Key-words:
Atypical - computed tomography - hyperdense - intracranial dermoid cyst - magnetic
resonance imaging - T2-weighted hypointensity - unusual
Introduction
Intracranial dermoids commonly occur in cisternal spaces and parasellar location with
characteristic imaging appearances. On computed tomography (CT) scan, typically dermoid
cysts appear as well defined, low attenuating (−20 to −140) due to their lipid content.
The sebaceous lipid material within a dermoid cyst has attenuation and signal intensity
characteristics that simulate those of fat on both CT scan and magnetic resonance
imaging (MRI), thus giving a characteristic hypodense appearance on CT, and are predominantly
T1-weighted (T1-W) hyperintense on MRI.[[1]] Calcifications may be present in the wall. Enhancement after intravenous gadolinium
administration is rare. Although not pathognomonic, this classic imaging appearance
is usually consistent with the diagnosis of a dermoid cyst. The differential diagnoses
to consider for a radiologically typical/classic dermoid cyst are craniopharyngioma,
epidermoid cyst, arachnoid cyst, teratoma, and lipoma.[[1]] The treatment of choice is total surgical resection with careful dissection between
the cyst capsule and the surrounding neurovascular structure.
Dermoids which appear as hyperattenuating lesions on CT studies are extremely rare
and present a diagnostic challenge. Dermoids which are hyperdense on noncontrast CT
are usually homogeneously hypointense on T2-W MRI. This awareness of unusual/atypical
radiological appearance of a benign lesion like dermoid cyst is important among clinicians
and radiologists since the surgical strategies often depend on the pathology of the
lesion. We report two such cases with atypical radiological features and intra-axial
location, which is a rare occurrence.
Case Reports
Case 1
A 30-year-old woman presented generalized tonic–clonic seizures for 1 year. On examination,
there were no neurological deficits. MRI [[Figure 1]]a, [[Figure 1]]b, [[Figure 1]]c, [[Figure 1]]d, [[Figure 1]]e, [[Figure 1]]f showed an intra-axial lesion in the left basifrontal region measuring 4.2 cm ×
5.2 cm × 5 cm (CC × AP × Trans) isointense on T1, homogeneously hypointense on T2-W
images (large arrow) with a mural nodule hyperintense on T1, T2 images (small arrow).
There is mild rim enhancement of the lesion with heterogeneous enhancement of peripheral
nodule. No restricted diffusion and no increased cerebral blood volume were noted
within the lesion on perfusion image. Noncontrast CT [[Figure 1]]g showed a homogeneously hyperdense lesion with an eccentric nodule having calcification
(arrow). The patient underwent left frontal craniotomy and excision of the lesion.
Histopathological examination [[Figure 1]]h showed keratinized stratified squamous epithelium with pilosebaceous cysts (arrows)
suggestive of a dermoid cyst. Postoperative recovery was uneventful.
Figure 1: (a-c) Axial T1, T2, postcontrast magnetic resonance imaging showing an intra-axial
lesion seen in the left basifrontal region measuring 4.2 cm x 5.2 cm x 5 cm (CC x
AP x Trans) isointense on T1, homogeneously hypointense on T2 (large arrow) with peripheral
nodular hyperintensity (small arrow) on T1, T2 images. There is mild rim enhancement
of the lesion with heterogeneous enhancement of peripheral nodule. (d) No increased
cerebral blood volume noted within the lesion on perfusion image. (e and f) Diffusion
and ADC (Apparent diffusion co-efficient) images showing no restricted diffusion.
(g) Axial noncontrast computed tomography revealing homogeneously hyperdense lesion
(large arrow) with eccentric nodule having calcification (small arrow). (h) Paraffin
section showing keratinized stratified squamous epithelium and pilosebaceous gland
(curved and straight arrows) (H and E, x100)
Case 2
A 36-year-old woman presented with a history of progressive gait unsteadiness for
3 months. There were no features of raised intracranial pressure. MRI scan [[Figure 2]]a, [[Figure 2]]b, [[Figure 2]]c, [[Figure 2]]d, [[Figure 2]]e showed an intra-axial lesion in the left cerebellar hemisphere along the left
paramedian location approximately measuring 50 mm × 57 mm × 46 mm (AP × ML × CC),
predominantly hypointense on T1 and significantly hypointense on T2-W images (large
arrow) with areas of heterogeneous T1 and T2 signal hyperintensities along the peripheral
nodule (small arrow). There was no enhancement of lesion on contrast study. No diffusion
restriction was noted. Noncontrast CT scan images [[Figure 2]]f showed homogeneously hyperdense lesion (large arrow) with a peripheral mural nodule
containing calcification (small arrow) and mild fat density (curved black arrow).
The patient underwent occipital craniotomy and excision of the lesion and histopathological
examination showed keratinized stratified squamous epithelium proliferated by pilosebaceous
cysts (arrows) consistent with the imaging diagnosis of dermoid cyst. The patient
recovered well in the postoperative period.
Figure 2: (a-c) Axial T1, T2, and postcontrast T1 magnetic resonance imaging showing a nonenhancing
intra-axial lesion in the left cerebellar hemisphere measuring 50 cm x 57 cm x 46
mm (AP x ML x CC). The lesion is hypointense on T1 and significantly hypointense on
T2 with peripheral mural nodule which shows heterogeneous signal. (d and e) Diffusion
and ADC images showing no restricted diffusion. (f) Axial noncontrast computed tomography
images showing homogeneously hyperdense lesion (large arrow) with peripheral nodule
with areas of calcification (small arrow) and mild fat density (curved arrow). (g)
Paraffin section showing keratinized stratified squamous epithelium with proliferating
sebaceous gland (arrow) (H and E, xl00)
Discussion
Intracranial dermoid cysts are rare benign neoplasms that are commonly located midline
and are caused by embryological malformation during the development of the neural
tube between the 3rd and 5th weeks.[[1]] These cysts are lined by squamous epithelium and contain skin appendages such as
hair follicles, sebaceous glands, nails, and teeth. The lesion enlarges as a result
of its increased content of glandular secretions and epithelial desquamation, and
as it grows, many symptoms result secondary to the compressed neural structures.[[1]]
Cyst with a mural nodule tumor (CMNT) is one of the well-known radiological patterns
of intracranial tumors. The common tumors that present with CMNT radiological appearance
are hemangioblastoma, pilocytic astrocytoma, ganglioglioma, and pleomorphic xanthoastrocytoma.[[2]] Uncommon tumors include tanycytic ependymoma, intraparenchymal schwannoma, desmoplastic
infantile ganglioglioma, and cystic metastasis.[[2]]
Raz et al.[[2]] in their study of radiologic–pathologic correlation of intra-axial CMNT of the
central nervous system did not encounter any dermoid cyst with CMNT radiological pattern.
Similarly, Jacob and Lee [[3]] in their study which evaluated the role of conventional, diffusion tensor, and
dynamic susceptibility contrast perfusion MRI in characterizing and differentiating
intracranial cystic tumors with the mural nodule did not report any cases of intracranial
dermoid with CMNT appearance. However, none of these cysts was homogeneously hypointense
on T2-W images (like in our cases).
Literature review shows only seven cases reported in the English language literature,
and none had an enhancing mural nodule except the one reported by Brown et al.[[4]] Tan et al.[[5]] also reported a case of hyperdense dermoid in suprasellar location, but no mural
nodule was noted unlike our cases. The common observation was that all these unusual
lesions, which were hyperdense on CT, are significantly hypointense on T2-W MRI.
This rare CT hyperdensity in combination with T2-W hypointensity on MRI of the dermoid
is thought to be due to combination of saponification of lipid/keratinized debris
with secondary microcalcification in suspension, partially liquefied cholesterol,
high protein content, and hemosiderin or iron–calcium complexes relating to previous
episodes of hemorrhage within the cyst.[[4]] In both our cases, the lesion was hyperdense on CT scan and hypointense on T2-W
MRI with only minimal fat density visualized in one of the cases.
On MRI sequences, typically, dermoid cyst appears T1 hyperintense (due to cholesterol
components) and is heterogeneous on T2-W images. On CT, typically, they are hypodense
with attenuation values equal to that of fat density. However, this may not always
be the reality. By virtue of their pathologic contents, rarely, dermoid cyst can show
atypical or unusual appearance. It is important to familiarize with this unusual appearance
of dermoid cysts described in our case series for the right preoperative diagnosis
and better surgical planning. T1 hyperintense (fat) droplets in the subarachnoid spaces
may be visible if there is rupture of dermoid cyst. Such rupture can either be spontaneous
or can occur at surgery resulting in chemical meningitis which may be severe leading
to vasospasm, infarction, and death.[[6]]
Conclusion
When a hyperdense lesion on CT shows significant hypointensity on T2-W MRI and has
a mural nodule with calcification/fat, even if they are not midline in location, it
is rewarding to consider dermoid cyst in the diagnosis. Decreased cerebral blood flow
on perfusion helps to confirm the benign nature of the lesion. Knowledge of this atypical
radiological appearance of dermoid cyst is essential to avoid diagnostic pitfalls
which may have a bearing on the surgical management.
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.
