Laboratory Studies
Lab tests are not helpful in the diagnosis of primitive neuroectodermal tumors (PNET).
Imaging Studies
Diagnosis of these clinical entities is confirmed or excluded by lumbar puncture and CT scan and/or MRI. Presence of a mass lesion on an imaging study precludes lumbar puncture because of the risk of herniation.
Clinical diagnosis of these tumors is not possible. Radiologic features unique to each type of tumor may be helpful, but the only possible absolute confirmation is by pathologic examination of the surgical specimen.
MRI
MRI is the imaging technique of choice. The typical tumor is a heterogeneous mass with ill-defined margins arising from the vermis, which fills the fourth ventricle.
Typical findings include moderate to intense enhancement of the tumor, which is not homogenous (see image below).
Cerebellar medulloblastoma. This MRI (axial view, T2-weighted image) demonstrates the heterogeneity of the tumor.
Accompanying hydrocephalus is common (see image below), and associated cystic changes can occur (see image below).
Cerebellar medulloblastoma. This sagittal view MRI without contrast demonstrates characteristic midline cerebellar location with mild obstructive hydrocephalus.
The entire neuraxis should be imaged to detect spinal metastases, which may occur via subarachnoid dissemination.
CT scan
In emergent situations, CT scan is preferred over MRI because of its easy accessibility. However, CT scan resolution is inferior to that of MRI. The mass is typically midline, relatively heterogeneous, and variably contrast enhancing. (See the image below.)
Cerebellar medulloblastoma. This axial view CT scan with contrast shows a partially enhancing mass arising in the midline from cerebellum and filling the fourth ventricle.
CT myelogram may be used to rule out spinal dissemination in cases in which MRI is contraindicated.
MR spectroscopy
As compared to the normal cerebellum, these tumors on MR spectroscopy reveal a heterogeneous picture with decreased N- acetyl-aspartate (NAA) and creatine peaks and increased choline peaks.
The technique is still considered experimental.
Procedures
See the list below:
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Ventriculostomy is rarely done preoperatively because of the risk of upward herniation.
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Preoperative lumbar puncture is avoided because of the risk of downward herniation.
Histologic Findings
See the list below:
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Primitive cells are observed growing in sheets or cords of dense cellularity with increased mitotic index and increased nuclear-cytoplasmic ratio.
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Formation of Homer-Wright rosettes (ie, neuroblastic rosettes consisting of tumor cell nuclei disposed in a circular fashion about tangled cytoplasmic processes) is typical but not always seen and is not essential for diagnosis. When present, it is frequently associated with marked nuclear pleomorphism and high mitotic activity.
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Associated gross pathologic findings may include cystic changes, although the tumors are usually solid. They may vary from soft to firm in consistency. Geographic areas of necrosis, vascular proliferation, or calcification are less common, while hemorrhage is rare.
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Immunohistochemical markers can confirm differentiation toward astrocytic or neuronal lineage.
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Unusual variants include those with melanin deposition, rhabdomyoblastic differentiation, or desmoplastic features, among others.
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Cerebellar medulloblastoma. This MRI (axial view, T2-weighted image) demonstrates the heterogeneity of the tumor.
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Cerebellar medulloblastoma. This sagittal view MRI without contrast demonstrates characteristic midline cerebellar location with mild obstructive hydrocephalus.
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Cerebellar medulloblastoma. This axial view CT scan with contrast shows a partially enhancing mass arising in the midline from cerebellum and filling the fourth ventricle.
