Hi Arushi,
Here is some interesting data other than spiders---all I can come up with at the moment; I read online that Arachnoid cysts are benign space-occupying lesions containing CSF.......(do a check online surfing the Web and check with your surgical team for the proper answers to your questions)
Internal Auditory Canal and Cerebellopontine Angle
Acoustic Neuroma
Acoustic neuroma or vestibular schwannoma is the most common mass of the cerebellopontine angle. It typically presents in the middle to late decades of life, and is usually unilateral. Bilateral disease or disease early in life should lead to suspicion of neurofibromatosis, a diagnosis which has implications on the treatment of choice for these lesions. Acoustic neuromas are benign masses. These masses tend to grow slowly over several years, with growth rates commonly from 0.2 – 2 mm per year. However, some tumors have been found to progress at rates up to 10 mm per year. These lesions may also remain stable for multiple years with no signs of growth on long-term follow-up. The masses themselves occur with equal frequency on the superior and inferior vestibular nerves. Schwann cells are more numerous in the region of Scarpa’s ganglion, and as a result, more tumors are found near this location. This is also why AN, as opposed to several other CPA masses, almost always has an intracanalicular component. While the vestibular portion of the vestibulococlear nerve commonly is involved, the cochlear portion is rarely involved with tumor. However, when the schwannoma arises from this nerve, the lesion may extend into the cochlea, due to the glial-schwann junction occurring distally at the modiolus.
The radiologic features of acoustic neuroma have been well documented. The anatomic location of the tumor is usually centered about the porus acousticus. The tumors usually have an intracanalicular component, and may extend into the CPA. Other lesions may arise completely within the CPA, and have no extension into the IAC. These lesions may grow to a larger size before diagnosis due to their lack of compression of the nerves within the IAC. Smaller lesions are becoming more common as new techniques for imaging are being found. The range of size at diagnosis therefore varies from mere millimeters up to 6-8 centimeters. As the tumors grow they generally enlarge the porus of the IAC. Although the diameter of the IAC varies greatly from individual to individual, the diameter of the contralateral IAC has been found to be nearly equal in normal patients. Therefore, an enlargement of the porus greater the 2 mm in comparison to the contralateral side is a relatively good predictor of AN. Since the lesion commonly arises near the area of the porus and enlarges in a spherical fashion, it is considered a concentric mass. Therefore, as the tumor enlarges, it will have acute angles to the remainder of the petrous portion of the temporal bone. Masses such as meningiomas, which are eccentric and tend to spread along the petrous area will have obtuse angles. Other features that may favor AN in the differential are: lesions rarely extend anteriorly and superiorly, they almost never penetrate into the middle cranial fossa, and they lack prominent vasculature, as well as dural tails.
CT scans of acoustic schwannomas tend to show the above features: a porus-centered mass, acute angles, IAC involvement. They also demonstrate the homogeneous nature of the mass. The majority of lesions, excluding previously treated tumors and very large tumors, will show uniform density on CT. Calcifications and central necrosis are rare, however, central clearing has been noted in some larger lesions. The density of AN on CT is similar to that of nearby brainstem, and more dense than surrounding CSF. If given IV contrast, the tumor will most likely show homogeneous uptake and turn very bright. Again, non-homogeneous uptake may be seen with previously treated lesions and large tumors.
MRI is the study of choice if the diagnosis of AN is in question. The T1 weighted exam with Gadolinium contrast has been shown not only to be 100% sensitive for the diagnosis of AN, but also to have the highest negative predictive value for the lesion as well. On standard T1 images, the tumor should be relatively isointense to pons but more intense than CSF. On T2 images, the lesion should be mildly brighter than pons, but darker than CSF. After Gadolinium, the T1 sequence should show a very intense lesion, brighter than all other surrounding structures.
Palace
