Medulloblastoma.
Originally classified as a glioma, medulloblastoma is referred to now as a primitive neuroectodermal tumor (PNET). This tumor accounts for approximately 7-8% of all intracranial tumors and 30% of pediatric brain tumors.
Pathophysiology: In the brain, medulloblastoma most often arises in the posterior fossa. The tumor has the propensity of spreading throughout the CNS. Systemic metastases of this tumor, especially to bone, also have been recognized.
Imaging Studies:
Computed tomographic scan
- Because most patients present with headache, a noncontrast head CT scan usually is performed because of its easy availability. These tumors typically are located midline in the cerebellum and extend into and fill the fourth ventricle.
- Prior to administration of intravenous (IV) contrast, the tumor is hyperdense to the brain as a result of its high cellularity. Preoperatively, high density on CT scan can help distinguish medulloblastoma from the hypodense appearance of a cerebellar astrocytoma. Medulloblastoma shows marked contrast enhancement. Surrounding hypodensity is indicative of vasogenic edema. Owing to compression of the fourth ventricle and outflow of CSF, marked hydrocephalus is the rule.
- Ependymoma is another hyperdense tumor that affects the posterior fossa of children. Unlike medulloblastoma, however, it often contains calcifications that can be recognized easily on CT scan. Choroid plexus papilloma usually arises in the trigone of the lateral ventricle in children; however, in adults it is most common in the fourth ventricle. Similar to ependymoma, choroid plexus papilloma commonly contains calcifications.
- Magnetic resonance imaging
- MRI with the administration of gadolinium DTPA is the diagnostic test of choice for medulloblastoma. Unlike CT scan, MRI can obtain multiplanar views without significant bony artifact in the posterior fossa.
- Nevertheless, with any increased intracranial pressure, MRI of children must be considered carefully. Younger children usually require sedation for this study. Without careful monitoring, cerebral carbon dioxide levels may increase, further aggravating intracranial hypertension.
- Tumor appears hypointense on pre-gadolinium T1-weighted images, usually seen expanding the fourth ventricle from its origin in the cerebellar vermis. Brain stem is compressed and shifted ventrally.
- Upon administration of gadolinium in children, homogeneous enhancement commonly occurs, whereas in adults, a more heterogeneous pattern usually is seen. Proton density and T2-weighted imaging displays a hyperintense mass with a surrounding area of edema.
- If the tumor extends upward into the cerebral aqueduct and third ventricle, marked hydrocephalus with transependymal reabsorption of CSF may occur. Extension also can be inferior into the cervical canal.
- Occasional areas of hemorrhage or cyst can be distinguished. Because calcifications are very rare, any area of signal loss must be considered a vascular flow void.
- MRI can help differentiate medulloblastoma from ependymoma: the latter extends further into the lateral recess of the fourth ventricle or even further into the cerebellopontine angle.
- MRI also can help distinguish between medulloblastoma and exophytic brainstem glioma (the latter having a broader attachment to the floor of the fourth ventricle).
- Adults, more frequently than children, can have the desmoplastic variant of medulloblastoma. This form of the tumor is situated laterally in the hemisphere with indistinct borders and small cystic or necrotic areas.
- Besides identifying the primary lesion, MRI is beneficial in detecting metastatic lesions. To rule out drop metastases, MRI of the spine is obligatory when medulloblastoma is either considered or diagnosed.
- Imaging of the spine is best performed prior to surgery in order to avoid postoperative artifacts, which may be interpreted as tumor metastasis. Metastases can occur in the basal cisterns. Both recurrent lesions and metastases show sparse enhancement.