Presentation
One-year history of intermittent left frontoparietal and occipital headache. Recurrent non-rotatory dizziness which initially responded to medications, but later became refractory.
Patient Data
There is a fairly well defined spherical lesion along the extra-axial space in the left posterior fossa with increased signal intensity on the T2, FLAIR and T1 weighted images and with foci of hypointense to intermediate signal.
The patient underwent left suboccipital craniectomy with excision of the tumor.
Histopathologic examination using H&E staining method. Left posterior fossa mass under scanning magnification. Fragments of keratin flakes (left) and cerebellar tissue (right) are seen. Consistent with an epidermoid cyst.
Representative axial images on follow up. T2, FLAIR, T1, GD now showing a rounded marginal enhancing lesion in the left cerebellum with hemorrhagic level and surrounding edema. Representative sagittal GD and T2 shows interval demonstration of a thick and defined marginal enhancement in the region of the inferior cerebellum/occipital region.
Subsequent follow-up one year later, MRI showed a rounded marginal enhancing lesion in the left cerebellum now with hemorrhagic fluid level observed within the lesion. Surrounding perilesional edema is also observed with mild mass effect to the adjacent fourth ventricle. In addition, there is now a thick and defined enhancement in the region of the inferior cerebellum/occipital region with an irregular enhancement observed in the overlying soft tissue compartment (sagittal Gd and T2 images).
Subsequent suboccipital craniectomy was performed. Histopathologic examination now showed squamous cell carcinoma with brain and dural invasion, the cerebellar hematoma is likewise positive for tumor cells.
Histopathologic examination of the posterior fossa mass and cerebellar hematoma using H&E staining method. (Leftmost) Sheets of malignant cells are seen invading the brain and the dura [LPO]. (Middle) Well-developed lamellated keratin pearls are appreciated [LPO]. (Rightmost) Tumor cells with pleomorphic, hyperchromatic nuclei, and prominent nucleoli [HPO].
Case Discussion
Malignant transformation within epidermoid cyst (ECs) resulting in the formation of squamous cell carcinoma (SCC) is a rare phenomenon. Recent literature search showed that there have been a little less than a hundred reported cases of malignant transformation of EC 1-7. The exact underlying mechanism causing malignant transformation is not definitely known. A systemic review conducted by Hamlat et al. 8, there are 5 types of malignant transformation, and the present case can be classified under malignant transformation from a remnant EC. In most of the reported cases, there was a history of previous surgery, ECs were subtotally resected and SCC developed from the residual lesion 1.
Diagnosis of malignant transformation of an EC primarily depends on postoperative histopathological examination. However, on imaging, typical MRI features of benign ECs include: irregular shape with good demarcation, low signal intensity, restricted diffusion and absence of edema. In malignant transformation, there is evidence of tumor aggressiveness with edema, invasion of adjacent structures, high intensity, and signs of cellular proliferation 1,8. In comparison to the reported cases, the lesions were mostly heterogeneous 1,2, hypointense on T2-weighted images due to high cellularity 1, and some hyperintensity 1,3,5,7 were observed probably due to necrosis; leptomeningeal carcinomatosis 6 and intracystic hemorrhage 4 were also reported. Tumors enhance mostly ring-like or peripherally but diffuse enhancement is also possible1. Providing support to these findings, interval appearance of a thickened surrounding wall with marginal enhancement in the area of the resection, areas of restricted diffusion and hemorrhagic fluid levels were also seen in the squamous cell carcinoma part of the tumor in this case.
Provided that transformation of a benign EC to SCC remains a rare entity, exclusion of the possibility of malignant transformation and metastatic infiltration is an important consideration on follow-up.