A Large Primary Frontal Sinus Cyst with Intracranial Extension

We report a large primary frontal sinus cyst with intracranial extension. A 60-year-old man presented with swelling of the forehead. Magnetic resonance imaging and computed tomography showed a large frontal sinus cyst that exceeded the base of the skull in size and extended into the intracranial region; the frontal lobe was also pressed against. The cyst also developed into the orbit and pressed against the eye-ball. However, no symptoms indicated frontal lobe impairment, and diplopia was not recognized. Since there was a possibility that the dura mater might be damaged by opening the cyst, it was opened by an extranasal approach.