250 CLINICAL IMAGING 1992;16:250-255

MRI SPECTRUM OF MEDULLOBLASTOMA

D.P. MUELLER. MD, S.A. MOORE, MD, PHD, Y. SATO, MD, AND W.T.C. YtiH, MD, MSEE

Medulloblastomas have been characterized as solid, homogeneously enhancing, midline masses of the posteriorfossa. To evaluate atypical MRIcharacteris- tics, we retrospectively reviewed 13 histologically proven medulloblastomas. MRI examinations were correlated with pathologic specimens and operative reports. Atypical features included cyst formation (77%), irregular enhancement (19%), extension through fourth ventricleforamina [15%), and presen- tation as a cerebellopontine angle mass (8%). We conclude that “atypical” characteristics are com- mon. In the child or young adult, medulloblastomas must be included in the differential diagnosis of any posterior fissa mass.

KEY woRns:

Medulloblastoma; Magnetic resonance imaging; Neoplasm; Brain

INTRODUCTION

Medulloblastomas have been characterized as solid, homogeneous, enhancing, mildine masses of the pos- terior fossa that may compress or extend into the fourth ventricle (l-3). Several investigators have de- scribed atypical characteristics that were identified during CT examination (1,4-6). Recently, variability in the MRI appearance of this tumor has been noted (7). This study was designed to evaluate the spectrum and frequency of atypical findings as identified by MRI examination.

From the Departments of Radiology (D.P.M., Y.S., W.T.C.Y.) and Pathology (S.A.M.), The University of Iowa Hospitals and Clinics, Iowa City, Iowa.

Address reprint requests to: W.T.C. Yuh, MD, MSEE, Depart- ment of Radiology, The University of Iowa Hospitals and Clinics, Iowa City, IA 52242.

Received March 2, 1992; accepted May 6, 1992. 0 19%’ by Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York, NY 10010 0899-7071/92/$5.00

MATERIALS AND METHODS

We retrospectively reviewed pathology records to identify proven cases of medulloblastoma. We corre- lated the radiologic records and identified 13 patients with preoperative MRI studies.

Nine patients were imaged on a 0.5-T Vista (Picker, Highland Heights, OH) unit. Three patients were im- aged on a 1.5-T Signa (General Electric, Milwaukee, WI) unit. One patient was imaged on a 1.5-T Vista (Picker, Highland Heights, OH) unit. Tl-weighted im- ages used a repetition time (TR) of 350-800 ms and an echo time (TE) of 16-26 ms. TZweighted images were obtained at a TR of 2000-2300 and a TE 80-100 ms. Contrast-enhanced MRI images were obtained immediately after an intravenous injection of gado- pentetate dimeglumine (0.1 mmol/kg) using the same parameters as those of the precontrast Tl-weighted examination.

MRI examinations, pathology reports, and surgical records were reviewed retrospectively. A neuropa- thologist (S.A.M) reviewed the specimens and pa- thology reports. The pathologic and surgical findings were then correlated with the preoperative MRI stud- ies. MRI examinations were evaluated for cyst forma- tion, tumor center, contrast enhancement, hemor- rhage, and extension through the foramina of Luschka and Magendie. Cysts were defined as sharply demarcated areas that have signal character- istics separate from the tumor mass that did not un- dergo contrast enhancement. Large cysts were de- fined as those equal to or greater than 10 mm in greatest diameter.

RESULTS

The MRI, and surgical and pathologic findings are summarized in Table 1. Four patients demonstrated a combination of large and small cysts (Figure l), four demonstrated only small cysts, and two demon- strated only large cysts. Large cysts were at the tumor

OCTOBER-DECEMBER 1992 MRI SPECTRUM OF MEDULLOBLASTOMA 251

TABLE 1. Summary of Patient Data and Findings

Case Age Sex Enhancementa Locationb cystsc Pathologyd Surgical report

1 5 M I V,FL S,P MC Extension into foramen of Luschka cyst 5 M D CH SC

L.P C 2

3 4

3 11

M D F NG

V V

N T

Vermis tumor Vermis tumor w

w L.C D CH 5 29 F N Center necrotic and

cystic Cysts, straw-colored

fluid Cerebellar hemisphere Vermis tumor Extension into foramen

of Luschka

6 11 F D V w SF

T

24 M D CH 6 M NG V 9 F I V

D N MC

7

8

9 S,P SC

M M

D CH D V,FM

S,P MC,N MC

10 11

20 2 Extension into foramen

of Magendie Tumor of CPA M D CPA LC

SF LP

MC,D 12 9

13 2 M D CH C,N cyst

a Enhancement: I = irregular enhancement; D = diffuse enhancement: NG = no contrast given. b Location: TUIIIO~ center-V = vermis; CH = cerebellar hemisphere; CPA = cerebellopontine angle cistern; Foraminal Extension-FL = foramen of

Luschka; FM = foramen of Magendie. ‘Cysts: S = small (< 1 cm); L = large (2 1 cm); P = peripheral; C = Central, ’ Pathology: MC = microcysts: C = cysts; N = necrosis; T = typical medulloblastoma; D = demoplasia.

periphery in four patients and were central in two. Four surgical reports mentioned cysts. The patho- logic specimens of 6 of these 10 patients demon- strated cystic or microcystic regions. Cyst size within the pathologic specimens ranged from less than 1 mm to approximately 5 mm. Cysts were identified in five of the seven patients with tumors centered in the cerebellar vermis. They were seen in five of the six patients with tumors centered in the cerebellar hemi- sphere or cerebellopontine angle cistern.

Cyst fluid signal was compared with that of cerebrospinal fluid (CSF). In 7 of these 10 patients, the cyst signal was slightly greater than CSF in both Tl- and T&weighted sequences (Figure 1) The cyst fluid of one of these patients was described surgi- cally as being “straw colored.” Only Tl-weighted images were available for one patient. The cyst signal was slightly greater than that of CSF. In one case, small cysts with a signal intensity similar to CSF during both Tl- and T&weighted sequences were identified. There was one case with increased signal from a large central cyst in both Tl- and T2- weighted sequences (Figure 2). The surgical report described the tumor as necrotic and the fluid as “brownish.” No hemosiderin was identified in the pathologic specimen.

Gadolinium was administered by intravenous in-

jection to 11 patients. Nine patients demonstrated diffuse enhancement, and two demonstrated nonho- mogeneous, predominantly peripheral enhancement (Figure 3).

Seven of 13 patients demonstrated a center within the vermis. The cerebellar hemisphere was the center in five. The average age of patients with midline tu- mors was 6.7 years. The average age of patients with hemispheric lesions was 16.0 years. One patient pre- sented with a cerebellopontine angle (CPA) mass (Figure 4). Surgically, the tumor was thought to be separate from the cerebellum. The pathologic speci- men demonstrated microcysts and desmoplasia.

We evaluated tumor extension through fourth ven- tricle foramina. There was one case of extension through the foramen of Magendie (Figure 5) into the cisterna magna and one case of extension through the foramen of Luschka (Figure 6) into the CPA and prepontine cisterns. Review of the surgical records identified an additional case in which extension into the foramen of Luschka was not identified preopera- tively.

DISCUSSION

Medulloblastomas are embryonic tumors of the cen- tral nervous system. A male predominance exists.

252 MUELLER ET AL. CLINICAL IMAGING VOL. 16, NO. 4

FIGURE 1. Case 4 (A) Coronal Tl-weighted MRI study demonstrates both large (straight arrow) and small (curved arrow) cysts. Cyst fluid signal is increased compared with that of CSF. (B) Axial T2-weighted MRI study demonstrates increased cyst fluid signal (arrow].

The median age at diagnosis is 9 years, and there is a rapid decline in incidence during the second and third decades (8). Medulloblastomas account for up to 25% of childhood intracranial tumors and are sec- ond in frequency to cerebellar astrocytomas in this age range (9).

Cystic appearing foci were common in our study. MRI examinations identified cysts in 10 of 13 pa- tients. The presence of cystic areas was confirmed by either pathologic examination or the surgical report in 9 of these 10. Cysts were common in both, tumors centered in the cerebellar vermis and those centered more laterally.

On Tl-weighted imaging, the signal of the cyst fluid was similar to or slightly greater than that of CSF in 9 of 10 patients. On T2-weighted imaging, the signal of the cyst fluid was always similar to or greater than that of CSF. One cyst demonstrated increased signal on both Tl- and T&weighted images. The cyst was large and central. The surgical report described

the tumor as necrotic and the fluid as “brownish.” Hemorrhage could result in an increased Tl signal. However, we feel that this is unlikely to be the source in this instance. The pathology specimen did not demonstrate hemosiderin and the surgical report does not mention hemorrhage. A possible alternative explanation would be increase in signal due to pro- teinaceous material within the cyst following tumor necrosis.

Our study indicates that cystic foci are not rare. Sandhu and Kendall (1) identified cysts or necrotic areas in 47% of their cases. In our study, 77% demon- strated cysts. The higher percentage may relate to the increased sensitivity of MRI when compared with CT in the detection of small posterior fossa cysts. Our study indicates that medulloblastomas must be in- cluded in the differential diagnosis of cystic cerebel- lar masses. It also demonstrates that tumor necrosis must be included in the differential diagrrosis of le- sions with increased signal on Tl-weighted images.

Tumor extension through fourth ventricle foram- ina has been associated with ependymomas (3, 10). MRI identified foraminal extension in two of our pa- tients: extension into the CPA cistern via the foramen of Luschka in one and into the cisterna magna via the foramen of Magendie in the other. In one additional case, extension through the foramen of Luschka,

OCTOBER-DECEMBER 1992 MRI SPECTRUM OF MEDULLOBLASTOMA 253

A FIGURE 2. Case 5. (A) Axial Tl-weighted MRI shows in- creased signal centrally (straight arrow). (B) Axial T2- weighted MRI. There is increased signal from the cyst fluid [straight arrow] compared with that of the CSF (curved arrow).

FIGURE 3. Case 1. Axial Tl-weighted MRI after intrave- nous administration of gadolinium. Contrast enhancement

identified surgically, was not appreciated preopera- tively. These findings emphasize the need to include medulloblastomas in the differential diagnosis of tumors exiting fourth ventricle foramina.

is irregular and predominantly peripheral.

CPA mass lesions usually are associated with me- ningiomas, acoustic neuromas, epidermoids, and me- tastases (3). In the child or young adult, medulloblas- toma must be included in this differential diagnosis. One of our patients presented with a CPA mass. Sur- gically, a plane was thought to be present between the tumor and the cerebellum. The pathologic specimen was characterized as desmoplastic. Rubinstein and Northfield (11) evaluated extra-axial medulloblasto- mas. It is their impression that these tumors arise from the cerebellar hemisphere. As these tumors in- vade the leptomeninges, they incite a desmoplastic connective tissue reaction.

Sandhu and Kendall (1) identified patchy en- hancement in 21% of CT examinations. This is simi- lar to the rate (18%) of irregular enhancement identi- fied in our series.

The tumor center was within the cerebellar hemi-

254 MUELLER ET AL. CLINICAL IMAGING VOL. 16, NO. 4

FIGURE 4. Case 12. Post contrast axial Tl-weighted MRI shows enhancing tumor in the left cerebellopontine angle cistern.

sphere in 5 of our 13 patients. The average age of these patients was 16.0 years, whereas the average age of patients with tumors centered in the cerebel- lar vermis was 6.7 years. These findings are consis- tent with previous reports indicating an increased

FIGURE 5. Case 11. Sagittal Tl-weighted MRI after intra- venous administration of gadolinium. Tumor extends through the foramen of Magendie into the cisterna magna.

FIGURE 6. Case 1. Axial proton density (TR 2300, TE 30) MRI shows tumor extension into the CPA cistern (arrow).

tendency for lateral localization of tumor with in- creasing age (9).

In conclusion, medulloblastomas can present in a wide variety of radiologic forms. They can assume characteristics that have been associated with cere- bellar astrocytomas, ependymomas, and even menin- giomas. Medulloblastomas must be considered in the differential diagnosis of any posterior fossa mass of the child or young adult.

REFERENCES 1. Sandhu A, Kendall B. Computed tomography in management

of medulloblastomas. Neuroradiology 1967;29:444.

2. Brant-Zawadzki M, Norman D. Magnetic Resonance Imaging of the Central Nervous Svstem. 1st edition. New York: Raven Press, 1967, p. 179-180.<

3. Rosenberg RN, Heinz ER. The Clinical Neurosciences, Vol 4, 1st edition. New York: Churchill Livingstone, 1984, pp 435-537.

4. Mahapatra A, Paul H, Sarkar C. Cystic medulloblastoma. Neur- oradiology 1989;31:369.

5. Zee C, Segall H. Less common CTfeatures of medulloblastoma. Radiology 1982;144:97.

6. Zimmerman R, Bilaniuk L. Spectrum of medulloblastomas demonstrated by computed tomography. Radiology 1978 126:137.

OCTOBER-DECEMBER 1992 MRI SPECTRUM OF MEDULLOBLASTOMA 255

7. Barkovich AJ. Pediatric Neuroimaging, 1st edition. New York: Raven Press, 1990, pp. 151-159.

8. Rosebud R, Lynch C, Jones M, Hart M. Medulloblastoma: A population-based study of 532 cases. J Neuropathol Exp Neu- rol 1991;50:134.

9. Russell DS, Rubinstein LJ. Pathology of Tumors of the Nervous System, 5th edition. Baltimore: Williams and Wilkin, 1989, pp 251-79.

10. Spoto G, Press G. Intracranial epdendymoma and subependy- moma: MR manifestations. AJNR 1990;11:83.

11. Rubinstein L, Northfield D. The medulloblastoma and the so- called “arachnoidal cerebellar sarcoma.” A critical re-exami- nation of a nosological problem. Brain 1964;87:379.

12. Rollins N, Mendelsohn D, Mulne A, Barton R, Diehl J, Reyes N, Sklar F. Recurrent medulloblastoma: Frequency of tumor enhancement on Gd-DTPA MR imaging. AJNR 1990;11:583.