One half of primary brain tumors in children originate in the posterior fossa. Medulloblastomas are highly malignant tumors; they are the most common malignant posterior fossa tumor in the pediatric population. They are characterized by their tendency to seed along the neuraxis, following cerebrospinal fluid (CSF) pathways, and they represent one of the few brain tumors, including ependymoma, pinealoblastoma, and lymphoma, to metastasize to extraneural tissues. Originally classified as a glioma, medulloblastoma is now referred to as a primitive neuroectodermal tumor (PNET). (See the images below.) [1, 2, 3, 4, 5, 6, 7]
Of medulloblastoma patients, 10-30% demonstrate CSF dissemination at diagnosis, mandating evaluation of the entire neuraxis with contrast-enhanced studies. Extra-axial metastases account for 5% of cases; most metastases are to the bone; less frequently, metastases are to the liver and lymph nodes.
Children with nondisseminated medulloblastoma have a high likelihood of long-term survival, with a 5-year survival rate of 80%. Intensified therapy has been shown to increase survival in children with disseminated disease. However, the quality of life in long-term survivors remains an important issue, because most survivors have neurologic and cognitive deficits. 
Medulloblastomas have been associated with basal nevus syndrome (Gorlin syndrome), Turcot syndrome, ataxia telangiectasia, xeroderma pigmentosum, and blue rubber bleb syndrome.
Standard treatment is surgery followed by radiation to the entire neuraxis. Medulloblastomas are radiosensitive. Gross total resection of the tumor, when possible, is the aim of surgery. Resection usually is achieved in 50% of patients, according to Thapar et al. 
Signaling pathways that regulate medulloblastoma tumor formation have been discovered. Advances in the molecular biology of medulloblastoma indicate that better understanding of the growth control mechanisms in medulloblastoma may lead to the development of new therapies for the disease. [10, 11]
Although medulloblastoma has a highly characteristic appearance on computed tomography (CT) scanning, magnetic resonance imaging (MRI) is the preferred tool. The multiplanar capability of MRI provides better 3-dimensional visualization of the extent of the tumor, as well as better visualization of edema and herniation, when present. MRI also is better for evaluating the remainder of the neuraxis for metastasis. In addition, MRI spectroscopy may help better delineate the tumor's boundaries. [12, 13, 14, 15] With CT scanning, only axial images can be obtained; by contrast, with MRI, any plane can be used for imaging. On CT scans, posterior fossa images often are degraded by beam-hardening artifacts. [16, 7, 17]
Medulloblastomas are highly cellular; therefore, on noncontrast CT, their classic appearance is that of a high-density midline mass. In most patients (90%), a varying degree of hydrocephalus is apparent (see the image below.) Variable amounts of asymmetric edema are seen in approximately 90% of patients.
On enhanced CT scans, a marked homogeneous enhancement of the tumor is seen (see the image below). In rare circumstances (13%), calcifications are found.
Metastatic nodular seeding may be seen in the supratentorial subarachnoid space on contrast-enhanced CT scans and in the spinal canal on CT myelography.
Necrotic, cystic areas and hemorrhage are seen in approximately 10-16% and 3% of patients, respectively.
In a child, especially a boy, the presence of a high-density midline posterior fossa mass with diffuse marked enhancement is highly suspicious for medulloblastoma.
CT scanning is superior to MRI in depicting small punctate calcifications.
Magnetic Resonance Imaging
On T2-weighted images, appearances may vary from isointense to hyperintense. (See the image below.)
Medulloblastomas classically demonstrate heterogeneous hypointense or isointense signal. Calcifications appear as areas of signal void on T2-weighted images.
The pattern of enhancement after intravenous injection of gadolinium is similar to that after injection of iodinated contrast material on CT. However, the greater sensitivity of MRI often enables appreciation of a slightly heterogeneous enhancing pattern that is not as readily evident with CT.
Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Systemic Fibrosis. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.
NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.
The blurring of cerebellar folia and fissures, representing tumor spread via CSF pathways (best depicted on midline sagittal images), is a helpful sign. Subarachnoid or intraventricular seeding usually demonstrates contrast enhancement.
Drop metastases appear as high signal foci on contrast-enhanced T1-weighted images in the extramedullary and intradural space; occasionally, they are subpial in location.
Proton spectroscopy demonstrates a nonspecific elevation of the choline peak, representing cell membrane turnover; a decreased aspartate peak, representing loss of neuronal tissue; and variable lipid and lactate.
Because of the age group in which medulloblastomas occur, as well as the location and general appearance of the tumors, the degree of confidence usually is high with MRI.
No findings specific to medulloblastoma have been described; however, single-photon emission CT (SPECT) and positron emission tomography (PET) scanning complement CT scanning and MRI. Although the mechanism of uptake is not clearly understood, 80% of pediatric tumors show uptake of thallium-201 chloride (201 TI). These techniques also are important in differentiating high-grade from low-grade tumors and residual tumor from postoperative changes.
Thallium SPECT and fluorine-18-flurodeoxyglucose PET are complementary in diagnosing gliomas, although thallium SPECT was found to correlate more significantly with malignancy. In a series of 19 patients, Kahn et al demonstrated that the sensitivity and specificity for tumor recurrence is 69% and 40%, respectively, for201 TI, and 81% and 40%, respectively, for PET. 
Angiographic findings are not diagnostic. Medulloblastoma may demonstrate abnormal neovascularity. Because medulloblastoma is a posterior fossa tumor, anterior displacement of the precentral cerebellar vein may be seen. Posterior and inferior displacement of the inferior vermian vein also may be seen. Angiographic findings are nonspecific for the diagnosis of medulloblastoma and are only indicative of a space-occupying lesion.