A case of dermoid cyst of the floor of the mouth: ultrasonography was more useful than computed tomography and magnetic resonance imaging in the preoperative diagnosis.

Dermoid cysts are rare in the floor of the mouth. This report presents a case of dermoid cyst (dermoid type) of the floor of the mouth in a 21-year-old man. The preliminary preoperative assessment was made by using three modalities: computed tomography (CT), magnetic resonance imaging (MRI), and ultrasonography (US). CT depicted the lesion as a well-circumscribed, unilocular, homogeneous low-density mass. MRI showed a mass lesion located in the submental region between the geniohyoid and mylohyoid muscles. The lesion exhibited homogeneous intermediate signal intensity on T1-weighted images and homogeneous high signal intensity on T2-weighted images. The wall of the lesion was well enhanced on gadolinium-enhanced MRI. CT and MRI could not demonstrate any fatty components within the lesion and only indicated that the lesion was cystic in nature. In contrast, on US, multiple echogenic foci that seemed to be caused by keratin and sebaceous material were detected within the lesion, and this US finding was useful information for the differential diagnosis. In this case, of the three modalities, only US suggested that the lesion might be a dermoid cyst.