Let’s examine several cases which will demonstrate how to distinguish between the four common types of extra-axial masses in adults using brain computed tomography (CT) and magnetic resonance imaging (MRI):
- Vestibular schwannoma
- Epidermoid tumor
- Arachnoid cyst
Vestibular schwannomas, meningiomas, and epidermoid tumors are the most common extra-axial tumors found in the posterior fossa (the space below the tentorium). Elsewhere in the brain, meningiomas remain the most common tumor. Keep in mind that meningiomas can be hard to distinguish from metastatic tumors in the dura or bone in patients with cancer.
Arachnoid cysts are not strictly tumors but can also have a mass effect like a solid tumor.
In our first case, the patient’s MRI and CT scans demonstrated a small vestibular schwannoma. These are histologically benign nerve sheath tumors that arise from the eighth cranial nerve and appear initially within the internal auditory canal.
When they grow larger, vestibular schwannomas can extend into the adjacent cistern and compress the brainstem while the tumor is still evident in the internal auditory canal.
In our second case, a patient’s MRI demonstrated an extra-axial mass that was largely on the right. The mass crosses the midline and doesn’t involve the internal auditory canal—findings that argue against the diagnosis of a vestibular schwannoma!
This mass proved to be a meningioma. All meningiomas and schwannomas enhance on MRI and both can have cystic areas that do not enhance.
In our third case, a patient had a left-sided extra-axial posterior fossa mass that can be seen displacing the middle cerebellar peduncle on MRI. This mass did not enhance, so a diagnosis of meningioma or vestibular schwannoma was not considered.
While the mass resembles cerebrospinal fluid (CSF) on this image, both arachnoid cysts and epidermoid tumors can also resemble CSF on both CT and MRI. In this case, the diffusion-weighted MRI established the diagnosis of an epidermoid tumor since an arachnoid cyst should be dark and resemble CSF.
Usually, the combination of CT and MRI (and the use of intravenous contrast) will allow you to correctly predict the nature of posterior fossa extra-axial tumors.
Consider our next patient who presented to the emergency room with dizziness. The CT scan demonstrated a low attenuation mass that was poorly seen on CT. But, based on the loss of the normal fourth ventricle contour, we know the tumor is there (Fig. 5). Based on this CT alone, the mass could be intra- or extra-axial in location.
The T1-weighted MRI of this patient demonstrated a sharp border between the tumor and the cerebellum. While the mass appears to be surrounded by brain tissue, be skeptical of this observation when based on a single view, especially with masses at the periphery of the brain and the midline.
Contrast-enhanced T1-weighted MRI shows that the mass does not enhance. This observation makes meningioma and vestibular schwannoma tumors unlikely since they both should enhance with intravenous contrast.
In the same patient, the midline mass has a very high signal intensity on the next diffusion-weighted MRI image. Considering the tumor’s lack of enhancement, the location of the tumor in the midline, and high signal intensity on diffusion-weighted MRI, you can conclude that this tumor is an extra-axial epidermoid tumor, even though it’s not in a typical location!
In our final case, the patient showed a cerebellopontine angle (CPA) mass at the level of the internal auditory canal on a T1-weighted MRI image. In this case, the mass on the diffusion-weighted MRI has the same signal as CSF. This is an arachnoid cyst—a collection of CSF that doesn’t usually require treatment.
While you will not be able to predict the type of every extra-axial mass, accurate localization of the mass to the extra-axial space will allow you to limit the possible diagnoses considerably.
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