F-18 Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography in the Evaluation of Intradiploic Epidermoid Cyst.

Epidermoid cysts are benign rare congenital cysts which typically present between 3rd and 5th decade of life. They mostly arise from cerebellopontine angle or parasellar region, but sporadic cases arise from cranial dipole. Here, we present a case of 42-year-old female with painful soft swelling in the left frontal region, which on F-18 fluorodeoxyglucose (FDG) positron emission tomography-computed tomography is well-circumscribed non-FDG avid lesion. The patient underwent total resection of the cyst, and the defect produced by space-occupying lesion was repaired by cranioplasty.

Introduction

Most of the epidermoid cysts arise in cerebellopontine angle or parasellar region. The uncommon location of presentation of epidermoid cyst is dipole of the skull.[1] The overall estimated incidence of epidermoid cyst accounts to 0.2%–1.8% of intracranial tumors.[2] Intradiploic epidermoid cysts are considered to be the inclusion of wrongly placed ectodermal embryonic tissue in neural groove or its vicinity[2] or may occur after trauma which results in implantation of epidermal cells into the bones.[3]

Case Report

A 42-year-old female presented with a single episode of unprovoked generalized tonic–clonic seizures. Review of her history revealed on and off episodes of a frontal headache over a duration of 4 months and painless swelling in the left frontal region for 6 months. On examination, the patient was conscious, coherent and denied any history of trauma, fever, gait instability, and visual disturbances. Neurological examination was normal.

Her history revealed benign meningioma 10 years ago, which was a well-circumscribed tumor and patient underwent left frontal craniotomy with complete resection of tumor followed by cranioplasty.

Whole-body F-18 fluorodeoxyglucose (F-18 FDG) positron emission tomography/computed tomography (PET/CT) is done under the suspicion brain secondaries in a patient with a headache. Scan findings revealed non-FDG avid circumscribed hypodense lesion in the left frontal bone, measuring 2.3 cm × 3.0 cm. The lesion is causing the destruction of inner table of left frontal bone and extending intracranially with tumor to gray matter ratio (TGR) of 0.3, suggestive of intradiploic epidermoid [Figures 1 and 2].

Figure 1

(a-c) A nonfluorodeoxyglucose avid well-circumscribed hypodense lesion in the left frontal bone measuring 2.3 cm × 3.0 cm causing destruction of inner table of left frontal bone and extending intracranially. (a-c) represent axial computed tomography, positron emission tomography, and fused positron emission tomography/computed tomography images respectively

Figure 2

(d-f) Represent coronal computed tomography, positron emission tomography, and fused positron emission tomography/computed tomography images, respectively

Under general anesthesia, total excision of cyst was performed, and the defect formed by space-occupying lesion was repaired by cranioplasty. Histopathology of cystic lesion revealed areas of laminated keratin with no foci of calcification or hemorrhage, confirming benign epidermoid cyst [Figure 3]. Postoperative recovery was uneventful, and the patient was discharged on the 6th postoperative day.

Figure 3

Histopathology of the lesion showing areas of laminated keratin with no foci of calcification or hemorrhage, confirming benign epidermoid cyst

Discussion

Epidermoid cysts are slow growing tumors. Most commonly, they are intradural in location[1] of which the most common site is cerebellopontine angle. They are ectodermal remnants, which during embryonic development remain within the skull bone[2] or they may also develop after iatrogenic trauma.[34] The most common site of bony epidermoid cysts is calvarium, and other sites include paranasal sinuses, mandible, maxilla, temporomandibular joint, tibia, femur, and distal phalanges.[5] In skull, intradiploic space constitutes 25% of all epidermoid cysts and they commonly present as subcutaneous swelling,[6] sometimes associated with headache,[7] tenderness, and focal neurological signs. Rarely, they may cause seizures due to their enormous growth and may undergo malignant transformation.[78]

Epidermoid cysts are lytic lesions with the rim of sclerotic bone and may erode into inner and outer tables of skull.[236910] These cysts may rupture, bleed, and contents may spill into cerebrospinal fluid leading to aseptic meningitis.[1112] Cysts may spread along and compress surrounding structures resulting in facial and trigeminal palsy.[12]

Diagnosis is based on clinical examination followed by CT and magnetic resonance imaging (MRI) and histopathology. CT reveals size, extent of cyst,[7] and dural infiltration.[6] On MRI, cyst mostly shows heterogeneous signal intensity on T1-Weighted images and hyperintense on T2-weighted images[6] without any contrast enhancement. Intracranial extension of the cyst can be identified.

F-18 FDG PET/CT normally shows non-FDG avid hypodense lesion in intradiploic cyst. It can differentiate enhancing tumors such as gliomas, meningiomas, metastatic tumors, central nervous system lymphoma from nonenhancing tumors such as dermoids, any radiation necrosis,[13] based on FDG avidity and TGR. TGR is nearly 1.05 in enhancing tumors, 0.85 in recurrence and further less in dermoids and radiation necrosis.[14] However, confirmation of diagnosis is based on histopathological findings.

The differential diagnosis includes hydatid cyst, aneurysmal bone cyst, cavernous hemangioma, eosinophilic granuloma, and Langerhans histiocytosis.[15] After proper surgical excision, the recurrence rate of 8.3%–25% has been reported.[16]

Conclusion

Intradiploic epidermoid cysts are rare. They can cause symptoms as they grow to a large size. Early evaluation including imaging followed by timely surgical resection is important to avoid potential complications. F-18 FDG PET/CT can be helpful in the differential diagnosis of intracranial lesions based on FDG avidity and tumor to gray matter ratio, thus improving the patient outcome.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.