Return to the Insights and Impressions home page Posterior Fossa Tumors Written by James J. Abrahams, MD Table of Contents Introduction Medulloblastoma and ependymoma Cystic astrocytoma and hemangioblastoma Brainstem glioma and brainstem infarct Acoustic neuroma and CP angle meningioma Epidermoid and arachnoid cyst Other lesions Figure Legend Introduction The purpose of this article is to present lesions of the posterior fossa in a simple, systematic, fashion that will allow the reader to more easily remember and differentiate the different tumors of this region. It is not intended to discuss, in detail, all of the characteristics of these lesions. How then, can we cluster lesions of the posterior fossa in a manner that will make it easier to remember? One can do this by dividing these lesions into couplets, in which two different lesions that have the same or similar radiographic appearance, are paired together. The couplets we will talk about will be as follows: Medulloblastoma and ependymoma Cystic astrocytoma and hemangioblastoma Brainstem glioma and brainstem infarct Acoustic neuroma and CP angle meningioma Epidermoid and arachnoid cyst Other lesions Medulloblastoma and Ependymoma The first couplet we will talk about will be the medulloblastoma and ependymoma. Radiographically, these two lesions can look exactly alike and can be difficult to differentiate. They both occur in the mid-line in the vicinity of the fourth ventricle; the meduloblastoma (Fig. 1, Fig. 2) from the inferior vermis and extending anteriorly to bulge into the ventricle and the ependymoma (Fig. 3) arising within the ventricle from the ependymal lining. The fourth ventricle is therefore effaced and barely seen if at all. Hydrocephalus frequently results from this obstruction. Radiographically, both lesions enhance. However, the medulloblastoma can be quite variable, being heterogeneous or not enhancing at all (Fig. 2b,2c). Ependymomas are reported to have calcifications, but I have not found this to be terribly common or helpful. Both lesions are bright on T2 weighted MRI. Many signs have been described to help differentiate these two lesions, however, I personally have found only one to be useful. This deals with the fact that the ependymoma grows within the ventricle and may extend inferiorly to exit the foramen of Magendie and Luschka. This is easily seen on the sagittal MR image (Fig. 3) and can be remembered, because the ependymoma arises from the epenmdymal lining and is more truly an intraventriclular lesion. If this sign is present, it highly suggests the lesion is an ependymoma. So, if this sign is absent, how else can we tell the two lesions apart? The answer is, if you can't tell them apart, call it a medulloblastoma! Why is this? Because statistically, medulloblastomas occur more frequently and this is why I got in the habit of listing medulloblastoma first when I list these two lesions. If you always do this, you'll remember that medulloblastoma is more common. Both lesions tend to be lesions of childhood and they also share another common feature that is extremely important to remember. They both spread or metastasize in the CSF and drop metastases to the spine (Fig. 4c), cisterns (Fig. 2c, Fig. 4a) and sulci (Fig. 4b) are therefore common. Any patient with one of these lesions also requires an MR of the spine to look for meningeal enhancement or drop metastases. After surgical resection, these patients will often return for follow-up imaging every 6 or 12 months. When they return, it is extremely important to look not only at the primary sight for recurrence, but also to look at the CSF spaces, including the cisterns and sulci, for evidence of drop metastases as illustrated in Figure 4. Key Points: 4th ventricular region tumors Medulloblastoma is more common Ependymoma grows down foramen of Magendie and Luschka Cystic Astrocytoma and Hemangioblastoma Both a cystic astrocytoma (polycystic astrocytoma) and hemangioblastoma can also appear identical radiographically. Typically, they appear as relatively large cysts in the posterior fossa with enhancing mural nodules (Fig. 5, Fig. 6). The enhancing nodule is the actual tumor rather than the cyst and treatment is directed toward removal of the nodule. Since these are benign lesions, successful removal of the nodule is usually curative. Occasionally tumor may spread in the wall of the cyst, suggesting a higher grade or more aggressive tumor (Fig. 7). In this situation more extensive surgery with removal of the cyst is required. It is important to observe and look closely at the cyst wall. In Figure 5 and Figure 6, what do you notice about the wall? Is it enhancing or not? It's not. This is an extremely important feature to note, because both of these lesions typically do not have enhancement of the cyst wall. If enhancement is present, then the differential diagnosis needs to be expanded to include more aggressive and malignant lesions such as metastases (Fig. 8) and glioblastoma. This principle also applies to other cystic lesions. In general, if cyst wall enhancement is absent, then think of benign lesions such as cystic meningioma, cystic astrocytoma, hemangioblastoma, etc. If the wall enhances, then think of more aggressive lesions such as glioblastoma, metastasis, abscess, or higher grade cystic astrocytoma. Although cystic astrocytomas do not usually demonstrate cyst wall enhancement, they occasionally can (Fig. 7). When present, this suggests a higher grade and more aggressive lesion. The surgeon should be warned of this, because the nodule, as well as the cyst and cyst wall, will then require resection. Hemangioblastoma, in addition to presenting as a cyst with a nodule, can also occasionally (40%) present as a relatively solid enhancing lesion (Fig. 9). Also, remember that 10-20% of hemangioblastomas can be associated with von Hippel-Lindau syndrome. So if these two lesions can look radiographically identical, how do we differentiate them? It's simple: the patient's age. Cystic astrocytomas occur in children and hemangioblastomas occur in adults. So when you see a cystic lesion of the posterior fossa with an enhancing nodule simply look at the age of the patient to figure out which of the two lesions it is. One other feature that may sometimes help differentiate the two lesions is the fact that the hemangioblastoma often abuts a pial surface (Fig. 6). Key Points: Cyst with enhancing nodule Child = cystic astrocytoma Adult = Hemangioblastoma If cyst wall enhancement, expand differential to include metastasis, GBM, abscess, or more aggressive cystic astrocytoma. Brainstem Glioma and Brainstem Infarct Most people have a fairly good concept of what an infarct looks like radiographically. In the brainstem it will look like a non-enhancing area of low attenuation with mass effect (Fig. 10). In the subacute phase, however, there may be enhancement. The radiographic features of a brainstem glioma or pontine glioma tend to be more difficult for people to remember, so simply remember that it looks just like an infarct! It's a low attenuating mass of the pons which does not enhance (Fig. 11a). This later feature, the lack of enhancement, is quite important for diagnosing this lesion, because most other lesions enhance. Prior to development of MRI, these lesions were very hard to see on CT because of their lack of enhancement. On MRI, these lesions are now easily seen because of their high signal on T2 and proton density images (Fig. 11b) and the ability to obtain sagittal images (Fig. 11c) So, if these two lesions can look identical radiographically, how do we differentiate them? Simple: clinically! Brainstem infarcts will present with an acute event while the brainstem glioma will present with gradual onset of symptoms. Also the brainstem glioma occurs in children while the infarct typically occurs in adults. Key Points Low attenuation without enhancement Brainstem glioma: child; gradual onset Brainstem infarct: adult; acute onset Acoustic Neuroma and CP Angle Meningioma Again, both of these benign lesions can look radiographically identical (Fig. 12a, Fig. 13a). They both enhance with contrast, and are extraaxial in the cerebellapontine (CP) angle. The key differentiating feature is that the acoustic neuroma grows into the internal auditory canal and often widens it by eroding bone (Fig. 12b), while the meningioma (Fig. 13b) tends not to do this. Meningiomas, however, may occasionally demonstrate hyperostosis which can help in the differential. Also, calcifications are more likely to be seen with meningiomas. Key Point: Acoustic neuroma widens acoustic meatus Arachnoid and Epidermoid Cysts Both of these extra axial lesions, on CT, have the same attenuation as CSF and do not enhance (Fig. 14a, Fig. 15). MRI can be helpful in differentiating them, because the epidermoid, which looks like CSF on T1WI and T2WI (Fig. 14b), often looks brighter than CSF on the proton density image (Fig. 14c). Also, diffusion weighted images are helpful because the epidermoid turns bright. In addition, the margins of the arachnoid cyst are smooth, while those of the epidermoid have a cauliflower-like appearance with crevices extending into the lesion. If intrathecal contrast is given, this may become apparent and can help differentiate the two lesions. Key Points: Lesions look like CSF, but epidermoid brighter than CSF on Protein Density Epidermoid: "Cauliflower" margin Arachnoid cyst: Smooth margin Other Lesions In addition to these couplets, it is useful to think of other lesions, such as meningiomas, which can arise from the tentorium and sometimes have the appearance of an intraaxial lesion . The coronal image is quite helpful in recognizing its origin from the tentorium. When there is a destructive lesion of the clivus, think about chordoma and metastases. When it's not destructive, think of meningioma, basilar tip aneurysm, etc. Other lesions to keep in mind for the posterior fossa include infection, abscesses, vascular malformations and bleeds, and infarct, which can enhance in the subacute phase and have somewhat bizarre appearances. In summary, the goal of this paper was not to provide a detailed description of all the characteristics of every lesion of the posterior fossa, but rather to present these lesions in a systematic fashion which will help the reader remember the lesions and more easily develop a differential diagnosis. Figure Legends Fig. 1. Medulloblastoma in 27 year-old female. Sagittal T1WI (a) and axial T2WI (b), shows 4th ventricle completely effaced by medulloblastoma (arrows). Figure 2. Medulloblastoma effacing 4th ventricle. Axial unenhanced CT (a) shows lesion effacing 4th ventricle (arrows). Axial CT at same level (b) as "a" shows heterogeneous contrast enhancement. Axial enhanced CT (c) again shows lesion in region of 4th ventricle. Note dilated temporal horns from obstructing hydrocephalus and tumor spread in CSF of suprasella cistern (arrow). Fig. 3. Sagittal enhanced T1W image shows ependymoma in 4th ventricle growing out foramen of Magendie (arrows). Figure 4. Follow-up scan after surgical removal of medulloblastoma shows recurrent tumor spread in CSF. Axial enhanced CT (a) shows encephalomalacia in region of tumor removal (thick arrow) and recurrent spread of enhancing tumor in the interpeduncular cistern (thin arrow). Coronal T1W enhanced image (b) shows enhancement in sulci from tumor spread (arrows). Sagittal T1W enhanced image (c) of cervical spine shows meningeal enhancement from drop metastases (arrows). Fig. 5. Axial T1W enhanced MR image of 6-year-old female with cystic astrocytoma shows enhancing nodule and cyst with walls that lack enhancement. Note dilatation of temporal horns and third ventricle from obstructive hydrocephalus. Fig. 6). Axial enhanced CT image of 59-year-old female with hemangioblastoma shows enhancing nodule (thick arrow) adjacent to pial surface and cyst with walls that lack enhancement (thin arrows). Fig. 7). Axial enhanced T1W MR image of 3-year-old male with cystic astrocytoma. Note enhancement of cyst wall which suggests a more aggressive tumor. Fig. 8. Axial enhanced T1W MR image of 35-year-old male with gastric metastasis. Note rim enhances unlike the benign cystic astrocytoma and hemangioblastoma seen in figures 5 & 6 respectively. Fig. 9. Axial enhanced T1W MR image of 35-year-old male with solid hemangioblastoma. Fig. 10. Axial enhanced CT image of 48-year-old male shows low attenuation in the brain stem (arrow) with mild mass effect from an infarct. Note lack of enhancement in acute phase. Figure 11. 8-year old male with pontine glioma. Axial enhanced CT image (a) shows effacement and posterior displacement of 4th ventricle (arrow) from mass in pons. Note lack of enhancement. Axial T2W MR image (b) shows high signal of lesion in pons. Sagittal T1W MR image (c) shows enlargement and low signal of pons from lesion which displaces 4th ventricle posteriorly. Figure 12. Acoustic neuroma widening internal auditory canal. Axial enhanced CT scan (a) shows enhancing lesion in cerebellapontine angle (CPA). Axial CT scan with bone window (b) shows widening of internal auditory canal (arrows). Figure 13. Meningioma of cerebellapontine angle cistern. Axial enhanced CT scan (a) shows enhancing lesion. Axial CT scan with bone window (b) does not show widening of internal auditory canal. Note lesion is centered slightly posterior to canal. Figure 14. Epidermoid. Axial CT scan (a) shows a low attenuating lesion (thin arrow) in CPA cistern which has same attenuation as 4th ventricle CSF (thick arrow). Axial T2W MR image (b) shows high signal of lesion. Axial proton density MR image (c) shows lesion to have higher signal than CSF. Figure 15. Axial CT scan shows arachnoid cyst of anterior cranial fossa (thick arrow). Note CSF in ventricle (thin arrow) has same attenuation as cyst. Figure 16. 74-year-old female with meningioma of posterior fossa. Axial enhanced CT image (a) shows uniformly enhanced lesion which has appearance of being intraaxial. Axial enhanced CT image cephalad to "a" (b), shows that lesion is growing off tentorium and is actually extra axial. Figure 17. 31-year-old male with chordoma of clivus. Axial enhanced CT (a) shows destruction of clivus (bold arrow) and growth of soft tissue tumor into sphenoid sinus (thin arrow). Sagittal T1W MR image (b) shows normal bright signal of clivus to be replaced by lower signal tumor which has also extended into sphenoid sinus. Note pituitary gland is normal excluding this as a cause for this lesion. Sagittal enhanced T1W MR image (c) shows enhancement of tumor in clivus. © 2000, RadiologyWeb