Abstract

Background: Epidermoid cysts are benign, slow-growing, and extra-axial lesions that typically develop between the brain structures. Intradiploic and intra-axial involvement is exceptionally rare. We present a case of an intradiploic epidermoid cyst with distinct clinical and radiological features. This case highlights an unusual location and radiological presentation of an epidermoid cyst located within the skull base and may be misdiagnosed as other pathologies.

Case Description: A 44-year-old male presented with a dull, aching frontal headache and slowly growing right frontal swelling, worsening over several months. On examination, the patient had a hard, non-tender, non-mobile right frontal swelling above the right orbit, with an unremarkable neurological examination. Computed tomography brain revealed an extra-axial hypodense lesion in the right frontal region, causing bony thinning and focal defects in the roof of the right orbit and the zygomatic process of the frontal bone. A mild mass effect on the superior rectus muscle and adjacent brain parenchyma was noted, with the left midline shift and effaced sulci. Magnetic resonance imaging (MRI) brain showed heterogeneous signals in both T1 and T2 as well as a central area of restricted diffusion in diffusion-weighted imaging, the blood products indicated by susceptibility-weighted imaging blooming artifacts, which were suggestive of hemorrhage. The radiological differential diagnosis included an epidermoid cyst, arachnoid cyst, dermoid cyst, abscesses, metastasis, and an aneurysmal bone cyst. The patient underwent a right frontal craniotomy and excision of the cyst. Intraoperative gross inspection and histopathological analysis confirmed the diagnosis of an intradiploic epidermoid cyst.

Conclusion: Epidermoid cysts occurring outside their usual locations are exceptionally rare, exhibiting atypical imaging characteristics, including unusual signal intensities on T1- and T2-weighted MRI sequences, along with the absence of the typical pattern of complete restricted diffusion. These findings may be indicative of a mixture of blood products and proteinaceous substances within the cyst. Subsequently, it may be misdiagnosed as other intracranial pathologies.

Keywords: Atypical, Case report, Epidermoid cyst, Intradiploic

INTRODUCTION

Epidermoid cysts are benign, slow-growing, extra-axial tumors that represent approximately 1% of all intracranial tumors.[ 12 ] These cysts arise from ectodermal inclusions during the closure of the neural tube between the 3^rd and 5^th weeks of embryonic development.[ 10 , 12 ] They are commonly found in the cerebellopontine angles and parasellar regions, where they progressively insinuate between brain structures.

In contrast, epidermoid cysts occurring in intradiploic or intraparenchymal sites are exceedingly rare, accounting for <5% of all intracranial epidermoid cysts.[ 1 , 2 ]

Here, we present an atypical case of intradiploic epidermoid cyst, which may be misdiagnosed as other intracranial pathologies, including arachnoid cyst, dermoid, neuroenteric cyst, abscess, and skull metastasis.

CASE DESCRIPTION

A 44-year-old male presented with complaints of a dull, aching headache, predominantly in the frontal region. In addition, the patient noted slowly-growing swelling in the right frontal area of the head. The symptoms had been progressively worsening over the past several months. There was no history of trauma or other neurological symptoms such as seizures, weakness, or sensory changes. The patient’s general medical history was unremarkable, and there was no significant family history of neurological conditions. On examination, the patient had a hard swelling over the right frontal region above the orbit. The swelling was non-tender and immobile, with the scalp moving over the swelling. Neurological examination was unremarkable. A non-enhanced axial computed tomography (CT) brain [ Figure 1 ] revealed a moderatesized extra-axial hypodense lesion in the right frontal region, measuring approximately 3.6 × 5.7 cm. The lesion showed adjacent bony thinning and remodeling, with focal defects in the right orbital roof and the zygomatic processes of the right frontal bone. A mild mass effect was noted on the superior rectus muscle, without intra-orbital extension, and there was focal compression of the adjacent brain parenchyma, though no significant brain edema was observed. Loco-regional sulcal and ventricular effacement were present, with a mild leftward midline shift of about 3 mm. Gray-white matter differentiation was preserved.

Figure 1:

A non-enhanced axial computed tomography brain (a) soft tissue and (b) bone windows showing right frontal non-aggressive looking intradiploic expansile lytic lesion with scalloped margins and focal sharply defined bony defect.

Preoperative magnetic resonance imaging (MRI) brain without and with gadolinium [ Figure 2 ] demonstrated a moderately-sized extra-axial non-enhancing lesion located in the right anterior inferior frontal convexity, measuring 5.3 × 4.4 × 4.3 cm. Similar to the CT findings, the MRI showed adjacent bony thinning and remodeling with focal defects in the roof of the right orbital and zygomatic process of the right frontal bone. There was a mild mass effect on the superior rectus muscle and a focal mass effect on the adjacent brain parenchyma but no evidence of brain edema. Mild leftward midline shift and sulcal and ventricular effacement were noted. The lesion exhibited heterogeneous signal intensity on T1 and T2 sequences, with areas of T1 hyperintensity suggestive of blood or proteinaceous products. In addition, susceptibility-weighted imaging (SWI) demonstrated blooming artifacts, which suggested the presence of blood products within the lesion.

Figure 2:

Magnetic resonance imaging brain axial cuts (a) T1-weighted image (T1W1), (b) T2WI, (c) diffusion-weighted imaging (DWI), (d) susceptibility-weighted imaging, and (e) T1WI with contrast showing right frontal non-enhancing lesion corresponds to the expansile intradiploic lytic lesion. The lesion demonstrates heterogeneous T1 and T2 signal intensities with elements of diffusion restrictions in DWI and variable intrinsic T1 hyperintense signal changes, which might reflect a combination of blood products and proteinaceous materials. In addition, susceptibility weighted imaging (SWI) demonstrated blooming artifacts, which suggested the presence of blood products within the lesion.

Both CT and MRI findings favored a diagnosis of a right frontal extra-axial lesion, with radiological features consistent with an intradiploic hemorrhagic epidermoid; however, other differentials considered were arachnoid cyst, dermoid, neuroenteric cyst, abscess, and skull metastasis.

The patient underwent right frontal craniotomy and excision of the lesion. Intraoperatively, the cyst was in the bone, causing thinning and invasion of the overlying dura [ Figure 3 ]. Consequently, the capsule of the cyst opened unintentionally during the removal of the bone flap as it was within the skull. Brownish, turbid fluid was aspirated from the cyst and was sent for cytology. It was reported as an amorphous eosinophilic material. A complete excision of the cyst was done. The patient tolerated the procedure very well. Postoperative CT brain [ Figure 4 ] revealed complete excision of the cyst.

Figure 3:

Intraoperatively image of the cyst showing the opened capsule and irregular, soft, pearly-white tissue.

Figure 4:

Computed tomography brain (a) axial and (b) coronal cuts showing post-surgical debulking of the right frontal lytic lesion with pneumocephalus.

The histopathological examination of the excised tissue confirmed the diagnosis of an intradiploic epidermoid cyst. The tissue consisted of irregular, soft, gray-white pieces measuring 6 × 4 × 1.5 cm. Microscopic examination revealed typical features of an epidermoid cyst, including the presence of cholesterol clefts and chronic inflammation [ Figure 5 ].

Figure 5:

Hematoxylin and eosin-stained slide (a) ×20 shows a cyst wall lined by squamous epithelium containing flaky keratin, with no associated adnexal structures or endodermal/mesodermal tissues, and (b) ×10 shows extensive mixed inflammation with lymphocytes and hemosiderin-laden macrophages, along with cholesterol clefts of varying sizes.

DISCUSSION

Atypical epidermoid cysts are rare, extra-axial lesions constituting <1.5% of all intracranial epidermoid cysts.[ 1 ]

Intradiploic epidermoid cysts (including congenital cholesteatomas) account for approximately 3% of the above group.[ 3 , 9 ] A recent systematic review[ 5 ] has reported only 41 cases of intradiploic epidermoid cysts in the skull.

Intradiploic epidermoid cysts arise from aberrant ectodermal remnants trapped after neural tube closure compared to intraparenchymal epidermoid cysts, which are believed to result from ectodermal inclusion occurring before the 3^rd week of embryogenesis, during the formation of the primary cerebral vesicle.[ 1 , 3 , 7 ]

Thus far, few series of atypical epidermoid cysts have been reported due to the low incidence of epidermoid cysts, especially those with atypical MRI appearance.

CT brain for the typical intradiploic epidermoid cysts includes a non-enhancing hypodense lesion with sharply demarcated bony defects and zones of calcifications that may alter the outer and/or inner tables of the skull,[ 7 , 11 ] a finding we observed in our case. However, the MRI features differ from the typical one. Most typical epidermoid cysts present with long T1 and T2 signals on MRI with a classic pattern of homogenous complete diffusion restriction.[ 8 ] However, atypical features are heterogeneous signal intensity with T1 hyperintense areas; variations in signal might suggest a more complex composition of the cyst, potentially due to prior hemorrhage or the presence of proteinaceous material, as in our case. This atypical finding, along with the blooming artifacts on SWI, suggests that the lesion may have contained elements of hemorrhage or chronic inflammatory changes.[ 7 , 11 ]

A study by Ren et al.[ 13 ] confirmed a propensity for spontaneous hemorrhage in atypical epidermoid cysts, with 21 out of 24 patients experiencing this complication. The hemorrhagic component in such cysts is attributed to an inflammatory reaction and is considered to be a risk factor for a rapid increase in the size of the lesion.[ 6 ]

The elevated protein content within the cyst, which led to significant heterogenous signals in both T1- and T2-weighted MRI, also contributed to the absence of the classic pattern of complete restricted diffusion within the whole lesion.[ 14 ] Although atypical epidermoid cysts do not exhibit consistent imaging characteristics, our findings indicate that intradiploic epidermoid cysts may present with atypical T1-weighted and T2-weighted signals, as well as an absence of a classic pattern of completely restricted diffusion within the whole lesion.[ 12 ]

Given the characteristics of our case, it could be mistaken for other lesions. In a study by Ren et al.[ 13 ] of 24 atypical epidermoid cysts, 14 lesions (58.3%) were misdiagnosed based on radiological findings. The radiological differential diagnosis for an epidermoid cyst includes arachnoid cysts, dermoid cysts, abscesses, metastases, an aneurysmal bone cyst, and slow-growing brain tumors. Fluid-attenuated inversion recovery and diffusion-weighted imaging sequences[ 7 ] have been suggested as helpful in distinguishing an arachnoid cyst from a typical intradiploic epidermoid cyst, as the signal intensity of an arachnoid cyst typically mirrors cerebrospinal fluid signal and lacks restricted diffusion. Dermoid cysts are generally located more centrally and often contain calcification foci, which helps differentiate them from epidermoid cysts.[ 14 ] Abscesses usually exhibit a characteristic contrast-enhancing rim, metastases are typically multiple with a known primary source and heterogeneous contrast enhancement, and cystic neoplasms often display a solid component with enhancement.[ 12 ]

Complete resection of epidermoid cysts provides a definitive treatment, effectively preventing recurrence or complications such as aseptic meningitis.[ 9 ] However, epidermoid cysts are frequently situated near critical neurovascular structures and essential brain parenchyma, which may occasionally necessitate a more conservative approach to resection. Given the slow-growing nature of these tumors, with a growth rate comparable to that of normal epithelial cells,[ 4 ] a less aggressive surgical strategy may be considered. Dural invasion and perforation need to be anticipated in such a large sized tumor. In cases of dural involvement, it is imperative that the surgeon excise the involved dura and do duraplasty, preferably autologous. Such a measure may also decrease recurrence.

CONCLUSION

Atypical intradiploic epidermoid cysts are extremely rare and display unusual imaging features, such as atypical signal intensities on T1- and T2-weighted MRI brain and the lack of the usual pattern of complete restricted diffusion. These characteristics could suggest the presence of a combination of blood products and protein-rich material within the cyst. As a result, the cyst may be mistakenly diagnosed as another type of intracranial pathology.

Ethical approval:

The Institutional Review Board approval is not required.

Declaration of patient consent:

Patient’s consent not required as patients identity is not disclosed or compromised.

Financial support and sponsorship:

Nil.

Conflicts of interest:

There are no conflicts of interest.

Use of artificial intelligence (AI)-assisted technology for manuscript preparation:

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

Disclaimer

The views and opinions expressed in this article are those of the authors and do not necessarily reflect the official policy or position of the Journal or its management. The information contained in this article should not be considered to be medical advice; patients should consult their own physicians for advice as to their specific medical needs.

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