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Neuroradiology Case of the Week

Case 67

Karin Westesson and P-L Westesson MD, PhD, DDS

Clinical Presentation: Patient is a 39-year-old male presenting with a right parietal tumor. A MRI was performed to evaluate.

Radiological Findings:  A large extra-axial lesion was present in the right parietal area that exerted a mass effect on the underlying parietal lobe of the brain. On the T1-weighted images, the lesion was primarily hypointense with scattered areas of high-signal intensity. On T2-weighted images, the lesion showed a high-signal with areas of low-signal within the lesion itself. On the enhanced SPGR images, there was no contrast enhancement and the diffusion weighted image was bright. At the inferomedial aspect, the lesion appears to broken thought the underlying dura.

Diagnosis: Epidermoid tumor

Discussion:   An epidermoid tumor, also known as a primary cholesteatoma, is a relatively benign cyst located intradurally in the basal subarachnoid spaces off the midline [3]. It forms at the time of the neural tube closure (week 3 to 5 of embryonic life) when ectodermal epithelial elements are included and it causes abnormal migration of epiblast cells [1,2]. It can also arise during the formation of secondary cerebral vesicles, but it is less common [3].
    The epidermoid is a well-delineated cystic lesion that generally insinuates along CSF cisterns. It has an irregular lobulated outer surface that resembles cauliflower with a shiny mother-of-pearl look [3]. It is filled with a soft, waxy, flaky keratohyaline which comes from the desquamation of the cyst wall [3]. There are no hair follicles, sebaceous glands or fat, as there is in a dermoid cyst [1]. It will expand where physical resistance is the least, such as burrowing into the crevices on the brain surface and extending around the brain stem into the contra lateral cistern which gives it a variable shape and irregular surface [2].
    On a CT scan, an epidermoid has a similar intensity as CSF, but is not enhancing and sometimes will calcify (10-25%) [2,3]. On an MRI, it will appear homogeneous isodense to CSF on T1-weighted and hyperdense on the T2-weighted. They resemble meningiomas, chondromas in their signal intensity, but do not enhance after contrast [2]. The ADC value of an epidermoid is similar to brain parenchyma whereas arachnoids have an ADC value similar to that of water.
    Since epidermoid tumors grow at the rate of normally proliferating epithelial cells, clinical symptoms usually do not appear until the 2nd to 4th decade of life [3]. When symptoms do arise, they vary according to the shape and growth pattern of the lesion. Most patients complain of dizziness. Some have headaches, trigeminal neuralgia, facial neuralgia and sensor neural hearing loss [1]. If the tumor is more in the supratentorial regions, seizures and headache are more prevalent. Vestibulocochlear dysfunction is more likely if the tumor is in the posterior fossa [4]. The CSF protein level is normal unless there is an associated case of meningitis [2].
    Treatment depends on the size of the lesion. Small lesions are readily cured by complete surgical removal [1]. However, larger lesions, which tend to grow upward and surround vessels and nerves, there have been cases of supratentorial herniation, making it more difficult to completely remove the lesion [1]. It can not be controlled with diuretics, which arachnoids can [2].

References:

1. Harnsberger HR, Hudgins PA, Wiggens, RH, Davidson HC. In: Pocket Radiologist: Head and Neck. W. B. Saunders, 2002.
2. Lo, WWM, Solti-Bohman, LG. Tumors of the temporal bone and cerebellopontine angle. In: Head and Neck Imaging, 3rd ed. Eds. Peter M. Som and Hugh D. Curtain. St. Louis: Mosby, 1998; 1461-1468.
3. Osborn AG. Miscellaneous tumors, cysts, and metastases. In: Diagnostic Neuroradiology. St. Louis: Mosby, 1994; 631-635.
4. Robertson R, Caruso, PA, Truwit CL, Barkovich AJ. Disorders of brain development. In: Magnetic Resonance Imaging of the Brain and Spine, 3rd ed. Ed. Scott W. Atlas. Philadelphia: Lippincott, Williams, Wilkins Publishers, 2002; 295-296.