Epidermoid Cyst with Malignant Transformation to Squamous Cell Carcinoma
Intracranial epidermoid cysts are benign, congenital inclusion tumors which account for 0.2 - 1.8% of all intracranial lesions (10). They commonly occur in the cerebellopontine angle, although the petrous apex and suprasellar region are also favored sites (3). They tend to be slow growing masses and presentation is dependent on the site of the lesion (7). Malignant transformation to a squamous cell carcinoma is an exceptionally rare event and should be thought of in the face of rapid progression of a patient's symptomatology (2,3).
Epidermoid cysts are best investigated with MRI where they appear hypo or isointense on T1, hyperintense on T2 and show minimal or no enhancement with gadolinium (6,8). They tend to be well demarcated and show an absence of edema (5). A dermoid cyst should also be included in the differential diagnosis while schwannoma or meningioma is far less likely as both tend to enhance brightly with gadolinium. Enhancement after gadolinium administration, rapid growth with an irregular margin along with rapid progression of the patient's signs and symptoms should alert the clinician to a possible malignant transformation (5,8).
On gross histology, epidermoid cysts have a pearly white appearance. Histopathologically, an epidermoid cyst is comprised of benign keratinizing stratified squamous epithelium lining (1) with a low proliferative index and desquamated cellular debris (8). In contrast, a squamous cell carcinoma (SCC) tends to have a grayish appearance with thickened walls, a solid component and unclear margins due to tumor infiltration of surrounding structures (8). On microscopic examination, dysplastic squamous epithelium with enlarged pleomorphic nuclei and a high proliferative index will be seen (9).
Malignant transformation of a benign epidermoid cyst was first described by Ernst in 1912 (5). This is a rare but well described event in the literature with our case being the forty eighth one.
In 1981, Garcia et al recommended definite criteria for the definition of a primary intracranial squamous cell carcinoma: a) the tumor has to be restricted to the intracranial, intradural compartment without invasion or extension beyond the dura or cranial bones, and b) there must be no extension or invasion through intracranial orifices, nor communication or connection with the middle ear, air sinuses or sella turcica and no evidence of nasopharyngeal tumor. These criteria exclude all possibilities of tumors arising outside the dura with secondary invasion of the brain (4).
In 2003, Hamlat et al classified these carcinomas into five groups (2):
Our case qualifies as type 2 according to Hamlat's classification.
In conclusion, we report a case of a primary intracranial squamous cell carcinoma due to malignant transformation of a previously partially resected CP angle epidermoid cyst. This is the forty eighth reported case in the literature. Mainstay of treatment is surgical resection followed by radiation (6). Despite this treatment, prognosis remains poor (10).
Contributed by Reena Baweja, Kesava Reddy, Anthony Whitton, John Proviasm, Boleslaw Lach