Imaging of Medulloblastoma

Fig. 15.1

Typical features CT: (a–c) unenhanced CT shows a well-defined, homogeneous hyperattenuated mass in the posterior fossa midline surrounded by vasogenic edema (asterisk), associated with hydrocephalus (yellow arrows). (d–f) Contrast-enhanced CT image shows intense, nearly homogeneous enhancement of the mass

Medulloblastoma has characteristic hyperattenuation compared with normal gray matter on unenhanced CT that reflects the high nuclear–cytoplasmatic ratio seen at histologic analysis; the degree of hyperattenuation is variable.

Variable amounts of surrounding low density consistent with asymmetric vasogenic edema are reported in 95 % of the cases, and hydrocephalus of various degrees of severity is evident in up to 95 % of the cases at presentation.

On enhanced CT scans, a marked enhancement of the mass, relatively homogeneous, occasionally patchy, is seen.

Nevertheless, variance from this characteristic features is common [3, 7, 8]. Several series have revealed atypical features, which include cystic or necrotic regions, calcification, ill-defined margins, and lack of enhancement. Intralesional areas of low density (usually <1 cm), consistent with intratumoral cystic and necrotic degeneration, can be depicted (Fig. 15.2).

Fig. 15.2

Variability of medulloblastoma on CT images: (a–c) unenhanced CT image shows heterogeneous hyperdense mass with cystic/necrotic areas (arrowheads) in the posterior fossa midline. (d–f) On the unenhanced CT image, the mass is hypo-hysodense compared with the surrounding normal cerebellum, and tumor has small cysts and a large cavity (arrowheads)

The prevalence of cysts ranges from 30 to 60 %; some tumor has multiple small cysts, others have single large cavity and also calcifications can be see (20–30 %).

Other less common atypical features include ill-defined margins, absence of vasogenic edema or hydrocephalus, hypoattenuation, hemorrhage, absence of enhancement on contrast-enhanced images, and the appearance of “primary” leptomeningeal dissemination.

Metastatic nodular seeding may be seen in the supratentorial subarachnoid space on contrast-enhanced CT scans and in the spinal canal on CT myelography. In the ventricle, typical area of metastases at diagnosis is the infundibular recess in the third ventricle.

Falcine calcification in children with medulloblastoma can suggest a nevoid basal cell carcinoma. For the reason that children with this tumor have a propensity to develop various basal cell carcinomas in irradiated fields, examination with CT in such patients is crucial, since it may influence therapeutic decision in the favor of chemotherapy or reduced-dose radiation therapy [9].

15.3 MR Imaging

The standard MRI protocol for pediatric intracranial tumors includes fluid-attenuated inversion recovery (FLAIR), T1-, T2-, axial diffusion-, and postcontrast T1-weighted sequences for brain and spine. Furthermore, MR angiography is regularly performed to better clarify the characteristics of the intratumoral circulation and to plan best way for the intervention. Susceptibility-weighted perfusion MRI and proton magnetic resonance (MR) spectroscopy are performed usually to distinguish posttreatment changes from recurrent tumor in the posttherapeutic setting, or occasionally when the neoplastic nature of a primary lesion is in question.

Preoperative evaluation of entire neuraxis and postoperative evaluation of surgical bed are key to prognosis (Fig. 15.3).

Fig. 15.3

Medulloblastoma. MR typical features: (a) On an axial T1-weighted MR image, the mass is hypointense compared with the surrounding normal cerebellum with well-defined margins. (b) On an axial T2-weighted MR image, the mass shows mild hyperintensity compared with surrounding normal brain tissue. (c) On an axial FLAIR MR image, the tumor shows hyperintensity compared with surrounding normal brain tissue with well-defined margins. (d) Sagittal T1-weighted MR image shows ill-defined mass originating from the inferior medullary velum compressing the fourth V and brainstem (arrows), causing obstructive hydrocephalus and projecting through the foramen magnum (arrowheads). Notice the hyposignal at C2–C3 level due to compression effect (asterisk). (e) On a coronal T2-weighted image, the mass shows hyperintensity and hydrocephalic dilatation of the lateral ventricles. (f) Contrast-enhanced axial T1-weighted MR image shows intense, nearly homogeneous enhancement of the mass

Bailey and Cushing in 1925 established the medulloblastoma principally as a tumor of the posterior vermis in children.

The medulloblastomas are most commonly situated in the region of the fourth ventricle as midline lesions involving the anterior portion of the vermis and can sometimes be seen originating from the inferior medullary velum; often the medulloblastomas are located in the middle and lower segments of the vermis. Less frequently can be seen originating from the superior zone of the vermis (Fig. 15.4) [10].

Fig. 15.4

Medulloblastomas in the region of the fourth ventricle originating from the inferior medullary velum (a–c) and mild vermis (d); medulloblastomas originating from the posterior portion of the vermis (e, f), involving the inferior zone of the vermis (e) and the superior zone of the vermis (yellow arrows) (f)

Medulloblastomas can be also off-midline tumors, located in cerebellar hemispheres (Fig. 15.5).

Fig. 15.5

Medulloblastoma in cerebellar hemisphere: desmoplastic MB in a 1-year-old girl (a) unenhanced CT shows a well-defined, homogeneous hyperattenuated right cerebellar hemispheric mass (arrows). (b) Axial DWI MR shows the tumor diffusivity to be increased to that in white matter. (c) On axial T1-weighted MR image, the lesion is hypointense compared with normal cerebellum with well-defined margins. (d) Axial FLAIR MR image shows hyperintensity compared with surrounding normal brain tissue. (e) Coronal T2-weighted MR image shows hyperintensity compared with surrounding normal brain tissue. (f) Axial T1 C+ MR and (g) coronal T1 C+ MR show intense, homogeneous enhancement of the mass

In 1930, Cushing included in an exhaustive survey of 61 cerebellar medulloblastomas, nine that were laterally situated, subsequently designated desmoplastic medulloblastoma (DMB).

Although much less common, medulloblastoma may also occur in adults, usually in the third and fourth decades of life, and the cerebellar hemispheres represent the most frequent location.

An off-midline location is common in desmoplastic medulloblastomas (DMB) and slightly more common in medulloblastomas with extensive nodularity variants (MB-EN).

Tumor margins are mostly convex and well defined on unenhanced spin-echo images. Lesions involving the vermis can occur as poorly defined zones of signal alteration on the long TR images only.

Foraminal extension from the fourth ventricle to involve the cerebellopontine angle, cisterna magna, and other cisternal compartments may occur but is not common [11].

Some reported cases of medulloblastoma involved the porus acusticus and simulated the imaging features of a vestibular schwannoma [12] (Figs. 15.6 and 15.7).

Fig. 15.6

Medulloblastomas with foraminal extension (arrows): (a) Axial T1- and (b) T2-weighted and (c) FLAIR images show foraminal extension from the fourth ventricle to the left foramen of Luschka

Fig. 15.7

Medulloblastomas with cerebellopontine angle cistern extension (arrows): (a, b) Axial T2-weighted images show foraminal extension from the fourth ventricle to the foramen of Luschka and to the cistern of left cerebellopontine angle. (d, c) Axial T2-weighted images shows metastatic MB (arrowsheads) with a lesion of the cistern of the left cerebellopontine angle

At conventional MR imaging, the classic appearance of a medulloblastoma is iso- to hypointense relative to gray matter with T1W sequences and variable signal intensity relative to grey matter with long repetition time pulse sequences [13]; the classic (CMB) and anaplastic medulloblastomas show commonly hyperintense signal on T2W images; DMB and MB–EN show commonly iso- to hyperintense signal on T2W images (Fig. 15.8) [14].

Fig. 15.8

Variability of signal intensity on T2-weighted images: (a) axial T2-weighted image shows a nearly heterogeneous hyperintense mass in the posterior fossa midline. (b) On an axial T2-weighted MR image, the mass is hypo-hysointense compared with the surrounding normal cerebellum with perilesional edema and well-defined margins. (c) Desmoplastic medulloblastoma: on an axial T2-weighted image, the mass, situated off-midline shows solid and fluid components, with iso-hyperintense signal of solid portion compared with surrounding normal brain tissue. (d) Medulloblastoma with extensive nodularity: on an axial T2-weighted image, the mass is heterogeneous hysointense with nodular aspect surrounding of vasogenic edema

On postcontrast images, majority of CMB and medulloblastoma variants have a marked enhancement; in CMB, subtle, marginal, or only linear enhancement is also possible.

Large cell (LC) and anaplastic variants show often inhomogeneous but marked enhancement, with or without necrosis and cystic parts; DMB will show a wide spectrum of enhancement patterns, inhomogeneous, marked homogeneous, nodular appearance [14].

The typical appearance of the MB–EN variant is strongly, multifocal, homogeneously enhancing, grape-like tumor nodules, and it is possible depict a central “scar-like” enhancement; the degree of uptake better than that seen in CM may indicate the neuronal differentiation known to be present within this variant [15] (Fig. 15.9).

Fig. 15.9

Variability of contrast enhancement: (a) On an axial T1-weighted postcontrast image, medulloblastoma infant shows nearly homogeneous enhancement of the mass. (b, c) On an axial T1-weighted postcontrast image, classic medulloblastoma shows patchy (b) and linear (c) enhancement. (d) Desmoplastic medulloblastoma: on an axial T1-weighted postcontrast image, the mass shows marked homogeneous enhancement with nodular aspect; notice nodular enhancement at the level of the right tonsilla cerebellar. (e, f) Medulloblastoma with extensive nodularity: on axial T1-weighted postcontrast images, the mass shows multifocal, heterogeneous enhancing, grape-like tumor nodules, and it is possible to depict a central “scar-like” enhancement

At the time of the diagnosis, MBs are commonly associated with hydrocephalus.

Sometimes, herniation of cerebellar tonsils resulting from the tumors is observed.

In majority of the cases, findings consistent with peritumoral white matter edema are seen; typically, the patients with MBs had mild to moderate perifocal edema; DMB can show marked edema.

Intratumoural cystic areas are sometimes depicted in CMB and MB variants; calcifications and hemorrhagic component are not common features in MB variants but can be occasionally seen in CMB (Fig. 15.10).

Fig. 15.10

Hemorrhagic medulloblastoma: (a) axial T2-weighted image. (b) Axial T2-GRE* image. (c) Axial T1-weighted image

However, conventional MRI has low sensibility and specificity in identifying specific medulloblastoma types and often cannot reliably differentiate between high-grade and low-grade tumors. Histopathologic evaluation of brain biopsies still remains the gold standard for definitive diagnosis.

Diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) maps, proton magnetic resonance spectroscopy (MRS), and perfusion imaging offer additional informations that can be helpful to determining tumor type and grade.

1H-MRS and DWI provide bio- and physicochemical information that cannot be obtained with conventional imaging techniques alone. Thus, an approach combining both spectroscopic and diffusion imaging techniques has become obvious in the field brain tumors.

Several reports have shown that ADC values could be used to differentiate some tumors or evaluate their cellularity.

The use of short echo time MRS, still a more demanding technique in terms of analysis than long echo time spectroscopy, allows the detection of metabolites which have been suggested useful in the diagnosis of specific tumor histology. Short TE single-voxel 1H-MRS combined with DWI fully discriminates the most frequent posterior fossa tumors in children.

DWI is a technique in which dedicated phase-defocusing and phase-refocusing gradients allows evaluation of the rate of microscopic water diffusion within tissues, where calculated ADC maps represent an absolute measure of average diffusion for each voxel [16].

It is used to distinguish necrosis from cyst formation or edema and has shown some efficiency in identifying different tumor types and delimiting their boundaries against normal cerebral tissue. DWI is helpful in distinguishing common pediatric cerebellar tumors on the basis of these tumors’ different ADC values. Low tumor ADC values compared with normal brain parenchyma have been linked to hypercellularity of the tumors. The high cellularity of medulloblastoma is a well-known histologic feature of these tumors [17, 18]. Medulloblastoma characteristically shows patternless sheets of small cells with small areas of necrosis.

There is relative restriction to the random movement of the water molecules in the small cells of medulloblastoma. The densely cellular nature of medulloblastoma and the high nuclear–to–cytoplasm ratio lead to restriction of the water diffusion, resulting in high signal on trace DWI and low ADC values (Fig. 15.11) [19]. Based on limited results, MB–EN and DMB may have a lower ADC compared to classic or LC/A variants. Studies with DWI measurements in larger numbers of patients with different MB variants are necessary to verify this and analyze further the value of DWI in distinguishing the variants of MB.

Fig. 15.11

(a, b) Diffusion-weighted MR images show extremely high intensity in the tumor. (c, d) On the axial ADC map, the tumors are well demarcated and isoitense to normal-appearing cerebellum. This finding is associated with high cellularity and small extracellular space at histologic examination

MRS analyzes the metabolic activity and chemical composition of the tissue studied through several major components such as choline-containing compounds (Cho), creatine plus phosphocreatine (Cr), N-acetylaspartate (NAA), and lactate (Lac).

Previous reports have demonstrated the ability of MRS to improve differentiation among tumors or even to identify specific histological tumor types.

These results have mainly been acquired at long echo time (TE 130 ms or longer), and only recently, MRS data obtained at short echo time (with a TE of 35 ms or less) have been published.

Most studies published have used long echo times (TE) and have focused on evaluating abnormalities of N-acetylaspartate (NAA), total choline (tCho), and lactate (Lac) by analysis of ratios relative to creatine (Cr). These studies have shown that brain tumors in general have elevated choline peaks, reduced N-acetyl aspartate and creatine peaks, and occasionally elevated lipid and lactic acid peaks, a characteristic spectrographic signature for a neuroectodermal tumor but not necessarily specific for medulloblastoma.

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