Cranial base approaches for endodermal cysts of the posterior fossa

Objective: Endodermal cysts of the posterior fossa are rare, but their location makes their excision difficult by conventional approaches. We reviewed the literature and added three new cases reported here to determine the clinical, pathological, radiological, and surgical characteristics of these lesions. Methods: Three patients with petroclival endodermal cysts were operated on over a three year period using cranial base approaches. These lesions were high signal on Tl and low signal on T2 without enhancement. One of these tumours was a dumbell lesion occupying the clivus region and the fourth ventricle, communicating via a sinus tract. At surgery, the lesions were greenish in colour with a cyst membrane, and whitish thick fluid and greenish-brown solid material inside. All these lesions were misdiagnosed as dermoid cysts preoperatively, and even on initial pathological examination. We have also reviewed another 46 cases reported in the literature between 1962 and 1996. Results: These cysts have a male predominance (61%) with a bimodal distribution (third to fourth decades and first decade). Clinically, headache is the most frequent complaint (17 patients). They can present also with recurrent aseptic meningitis (10 patients). On immunohistopathology, endodermal cysts were reactive for EMA, and keratin immunostains whenever the latter were performed (17 and 10 cases respectively). They were reactive for ' CEA in 9 out of 11 cases. They were frequently hypodense on CT scan ( 16 cases), and in 5 cases, the lesion was even missed. MRI appearance is variable: the signal was high on T2 weighted images in 8 cases, and it was also high on Tl weighted images in 9 cases. They are most frequently located anterior to the brainstem (51%), thus making cranial base approaches ideal for their resection. They are rarely associated with bony clivai anomalies (One case of bifid clivus). They can be occasionally adherent to surrounding neurovascular structures over a focal (8 cases) or diffuse (9 cases) area. Growth as well as recurrence (3 cases) have been documented.The different theories regarding embryogenesis are also discussed. Conclusion: Endodermal cysts of the posterior fossa are rare lesions that occur in the petroclival region. Total excision with preservation of neurological function should be the goal. Cranial base approaches may be helpful in surgical access to some of these lesions, especially those located anterior to the brainstem.