|Year : 2012 | Volume
| Issue : 2 | Page : 98-102
Intradiploic epidermoid tumor of temporal bone X-ray, CT, MR Imaging
Shibani Mehra, Garga Umesh Chandra, Suresh Kumar
Department of Radiology, Dr. Ram Manohar Lohia Hospital and PGIMER, New Delhi, India
|Date of Web Publication||6-Sep-2012|
Department of Radiology, Dr. Ram Manohar Lohia Hospital and PGIMER, C1/36, Safdarjung Development Area, New Delhi-110016
Source of Support: None, Conflict of Interest: None
Intradiploic epidermoid tumors are extremely uncommon. About 200 cases have been reported in literature. These lesions are caused by inclusion of ectodermal cells in the bone tissue during neural tube closure. These tumors either remain asymptomatic or are incidentally detected or they may present as a palpable lump. They can erode the bone and involve the brain parenchyma due to their proximity to the brain. Radiological imaging is very helpful in accurate diagnosis of these lesions and in differentiating intradural from intradiploic varieties of epidermoids.
Keywords: CT, DWI, Epidermoid tumor, FLAIR, MR, T1W, T2W
|How to cite this article:|
Mehra S, Chandra GU, Kumar S. Intradiploic epidermoid tumor of temporal bone X-ray, CT, MR Imaging. Indian J Otol 2012;18:98-102
| Introduction|| |
Intracranial epidermoid tumors are benign, congenital tumors composed of epithelial cell remnants, rich in cholesterol. These rare tumors account for 0.2-1.8% of intracranial tumors.  Although, most intracranial epidermoids are intradural and seen mostly in the cerebellopontine angle, intradiploic intraosseous epidermoids have been known to occur. These intradiploic epidermoids are extremely rare tumors and have been found within the frontal, temporal and occipital bones in the skull.  Cushing was the first to describe the radiologic pattern of intradiploic epidermoids as areas of bone destruction with smooth sclerotic margins.  The possible complication of the intradiploic epidermoids is spontaneous rupture into the subarachnoid space causing aseptic meningitis due to the discharge of cholesterol contents, subsequently leading to chronic granulomatous arachnoiditis.  These tumors pose a diagnostic challenge and preoperative diagnosis is imperative both to plan surgery at an appropriate time and to avoid the development of complications. Imaging enables an accurate diagnosis of intradiploic epidermoids, with plain X-ray Skull, CT and MR, each playing an important role. We present the radiological findings in an intradiploic epidermoid of the temporal bone.
| Case Report|| |
A 21-year-old male presented with a tender lump in the Right mastoid region. The lump had been present since 8 years and had been gradually increasing in size. Dull pain and tenderness over the swelling had developed since the past 8 months. The patient also complained of hearing loss. Plain X-rays skull AP, Townes, Lateral view demonstrated a well defined, expansile lytic lesion with sclerotic margins in the right temporal bone [Figure 1]a-c.
|Figure 1: (a) X-ray skull AP view shows an expansile lesion in the right temporal bone. (b) X-ray skull lateral view shows the lesion is lytic. (c) X-ray skull Towne's view shows the lytic lesion in the temporal bone has sclerotic margins|
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Contrast enhanced CT was performed on Philips Brilliance Spiral Scanner and axial and multi-formatted images of the temporal bone and brain were obtained. There was a predominantly hypodense fluid attenuation, expansile, intradiploic lesion in the right temporal bone, mastoid region, involving the inner table of the diploic space with bony septations [Figure 2]a, b. The walls of the lesion were thick and revealed enhancement on contrast administration [Figure 2]c. No involvement of the middle ear ossicles was seen [Figure 2]d.
|Figure 2: (a), Axial CT image, bone window, shows intradiploic lesion involving the right temporal and mastoid region of the skull. (b) Coronal CT images of the skull on bone window settings demonstrate erosion and involvement of the inner table of the skull in the vicinity of the lesion. (c) Axial CT image, soft tissue settings shows a hypodense lesion in the diploic space of temporal mastoid region of the skull. (d) Axial CT image shows the lesion is clear of middle ear cavity and normal ossicles seen in the middle ear, and is intradiploic in location|
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MR imaging of brain was performed on the Siemens Somatom Balance 1.5 Tesla Scanner using phased array head coils. Imaging was done in axial, coronal and sagittal planes using T1-weighted, T2-weighted sequences. FLAIR, GRE and Echoplanar Diffusion-weighted images were also obtained. The MR images demonstrated a T1 hypointense, T2 hyperintense lesion in the right temporal bone [Figure 3]a, b. On T1-W image the signal intensity was greater than that of CSF. The lesion did not show signal suppression on fat saturated sequences ruling out presence of true fat or lipid [Figure 3]c. On FLAIR images, the lesion appeared hyperintense [Figure 4] and exhibited bright signal on diffusion weighted images [Figure 5]a, b. Rim of the lesion showed contrast enhancement after intravenous gadolinium administration [Figure 6]a, b.
|Figure 3: (a) T1-Weighted MR image shows a hypointense well marginated lesion in right temporal diploic space, with few areas of high signal intensity within it. (b) The lesion on T2-Weighted MR image appears hyperintense. (c) Fat saturated MR image shows suppression of high intensity areas of the T1 image but the non fatty major component does not show signal suppression|
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|Figure 4: FLAIR MR image demonstrates high signal intensity within the lesion indicating the fluid composition of the lesion|
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|Figure 5: (a) Diffusion weighted MR image shows hyperintense signal intensity if the entire lesion, which signifies that though the contents are of fluid composition, they are different from CSF, and therefore demonstrate Diffusion restriction. (b) ADC component of the Diffusion weighted image shows diffusion restriction in the lesion confirming the non CSF Contents of the cystic lesion|
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|Figure 6: (a) Post-gadolinium axial MR images show thick enhancing rim of the lesion, indicating that the lesion is infected. (b) Sagittal MR image after gadolinium shows enhancement of the margin denoting secondary infection.FG|
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The well defined marginal sclerosis and expansile nature on X-rays, the temporal bone intradiploic location along with the lack of middle ear involvement, the hypodense fluid attenuation on CT and the intradiploic location with intact middle ear cavity and ossicles with fluid intensity of the contents which was higher than that of true fat and that of CSF on MR, allowed the accurate radiological diagnosis of a temporal bone intradiploic epidermoid. DW and FLAIR images were especially helpful in diagnosing the lesion as epidermoid. The presence of thick rim which showed enhancement on CT and MR contrast administration indicates infection in the lesion. Histopathology on surgical resection confirmed the keratin contents and the squamous cell lined wall of the enucleated cyst.
| Discussion|| |
Epidermoids are not true tumors but are congenital inclusion cysts of ectodermal origin. These are composed of desquamated epithelial cells, keratin in concentric layers and cholesterol in solid crystalline state. These lesions can be seen anywhere along the neural axis with 90% of intracranial epidermoids being intradural, located at the cerebellopontine angle in 40%, at the fourth ventricle in 20% and in the middle cranial fossa in 15%; 10% of intracranial epidermoids are extradural, being encountered most often in the skull as intradiploic masses in the temporal, occipital, parietal and frontal bones.  They result from incomplete cleavage of the neural from the cutaneous ectoderm in the neural groove at the time of neural tube closure, along with retention of ectopic ectodermal cells, during 3 rd and 5 th week of gestation.  The intradiploic epidermoids result from trapping of ectodermal embryonal remnants within the cranial bones or rarely due to trauma resulting in invagination and implantation of epidermal cells into the bones.
An epidermoid that involves the middle ear and temporal bone is often called a cholesteatoma due to it is cholesterol content. Congenital cholesteatomas develop due to migration of epithelial cells during embryogenesis from the external auditory canal into the middle ear and present with conductive hearing loss. On imaging these are seen located in the antero-superior mesotympanum.
Epidermoids commonly present as a long standing painless scalp swelling at any time between the first to seventh decades. As result of their slow growth and benign nature, signs and symptoms develop in these congenital tumors only after years of growth or when the lesion ruptures. There is a slight male predilection. The symptoms include headache and focal epileptic seizures more frequently; cranial nerve deficits due to nerve compression, raised intracranial tension and cerebellar symptoms due to mass effect are seen less commonly.  Rupture, bleeding and vsuperinfection can complicate these lesions. The chronic inflammatory meningeal reaction due to their rupture can additionally lead to the development of trigeminal neuralgia and other neurologic deficits. Malignant transformation of epidermoids too has been reported.  Imaging is essential prior to any surgical intervention. Further, imaging helps in postoperative follow-up of these tumors as a recurrence rate of 0.3-0.25% has been reported after improper surgical enucleation.
The diagnosis is first suggested on plain skiagrams of the skull by identifying an expansile, lytic lesion with well defined sclerotic borders. They expand the inner and outer tables of the skull. Erosion of both inner and outer tables of skull is seen in 46% cases; with outer table involvement alone being seen in 36%, more frequently than the inner table involvement.  The sclerotic borders are characteristic of these benign lesions and differentiate them from eosinophilic granulomas which lack marginal sclerosis due to their aggressive and malignant nature.
Both CT and MRI are comparable modalities that enable the preoperative diagnosis of these tumors with good accuracy. However, CT imaging is of greater value than plain radiography as the level of bony detail seen on CT is better. CT allows better assessment of both skull involvement as well intracranial extensions. Intraparenchymal extension of intradiploic epidermoids is known to occur. The extent of involvement of the inner and outer tables, presence of table destruction and of calcification, the nature and density of the lesion, cystic, solid or otherwise, as well as the sclerotic margins are best exhibited on CT.  Imaging appearance depends on the ratio of keratin (protein) to cholesterol (lipid) in the lesion. Epidermoids are hypodense typically on CT. An attenuation ranging between -20 to +20 HU is seen commonly, due to their cholesterol content. When the cellular debris within the lesion is greater than the lipid content, the attenuation roughly parallels that of water or CSF and they appear similar to arachnoid cysts. Epidermoids lack enhancement on contrast administration. Crenated margins of the lesion when seen on CT images are characteristic of epidermoids.  Calcification is present in 25% epidermoids and easily detected on CT. Very rarely, wall enhancement may be seen in the presence of superadded infection.
MRI is superior to CT in demonstrating early lesions, which do not have associated bone destruction and also those lesions that have associated parenchymal involvement. MR due to it's multiplanar imaging capability helps to differentiate intradiploic extradural from intradural location of these lesions and delineates the anatomic boundaries for planning surgical resection very well.  On MR, epidermoids demonstrate long T1 and T2 relaxation and show intermediate signal intensity between that of brain and CSF on T1-weighted sequences and greater signal intensity than CSF on T2-weighted images thereby exhibiting T2 hyperintense signal consistently and appearing iso - hypointense to CSF on T1-Weighted and Proton Density images.  The T1 signal intensity depends on the lipid content of the lesion, and occasionally T1 hyperintense signal is identified when the lipid content within the cyst is high or due to predominance of keratin which is highly proteinaceous. Intracystic hemorrhage too results in increased T1 signal intensity. Internal heterogeneity is found in the center in 65% of lesions.  Epidermoids exhibit well-defined margins. The wall of the cyst is nonvascular and so mostly these tumors do not enhance on gadolinium administration. Enhancement of portions of the rim may occur when the cyst is superinfected. 
MR FLAIR images show hyperintense signal in epidermoids due to their cholesterol and desquamated epithelial debris content. Flair MR allows signal differentiation between CSF and other cyst contents and is extremely useful in differentiating epidermoids from arachnoid cysts.  Arachnoid cysts always demonstrate CSF signal intensity on FLAIR images.
On Diffusion-weighted MR imaging epidermoids show bright signal intensity due to the T2 shine through effect. Echoplanar Diffusion MR Imaging is extremely helpful in confirming the distinction between arachnoid cysts and epidermoids, the latter demonstrating restricted diffusion in the form of hyperintense bright signal on DW image with a corresponding isointense or hypointense signal on ADC maps. No such diffusion restriction is demonstrable in the CSF containing arachnoid cysts in DWI, the free water in them showing low signal intensity.  Diffusion imaging is also useful in assessing residual epidermoid tumor tissue after surgical resection.
Middle ear Cholesteatomas too demonstrate increased DW signal intensity for the same reason as epidermoids, namely their epidermal contents and ADC mean values of approximately 1.070=103 mm 2 /s [range 1.280-0.807=10-3mm 2 /s] have been described in those.
Intradiploic epidermoids are to be differentiated from dermoid cysts, eosinophilic granulomas, cholesterol granulomas and hemangiomas on the CT and MR images. aneurysmal bone cysts, fibrous dysplasia and eosinophilic granuloma are also included in the differential diagnosis of expansile lytic lesions in young adults and children.  The MR signal characteristics are specific for and can differentiate CSF, proteinaceous or lipid contents, and can identify blood-blood or blood-fluid levels within lesions thereby giving a more accurate insight into the probable histology and aetiology. Fibrous tissue as well as osteoid tissue can be differentiated by their MR signal characteristics.
Aneurysmal bone cysts are expansile, lytic, multiloculated lesions with low and high signal areas both, characteristic of blood content of these cysts. The lesion in fibrous dysplasia on X-ray shows ground glass pattern, with well defined non sclerotic borders, with an hypointense MR signal intensity on both T1 and T2-weighted images characteristic of fibrous composition of tissue and enhances homogenously with gadolinium. Calvarial hemangiomas are solitary lesions usually encountered in middle age women, appear lytic on X-ray and CT imaging and on MR, they exhibit T1 hyperintensity. An important radiological finding is these lesions only affect the outer table of the skull, sparing the inner table. Bony striations radiating from the center of the lesion towards the periphery forming a sunburst or honeycomb pattern are a characteristic feature of hemangiomas.
Dermoid cysts are midline, unilocular masses with characteristic T1 and T2 high signal intensity on MR and extremely low attenuation below -60 HU on CT signifying their true fat content. Geographic punched out lytic lesions of eosinophilic granuloma have ill-defined margins on X-ray and on CT destruction of both outer and inner skull tables is seen. On MR the lesions are hypointense on T1 with heterogeneous intermediate to hyperintense signal intensity on T2 image, and moderate contrast enhancement of the lesion is seen.
In the present case reported, the well circumscribed, expansile, lytic lesion with sclerotic margins in the temporal bone on X-ray, the intradiploic location with both inner and outer table involvement on CT with hypodense attenuation of the contents and the characteristic T1,T2, FLAIR and DWI appearance and intradiploic location on MR supported the diagnosis of an intradiploic epidermoid cyst. The presence of rim enhancement on CT, favored presence of superinfection in the epidermoid. The lesion in our patient was seen to spare the middle ear on CT and lay in the temporal bone, ruling out cholesteatoma. Surgical enucleation was performed and cholesterol contents within a squamous epithelial lined cyst were confirmed on histopathology.
| Conclusion|| |
Intradiploic epidermoid tumors are benign lesions with a slow linear growth rate. These should be completely excised for a good long-term prognosis. Surgery enables excellent cure for the patient. Complete resection of the cyst lining cannot be overemphasized, as incomplete removal predisposes the patient to the risks of recurrence, inflammation and possibility of malignant transformation. Intradiploic epidermoids can pose a diagnostic challenge and imaging with MRI and CT allows evaluation with correct diagnosis preoperatively.
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