In this video, taken from our Brain CT Essentials course, you will learn how to recognize the most common adult extra-axial tumors in the posterior fossa—and avoid misdiagnosis—and why you can't use CT scans alone to do this.
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Magnetic resonance imaging is just better than CT when it comes to demonstration of masses and the posterior fossa. That's because of the artifacts that appear on head CT arising from the dense bones of the skull base, and dental implants or fillings. These artifacts will degrade the imaging frequently in non-intuitive ways.
The problem is quite evident in this patient with a known posterior fossa mass. These alternating white and dark lines are due to artifacts arising from the skull base and fillings in the teeth and they obscure the tumor. The three most common adult extra-axial tumors in the posterior fossa, that is the space below the tentorium, are the meningioma, vestibular schwannoma, and epidermoid.
Elsewhere on the brain meningioma remains the most common extra-axial tumor. Although you should keep in mind that can be hard to distinguish from metastatic tumors involving the dura or bone in patients with cancer. Arachnoid cysts are not strictly tumors but can have mass effect like solid masses.
This magnetic resonance image demonstrates a small vestibular schwannoma. These are histologically benign nerve sheath tumors arising from the eighth cranial nerve. These tend to occur initially within the internal auditory canal as shown here. When they grow larger, they can extend into the adjacent cistern, and compress the brainstem while the tumor is still evident in the internal auditory canal.
This patient's magnetic resonance scan demonstrates an extra-axial mass that is largely on the patient's right, but crosses the midline and does not involve the internal auditory canal. These findings argue against the diagnosis of a vestibular schwannoma and this proved to be a meningioma.
Keep in mind that all meningiomas and schwannomas enhance on magnetic resonance imaging, but can have cystic areas that do not enhance, This patient has a left sided posterior fossa mass that displaces the middle cerebellar peduncle. This mass did not enhance so meningioma and vestibular schwannomas are not considerations.
While the mass resembles cerebrospinal fluid on this MRI image, both arachnoid cysts and epidermoid tumors can resemble fluid on both CT and MR. In this case, the MR diffusion scan established the diagnosis of an epidermoid tumor, since an arachnoid cyst should be dark, and resemble CSF elsewhere on the image.
This patient chose a different CP angle mass at the level of the internal auditory canal, seen here on the T1 weighted MR image. But in this case, the mass on this diffusion weighted image has the same signal as CSF in the fourth ventricle. This proved to be an arachnoid cyst, which is simply a collection of cerebrospinal fluid that usually does not require treatment.
Usually the combination of CT and MR and the use of intravenous contrast will allow you to correctly predict the nature of posterior fossa extra-axial masses. Consider this patient who presents to the emergency room with dizziness. His CT scan demonstrated a low attenuation mass but poorly seen on the CT scan.
But you know it's there by the loss of the fourth ventricular contour. Based on the CT scan alone, this mass could be intra or extra-axial in location. The T1 weighted MR scan of this patient demonstrated that the lesion has a sharp border with the brain. |On the mass appears to be surrounded by brain, you now know to be skeptical of this observation when based on a single view.
That's especially true with masses at the periphery of the brain and at the midline. In contrast, enhanced MR imaging have demonstrated that this lesion did not enhance at all. That observation makes meningioma and schwannoma very unlikely, since both usually enhance with intravenous contrast. This midline mass has very high signal intensity on this diffusion weighted MR image.
When you consider that the lesion does not enhance, its location in the midline, and it's high signal intensity on diffusion weighted imaging, even though this is not a typical location for it, you can conclude that this tumor is an extra-axial epidermoid. 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 possibilities considerably.