We present the case of a patient who was diagnosed preoperatively as a case of eosinophilic chronic rhinosinusitis with supra agger frontal cell (SAFC) cyst. In fact, there was no cyst, but rather bone thinning of the anterior cranial floor caused by nasal polyps. Therefore, the cyst wall was perforated, but it was actually the dural membrane, and spinal fluid leakage occurred. Intraoperative MRI was used to minimize complications. The dura was repaired using a free mucosal flap and the postoperative course was satisfactory. Even polyps, such as in case of eosinophilic chronic rhinosinusitis, can cause thinning and loss of bone in the skull base, and intraoperative MRI, which can confirm dural membrane damage and anatomic location, is useful if such a finding is present.
