Evaluation of Supratentorial Primitive Neuroectodermal Tumors in Children

Examination

A routine physical and neurological examination is performed. There will usually be:

• Evidence of elevated ICP: Bulging fontanelle, enlarged head, sixth nerve paresis, and sunsetting eyes may be present in an infant. Papilledema and sixth nerve palsy may be present in older children.
• Neurological deficits: The presence of neurological deficits depends on the location of the tumor. The most common neurological deficit is hemiparesis.

Laboratory Tests

The results of laboratory tests are usually normal.

Radiologic Tests

Ultrasound

• Can visualize tumor in infants: In infants with an open anterior fontanel, the initial diagnosis of the mass lesion or associated hydrocephalus can be made with transcranial ultrasound. In this situation, ultrasound may identify the appearance of an abnormal mass during the investigation of an enlarged head circumference and thus prompt CT and MRI scanning.

CT Scan

• Well circumscribed: These tumors are typically well circumscribed.
• Hyperdense: They usually appear hyperdense due to their hypercellularity and increased vascularity, but there is minimal surrounding edema. 
• Heterogeneous: They may contain calcium, necrosis, cysts, and hemorrhage that can give them a heterogeneous appearance.
• Enhancement nonhomogeneous: These tumors have brilliant but nonhomogeneous contrast enhancement. 

CT scan of a supratentorial PNET:

Enhanced CT scan of a supratentorial PNET: Note brilliant but nonhomogeneous enhancement.  

MRI

• Heterogeneous: The MRI will show a heterogeneous mass with well-defined margins. 
• Hypointense on T1-weighted imaging: On T1-weighted imaging, supratentorial PNETs typically are hypodense, but increased blood in the tumor can make them hyperdense. 
• Isointense to hyperintense on T2-weighted imaging: On T2-weighted imaging, supratentorial PNETs are typically isointense but may be hyperintense.  Calcification, necrosis, cysts, and hemorrhage may be present.
• Gadolinium enhancement: The tumor typically has variable contrast enhancement and frequently demonstrates increased vascularity, with minimal surrounding edema.
• Isointense FLAIR and restricted diffusion: The solid portion of the tumor will be isointense on FLAIR imaging and will exhibit restricted diffusion (5). 
• Spectroscopy will show an elevated choline peak, a decreased N-acetyl aspartate peak, and an elevated taurine peak (2).

Gadolinium-enhanced T1-weighted MRI of a supratentorial PNET:

T2-weighted MRI of a supratentorial PNET: The solid portion of the tumor appears hyperintense to surrounding normal brain.

Flair MRI of supratentorial PNET: The tumor is isointense to the brain, a distinguishing characteristic as compared to other gliomas and choroid plexus carcinomas.

• Spinal MRI should be obtained to assess for leptomeningeal spread to the spine.  This is ideally done prior to surgery to avoid any disease being obscured by blood or other tissue products that result from the surgery.

Staging extent of disease spread

• Up to 40% incidence of metastasis: Staging the extent of disease in supratentorial PNET is critical for devising an appropriate treatment plan. Metastatic dissemination of supratentorial embryonal tumors within the CNS is observed in 5–39% of patients (1, 4, 8, 9). Staging of supratentorial PNETs can be done on the basis of the criteria proposed by Chang et al. (23) as follows: M0 (no tumor dissemination beyond the local site), M1 (positive CSF cytology), M2 (intracranial dissemination), M3 (intraspinal dissemination), and M4 (systemic dissemination).
• CSF cytology: Like medulloblastomas, supratentorial PNETs often disseminate along CSF pathways. CSF for cytology obtained by lumbar puncture may be helpful in ruling out occult seeding of the neuraxis. Careful thought should be given to timing for obtaining this specimen since performing a lumbar puncture in the setting of a posterior fossa tumor risks herniation.
• Spinal MRI is required to evaluate for the presence of leptomeningeal dissemination. This is ideally done preoperatively so as to avoid a postoperative study that may be obscured by blood and other tissue products.

Nuclear Medicine Tests

• Radionuclide bone scanning and bone marrow aspiration have been used in tumor staging, for detection of extraneural tumor. However, in a recent study, the examination of aspirated bone marrow proved negative in 13 patients with supratentorial PNETs, bone scans were negative in 11 (1), and no patient (of 48 patients studied) had M4 disease (extraneural metastasis).

Correlation of Tests

• CT and, more importantly, MRI scans allow excellent localization of supratentorial PNETS but do not allow a precise diagnosis. Typically the diagnosis will be of a malignant supratentorial tumor, but the differential diagnosis would include tumors such as glioblastoma multiforme in addition to PNET. One of the goals of surgical resection is to obtain a tissue diagnosis.