Cell differentiation

Common

  1. glial lines
  2. neuronal lines

Less common:

  1. mesenchymal lines

    (“medullomyoblastoma”)

 

Classic medulloblastoma is a highly cellular tumor

  • Composed of cells with round, oval or angulated (‘carrot-shaped’) nuclei and minimal cytoplasm.
  • The tumor cells are in sheets, rows or nodules.
  • Homer-Wright rosettes are present in <40% of cases and consist of nuclei of rosette forming cells arranged in a circular fashion around fibrillary processes.
  • Medulloblastomas are mitotically active tumors, often show single cell necrosis and less commonly geographical areas of necrosis.
  • Occasionally multinucleated giant cells or cells with large nuclei are seen within the tumor.

The slide above shows a classic hypercellular medulloblastoma with several mitotic figures.

 

The slide above shows tumor cells in higher magnification. There are cells with angulated hyperchromatic nuclei. There is cell moulding.

 

Desmoplastic form of medulloblastoma can be histologically differentiated from the classic form.

  • Desmoplastic medulloblastoma is defined by the presence of several prominent nodules, or “pale islands”, of tumor.
  • These areas are of lower cellularity, reticulin –free, show nuclear uniformity and are in a background of collagen-rich, highly-proliferative tumor.
  • This form also tends to be more discrete than the classic variety and is often located in the cerebellar hemispheres.
  • There has been little success in finding a histopathologic grading system that gives an accurate prognosis of patients with medulloblastoma.
  • The degree of anaplasia of the tumor cell may be a predictor of the patient’s outcome.
  • Tumors that are well-differentiated and extensively nodular are found to have better clinical outcomes than those with large-cell anaplasia.

The slide above shows a nodular desmoplastic medulloblastoma.

 

The slide above shows reticulin staining in the same desmoplastic medulloblastoma. The reticulin surrounds the nodules and is in the internodular areas.

 

The slide above shows tumor cells infiltrating through the molecular layer of the cerebellum. Tumor is present in the subarachnoid space.

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