Epidermoid tumour within the collateral sulcus: a rare location and atypical presentation.

Epidermoid tumours (ETs) are uncommon benign lesions that may grow by spreading in the subarachnoid space of the basal cisterns and expanding to conform to the shape of specific sulci and fissures. A temporobasal location is very rare, and there have been no reports regarding single sulcus involvement of ETs. We describe the case of an ET located on the basal surface of the temporal lobe, predominantly within the collateral sulcus, which separates the parahippocampal gyrus medially from the fusiform gyrus laterally. We report the case of a 25-year-old woman with complex visual hallucinations. MRI of the brain revealed a right temporobasal mass lesion, hypointense on T(1)-weighted and hyperintense on T(2)-weighted images, with minimal contrast enhancement, on the basal surface of the temporal lobe. Right-sided anterior temporal lobectomy, along with microsurgical removal of the collateral sulcus ET were performed with consequent resection of mesial temporal structures (the region of the hippocampus, parahippocampal gyrus, and amygdala). It is important to consider ETs when treating lesions on the basal temporal lobe, since the inferior surface of the temporal lobe, more specifically the collateral sulcus, may be a convenient space for ETs to localize. Total surgical resection should be the goal in these cases; however, the surgical approach may be tailored to include the resection of mesial temporal lobe structures when seizure is the predominant presenting symptom.