Clinical History
A 48-year-old lady presented with vertigo and headache for 2 weeks.
Imaging Findings
CT brain revealed a space occupying lesion with intra-lesional calcification and macroscopic fat in the left posterior cranial fossa. The lesion is mainly extra-axial but its interface with the left cerebellar hemisphere was indistinct. Mild upstream hydrocephalus was noted.
A tiny fat component was seen along the left tentorium.
An MRI was subsequently performed, showing an enhancing mass with signals corresponding to macroscopic fat and a cystic area. Part of the lesion showed restricted diffusion. A tiny T1W hyperintense deposit was noted along the left tentorium, corresponding to the macroscopic fat in the CT scan. This lesion shows typical restricted diffusion for a dermoid cyst. There was heterogeneous intra-lesional gadolinium enhancement.
Peri-lesional edema with a mass effect onto the fourth ventricle was demonstrated. The lesion was mainly extra-axial with suspicious component of intra-axial invasion, favouring an aggressive lesion. No leptomeningeal enhancement was demonstrated.
Discussion
Background
Intracranial dermoid cysts are rare lesions, constituting less than 0.5% of primary intracranial tumours. Dermoid cysts are congenital ectodermal inclusion cysts, arising from the inclusion of ectodermally committed cells at the time of neural tube closure (3rd–5th week of embryogenesis)[1].
These cysts increase in size by means of glandular secretion and epithelial desquamation. Growth can lead to rupture of the cyst contents. Malignant transformation of intracranial dermoid is rare according to the literature [2].
Imaging Perspective
Presence of intra-lesional calcification and macroscopic fat is supportive of dermoid cysts. The best diagnostic clue of a ruptured dermoid cyst is fat-like droplets in the subarachnoid cisterns, sulci, and ventricles. In this case, a fat containing lesion along the left tentorium likely represent previous rupture, which is typical for a dermoid cyst.
However, this extra-axial lesion shows component of intra-axial invasion which favours a malignant lesion, which is confirmed in subsequent histopathology.
Outcome
The main stay of treatment would be surgery followed by adjuvant radiotherapy, which was given to this patient.
Left retromastoid craniotomy with en bloc excision was performed. Intraoperatively, the cerebellar cyst was found to be tightly adhered to a small area of dura and overlying hyperostotic skull bone. The cyst content was heterogenous, in which sebaceous content intermingled with hairs and fibrous tissue were found. The overlying skull bone was also sectioned for histopatological examination.
The lesion histology reviewed irregular islands of atypical squamous cells, with hyperchromatic and pleomorphic nuclei, prominent nucleoli and rare mitosis, with invasion into the adjacent cerebellar tissue in small irregular islands. Non-dysplastic keratinizing squamous epithelium with epithelial type keratinization, bilayer non-ciliated columnar epithelium, melanin pigment, hair, foreign body type giant cell reaction, bone, associated skin adnexae and piloid gliosiswere observed focally. The overall features were compatible with squamous cell carcinoma, arising from preexisting dermoid cyst or teratoma.
Patient was well after operation with no added deficits with Karnofsky Performance Status (KPS) of 90. Whole body PET-CT was performed which did not review any distant metastases. Patient was treated with adjuvant radiotherapy with 60Gy/30Fr. Patient was asymptomatic at 6 month follow up.
Differential Diagnosis List
Posterior cranial fossa dermoid cyst with malignant transformation
Metastasis (less likely)
Simple dermoid cyst (unlikely due to aggressive feature)
Final Diagnosis
Posterior cranial fossa dermoid cyst with malignant transformation
