Introduction

Supratentorial primitive neuroectodermal tumours(PNETs) belong to an inhomogeneous group of embry-onal, largely undifferentiated tumours generally occur-ring in children, and including medulloblastoma, neu-roblastoma, pineoblastoma, ependymoblastoma andmedulloepithelioma [1]. The designation PNET is con-troversial [2, 3]. Medulloblastoma [15±25 % of primarycentral nervous system (CNS) tumours in children] isthe prototype of infratentorial PNET, whereas cerebralneuroblastoma is the prototype of supratentorialPNETs. Supratentorial PNETs are rare, accounting forless than 1 % of primary CNS tumours. More than 50%are manifest in the first 5 years of life [4, 5]. Supra-tentorial PNETs are typically bulky hemisphere masseswhich appear sharply circumscribed. They are typicallysituated deep within the frontal or parietal lobes, ad-

jacent to the lateral ventricle and frequently show hae-morrhage, necrosis, calcification and cysts but only mildoedema. They are thought to originate from primitive orundifferentiated neuroepithelial cells with the capacityto differentiate into glial or neuronal lines. They aredensely cellular with numerous mitoses and frequentlyshow regions of cell clusters around a fibrinoid matrix(Homer-Wright rosettes) [6, 7]. On immunohistologicalexamination they may be immunoreactive for synapto-physin, neuron-specific enolase or glial fibrillary acidprotein. On MRI the tumours give inhomogeneouslylow signal on T1-weighted images, usually showingstrong contrast enhancement. Some areas reveal highsignal on both T 1- and T 2-weighted images, suggestingmethaemoglobin [8, 9]. PNETs frequently seed withinthe CNS via the cerebrospinal fluid (CSF) and may bemulticentric at the time of diagnosis. Because of me-ningeal spread and subarachnoid seeding, the spine

Neuroradiology (2000) 42: 393±398Ó Springer-Verlag 2000 PAEDIATRIC NEURORADIOLOGY

J. KlischH. HusstedtS. HenningsV. v.VelthovenA. PagenstecherM. Schumacher

Supratentorial primitive neuroectodermaltumours: diffusion-weighted MRI

Received: 27 January 1999Accepted: 3 September 1999

J. Klisch ()) × H. Husstedt × S. Hennings ×M. SchumacherDepartment of Neuroradiology,University of Freiburg,Breisacher Strasse 64, 79106 Freiburg,Germanye-mail: Klisch@nz11.ukl.uni-freiburg.de,Tel.: + 49-761-2 705180,Fax: + 49-7 61-270 5195

V.v.VelthovenDepartment of Neurosurgery,University of Freiburg, Freiburg, Germany

A. PagenstecherDepartment of Neuropathology,University of Freiburg, Freiburg, Germany

Abstract We report the clinical andpathological findings of supra-tentorial primitive neuroectodermaltumours (PNETs). These are rare,poorly differentiated, highly malig-nant neoplasms occurring primarilyin young individuals. They fre-quently show dissemination to thespinal cord and sometimes also be-yond neuraxis. Preoperative radi-ological diagnosis is difficult, due tothe nonspecific CT and MRI char-acteristics. Our findings indicatethat diffusion-weighted imaging(DWI) can be used to show the solidportion of the tumour pre-operatively and to monitor post-surgical recovery. We describe theMRI findings in three patients with

histologically confirmed supra-tentorial PNET, focussing on therole of DWI for improving the spe-cificity of radiological diagnosis.

Key words Primitiveneuroectodermal tumours × Brain,neoplasms × Magnetic resonanceimaging, diffusion weighted

should be screened [10±12]. Despite hyperfractionatedcraniospinal radiation and adjuvant chemotherapy fol-lowing surgery, the prognosis is poor, due to CSF dis-semination and local recurrence [13, 14]. To determinethe value of diffusion-weighted imaging (DWI) in clin-ical practice we used it as a screening method in morethan 100 patients with brain tumours from 1997. High-speed multislice DWI was performed in two planes. Wethink two measurements are sufficient in most clinical

settings to show areas of low diffusion due to anisotropy,which is not present in all directions, while isotropicdiffusion is evident in all three acquisitions.

Case reports

Case 1

A 7-year-old girl had a 6-week history of occipital headaches, pro-gressive nausea and vomiting. Examination revealed a left hemi-paresis, papilloedema and a left homonymous hemianopia. MRI(Fig.1a±e) showed a temporal and parietal, inhomogeneously en-hancing, solid mass. The most intensely enhancing, solid part of thetumour gave moderately intense signal on T2-weighted images(Fig.1d), while most apparently parts also gave high signal on T1-weighting. DWI revealed strong signal in the solid tumour, whereasthe cystic components gave low signal (Table 1). There was no evi-dence of metastatic lesions. The patient underwent surgery; histol-ogy confirmed an ependymoblastoma (PNET; WHO grade IV).

Case 2

A 2-year-old boy had a 2-month history of progressive left tremor,nausea and vomiting. Examination showed a mild left hemiparesis.MRI revealed a right temporal mass with inhomogeneous contrast

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Fig.1 a T1-weighted spin-echo (SE) (TR/TE 570/14 ms, field ofview (FoV) 173 � 230 mm, acquisition time (TA) 1 min 53 s, matrix192 � 256) image: different signal intensities in the mass (high inthe central cystic part, low in the solid part). b The solid portionshows contrast enhancement, and c extends to the base of thetemporal lobe. d Sagittal T2-weighted fast spin-echo (TR/TE4500/120 ms, FoV 220 � 220 mm, TA 3 min 40s, matrix 256 � 256)image: the solid portion gives lower signal than the cystic portion.e Coronal contrast-enhanced image shows extensive midline shift.f Coronal diffusion-weighted imaging (DWI) (echo-planar (EPI)spin-echo multislice, TR/TE 0.8/123 ms, flip angle 90�, b 1090 s/mm2, FoV 240 � 240 mm, TA 5.66 s, matrix 128 � 200, 20 slices,5 mm thick): high signal in the solid basaltemporal portion of thetumour

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Table 1 Histological and imaging data

Case Histology Site MRI characteristics

Inhomo-geneity

Bloodproducts

Cysts Signal intensity

T 1-weighted

Contrastenhance-ment

T2-weighted Diffusion-weighted

Solid Cysts Solid Cysts

1 Ependymo-blastomagrade IV

Temporo-parietal

Marked ++ Yes Isointense ++ Low High High Low

2 Neurobla-stomagrade IV

Temporallentiformnucleus

Marked ++ Yes Isointense ++ Slightlyhigh

High High Isointense

3 Neurobla-stomagrade IV

Temporal Marked No No Isointense Tumour ++Resectionmargin �

Low ± High ±

Fig.2 a T1-weighted SE (TR/TE 570/14 ms, FoV 201 � 230 mm,TA 2 min 11 s, matrix 224 � 256) image: differing signal intensities(high in the central cystic part, low in the solid part). b Contrastenhancement of the solid portion. c T2-weighted fast SE (TR/TE4500/120 ms, FoV 201 � 230 mm, TA 3 min 22 s, matrix 330 � 512)image: the solid component gives lower signal than the cystic por-

tion. d On a coronal section, the enhancing mass extends to thebase of the temporal lobe. e, f Coronal and axial DWI (EPI spin-echo multislice, TR/TE 0.8/123 ms, Flip angle 90�, b 1090 s/mm2,FoV 240 � 240 mm, TA 4.62 s, matrix 128 � 200, 20 slices, 5 mmthick): high signal from the solid component of the tumour

enhancement, infiltrating the lentiform nucleus (Fig. 2a±d). Themost strongly enhancing portion gave low to moderately high sig-nal on T2-weighted images; most apparently cystic areas also gavehigh signal in T 1-weighting, suggesting methaemoglobin. DWI(Fig.2e, f) revealed high signal in the solid, enhancing tumour; thecystic components gave high or low signal (Table 1). There was noevidence of metastatic lesions. Histology revealed a neuro-blastoma (PNET; WHO grade IV).

Case 3

A 9-year-old girl had a 3-week history of progressive headache, aleft hemiparesis occurring within 2 days. Rapid deterioration, withmydriasis and stupor, indicated urgent neurosurgical treatment

without preoperative MRI, but CT (Fig.3a) revealed a contrast-enhancing right temporal tumour in contact with the trigone. Re-section was incomplete, and histological examination revealed aneuroblastoma (PNET; WHO grade IV) (Fig.4). The patient un-derwent chemotherapy and radiation. MRI, including DWI, wasperformed within 24 h of surgery to define the residual tumour(Fig.3b±d). On T1-weighted contrast-enhanced images, the re-maining tumour showed strong enhancement in the region of thetrigone and the dorsal margin of the resection, with high signal onDWI. On DWI, both margins of the resection and the residual tu-mour gave high signal (Fig.3e, f).

Discussion

Although there are numerous publications about theimaging characteristics of supratentorial PNETs usingstandard pulse sequences, there have been none on theuse of DWI for diagnosis of these tumours. Our casessuggest that DWI could be used for showing the solidportion of the tumour preoperatively and for follow-up.

The solid components of supratentorial PNETs havethe following MRI features: low signal on T 1-weightedimages; strong contrast enhancement; low signal on T 2-weighted images; and high signal on DWI (Table 1).

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Fig.3 a Contrast-enhanced CT bulky right hemisphere tumour incontact with the trigone and compressing the lateral ventricle.d T2-weighted fast SE (TR/TE 4500/120 ms, FoV 201 � 230 mm,TA 3 min 22 s, matrix 330 � 512) image within of 24 h surgery: highsignal of oedema surrounding the resection. The solid tumour giveslow signal. b, c Subtracted SE (TR/TE 570/14 ms, FoV201 � 230 mm, TA 2 min 11 s, matrix 224 � 256) images: enhancingtumour in the dorsal and medial margins of the resection, near thetrigone. e, f DWI (acquisition parameters as in Fig.2): high signalin the tumour and resection margins

With increasing use of high-speed echo-planar ima-ging, DWI has become more widely used in clinicalpractice. The clinical utility and indications of DWI arestill being investigated and there are few reports of theuse of DWI for differential diagnosis of brain tumours[14, 15]. The advantages of DWI in showing epi-dermoids are well known [16]: the diffusion character-istics of CSF within arachnoid cysts differ considerablyfrom those of the solid and semisolid pearly matrix ofepidermoids. It is also claimed that DWI can also sup-port discrimination of brain abscess from necrotic orcystic tumours [17], assessment of traumatic brain injury[18], differentiation of dys- and demyelination [19] anddifferentiation between vasogenic and cytotoxic oede-ma in eclampsia [20].

The clinical course and MRI findings on standardsequences in our cases were in accordance with otherpublished cases [7, 10, 11]. Except for the rapid onsetand deterioration, the clinical presentation with in-

creased intracranial pressure was nonspecific. On T 1-weighted images supratentorial PNETs are usually iso-intense with a lower signal than cortical grey matter(Figs. 1 a, 2 a) and their solid components enhance [1].The signal from the solid component is much lower thanthat from the cystic parts, which are markedly in-homogeneous, ranging from low to high signal, even ifthere are tumour cysts containing extracellular blood ordeoxyhaemoglobin, methaemoglobin or haemosiderin[21]. As in the literature [4], we found little perifocaloedema on T 2-weighted images.

There was a strong DWI signal in the well-definedsolid portion, while the cystic portions gave weaker sig-nals. Although PNETs are apparently well circum-scribed on T1- and T 2-weighted images, they are in-filtrative tumours, with malignant cells extending be-yond the margins defined by conventional sequencesand by contrast enhancement. DWI was not helpful indefining the zone of infiltration more accurately. As inour case 3, acute haemorrhage could influence DWI,mimicking solid tumour. Imaging within 24 h of surgery(case 3) precisely differentiated tumour and post-operative parenchymal defect, by the use of T1-weigh-ted images before and after contrast medium and DWI,as in the other two cases: diffusion was reduced in thesolid portion near the trigone, which showed strongcontrast enhancement. The resection margin, contain-ing acute extracellular blood products, showed highDWI signal without contrast enhancement.

The pathophysiological background of the DWIfindings is still controversial. Although there are histo-logical differences between an ependymoblastoma and aneuroblastoma, both are highly malignant tumours thatcorrespond to WHO grade IV [1, 2]. Both showed highsignal on DWI, like epidermoids. The dense packing oftumour cells in PNETs may be responsible for the signalcharacteristics; similar DWI findings should be presentin tumours with a comparable density of tumour cells,e. g. lymphomas. It has been reported that the signal onDWI and apparent diffusion coefficient maps dependson cell density (e.g. lymphoma, medulloblastoma andadenomas), low extracellular water content and contrastenhancement of the tumour [21]. The high signal inPNETs, present within the intensely enhancing growthzone, can be explained mainly by the high cell densityand less by the tumour matrix itself. Epidermoid cystsare not nearly as cellular as PNETs. They are filled withmembrane-rich shed epithelium which might have thesame effect on the DWI as high cell density. These tu-mours often are immunopositive for synaptophysin andneuron-specific enolase [1] (Fig.4), but in our casesthese markers did not correlate with the findings onDWI. The small amount of tumour matrix and the poordifferentiation into neuronal lines are not specific histo-logical findings for PNETs and cannot be regarded asresponsible for the DWI findings.

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Fig.4a, b Patient 3. a A micrograph reveals a highly cellular tu-mour with abundant mitoses (arrowheads) and nuclear poly-morphism (haematoxylin and eosin, original magnification 40 �).b The tumour shows focal clusters of tumour cells immunoreactivefor the neuronal marker synaptophysin (arrowheads). Note strongnegative contrast of the blood vessels (original magnification 40 �)

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