Medulloblastoma Imaging 

Updated: Oct 09, 2020
  • Author: Djamil Fertikh, MD; Chief Editor: James G Smirniotopoulos, MD  more...
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Practice Essentials

Medulloblastomas are highly malignant tumors and 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). Although the overall cure rate is around 70%, high-risk disease is associated with poor outcome long-term morbidity. Medulloblastoma constitutes about 20% of all pediatric brain tumors, and mortality during the first few years after diagnosis is approximately 15%. [1, 2, 3, 4, 5, 6]

The World Health Organization classifies medulloblastomas into 2 general designations: (1) MB, histologically defined and (2) MB, genetically defined. Histologically, medulloblasoma is divided into classic, desmoplastic/nodular (DN), and large cell/anaplastic (LCA). A special group of DN tumors in the infant population is designated MB with extensive nodularity (MBEN). Genetically, medulloblstoma is separated into WNT‐activated; SHH‐activated and TP53‐mutant; SHH‐activated and TP53‐wild‐type; and non‐WNT/non‐SHH groups (this last group is further subdivided into group 3 and group 4). [7, 1, 5, 8, 9]

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. 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. [10, 11, 12, 13, 7, 14, 15, 16, 17, 8, 9]

MRI is performed for all patients with brain tumors and is key in the diagnosis, surgical guidance, and follow-up in patients with medulloblastomas. MRI with morphologic sequences is the first important step in identifying tumor location and extension and in differentiation from other tumor histotypes. MRI sequences such as diffusion-weighted imaging (DWI) with apparent diffusion coefficient (ADC) maps and spectroscopic studies have improved quantitative analysis and provided potential for correlating image findings with medulloblastoma molecular subtypes. [17]

Several studies have shown that MRI could be a promising tool for early detection of medulloblastoma subgroups (wingless [WNT], Sonic Hedgehog [SHH], group 3, and group 4). Each of these subgroups are associated with specific genetic aberrations, typical age of onset, and survival prognosis. Medulloblastomas may present with heterogeneous imaging aspects and specific phenotypic radiologic features that may reflect tumor histological and biological characteristics. [7, 14, 17, 18]

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.

(See the images below.)

Medulloblastoma. Unenhanced CT shows a high-densit Medulloblastoma. Unenhanced CT shows a high-density midline tumor in the posterior fossa with a small amount of surrounding vasogenic edema exerting mass effect on the fourth ventricle, with a moderate degree of hydrocephalus.
Medulloblastoma in a 27-year-old man. Nonenhanced Medulloblastoma in a 27-year-old man. Nonenhanced sagittal T1-weighted image shows a poorly defined, laterally situated, hypointense cerebellar mass with a small cystic area.

The WNT-activated group (wingless) has very good long-term prognosis as compared to the other groups. Long-term survival rates for patients with WNT medulloblastoma have been reported to exceed 90%. [1, 2]

SHH-activated medulloblastomas are named after the Sonic Hedgehog signaling pathway, which is thought to drive tumor initiation in many, if not all, such cases. [1, 2]  

Medulloblastoma groups 3 and 4 account for 25% and 35% of pediatric medulloblastoma cases, respectively; these tumors lack involvement of any clearly defined signaling pathway. Group 3 tumors occur more commonly in males than females and are found in infants and children but almost never in adults. Group 4 medulloblastoma is considered the prototypical medulloblastoma. Loss of the X chromosome is seen in 80% of females with group 4 tumor. [1, 2]

Adult tumors are often located in the cerebellum. SHH-activated tumors are the most common type found in adults, and TP52-mutant tumors have a worse prognosis than TP53-wildtype. [1]  

A study of US SEER (Surveillance, Epidemiology, and End Results) data reported that overall median survival for adults with medulloblastoma is 60 months, but survival dropped to 37 months when tumors were located outside the cerebellum. [19]

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. [20]  The Childhood Cancer Survivor Study of over 1300 5-year survivors of medulloblastoma reported increased risk for subsequent neoplasms, hearing loss, and cardiovascular disease. [21]  

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. [22]

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. [3, 23]

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Computed Tomography

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.

Medulloblastoma. Unenhanced CT shows a high-densit Medulloblastoma. Unenhanced CT shows a high-density midline tumor in the posterior fossa with a small amount of surrounding vasogenic edema exerting mass effect on the fourth ventricle, with a moderate degree of hydrocephalus.

On enhanced CT scans, a marked homogeneous enhancement of the tumor is seen (see the image below). In rare circumstances (13%), calcifications are found.

Medulloblastoma. Following intravenous injection o Medulloblastoma. Following intravenous injection of contrast material, the tumor shows marked diffuse and homogeneous enhancement.

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.

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Magnetic Resonance Imaging

MRI is performed for all patients with brain tumors and is key in the diagnosis, surgical guidance, and follow-up in patients with medulloblastomas. MRI with morphologic sequences is the first important step in identifying tumor location and extension and in differentiation from other tumor histotypes. MRI sequences such as diffusion-weighted imaging (DWI) with apparent diffusion coefficient (ADC) maps and spectroscopic studies have improved quantitative analysis and provided potential for correlating image findings with medulloblastoma molecular subtypes. [17]

Medulloblastomas are hypointense to isointense on T1-weighted images. 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. [10, 11, 12, 13, 7, 14, 15, 16, 17, 8, 9]

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 is usually high with MRI.

(See the images below.)

Medulloblastoma in a 27-year-old man. Nonenhanced Medulloblastoma in a 27-year-old man. Nonenhanced sagittal T1-weighted image shows a poorly defined, laterally situated, hypointense cerebellar mass with a small cystic area.
Medulloblastoma. Axial MRI of the posterior fossa Medulloblastoma. Axial MRI of the posterior fossa shows a right-sided laterally located mass with mixed isointense and hyperintense signal intensity. Surrounding edema is seen as high signal intensity.
Medulloblastoma. Coronal T1-weighted postcontrast Medulloblastoma. Coronal T1-weighted postcontrast image shows a markedly enhancing peripheral tumor.
Medulloblastoma in a 6-year-old boy. Sagittal T1-w Medulloblastoma in a 6-year-old boy. Sagittal T1-weighted image shows a slightly hypointense mass in the region of vermis with compression of the fourth ventricle and obstructive hydrocephalus. The brain stem is also compressed by this mass. The lateral and third ventricles are dilated.
Medulloblastoma. Axial T1-weighted image shows a h Medulloblastoma. Axial T1-weighted image shows a hypointense mass in the midline, just posterior to the fourth ventricle. The fourth ventricle and brain stem are displaced anteriorly and compressed.
Medulloblastoma. Axial T1-weighted postcontrast im Medulloblastoma. Axial T1-weighted postcontrast image demonstrates an irregular, heterogeneous enhancing mass. Note the dilatation of both temporal horns, indicating obstructive hydrocephalus.

On T2-weighted images, appearances may vary from isointense to hyperintense. (See the image below.)

Medulloblastoma. Axial T2-weighted image reveals a Medulloblastoma. Axial T2-weighted image reveals a predominantly isointense mass to gray matter with small foci of cystic changes.

In a retrospective study of 38 patients with medulloblastoma to determine whether apparent diffusion coefficient (ADC) and other MRI features can be used to predict medulloblastoma histologic subtypes, the large or anaplastic medulloblastoma subtype was found to be associated with increased ADC and with ring enhancement, the latter of which correlated with tumor necrosis. [14]

In a prospective study, by Duc et al, to evaluate the function of diffusion tensor imaging (DTI) metrics in differentiating between cerebellar medulloblastomas and brainstem gliomas in children (medulloblastoma n=25; glioma n=15), the fractional anisotropy (FA) value for medulloblastomas was significantly higher than that for brainstem gliomas (P< 0.05). In contrast, the diffusivity and tumor-to-parenchyma ratios for medulloblastoma were significantly lower than those for brainstem gliomas (P< 0.05). [24]

Several studies have shown that MRI could be a promising tool for early detection of medulloblastoma subgroups (wingless [WNT], Sonic Hedgehog [SHH], group 3, and group 4). Each of these subgroups are associated with specific genetic aberrations, typical age of onset, and survival prognosis. Medulloblastomas may present with heterogeneous imaging aspects and specific phenotypic radiologic features that may reflect tumor histological and biological characteristics. [7, 14, 17, 18]

Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). 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.

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Nuclear Imaging

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 (201TI). 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 (FDG) 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, for 201TI, and 81% and 40%, respectively, for PET. [25]

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