Interdural dermoid cyst located in the posterior fossa: A rare pediatric case report

Background: Intracranial dermoid cysts are rare congenital lesions arising from ectodermal inclusions during neural tube closure. They are usually located along the midline and most commonly present as intradural lesions. Posterior fossa localization is uncommon in children, and interdural localization in this region is exceptionally rare. Case Description: A 12-year-old female patient presented with worsening headache, nausea, and vomiting. Neurological examination revealed no focal deficit. Cranial magnetic resonance imaging demonstrated a large, well-circumscribed posterior fossa mass containing fatty components, producing marked mass effect, extending through the foramen magnum, and compressing the brainstem. Magnetic resonance venography showed nonvisualization of the left transverse sinus, considered secondary to mass compression. Computed tomography demonstrated a dermal sinus tract traversing a midline occipital osseous defect toward the subcutaneous tissue. There was no history of occipital discharge or recurrent infection. The patient underwent surgery through a median suboccipital approach. Intraoperatively, the tract was followed from the subcutaneous tissue through the osseous defect to the cyst capsule and excised with the lesion. The cyst was located in the interdural plane and contained gelatinous material and hair-like structures. Total excision was achieved, and opening of the inner dural layer confirmed no intradural extension. Histopathology confirmed a dermoid cyst. The postoperative course was uneventful, with complete symptom resolution and no residual lesion. Conclusion: Posterior fossa interdural dermoid cysts are exceptionally rare in children. A dermal sinus tract traversing a midline occipital osseous defect supports a dysraphic origin. Careful preoperative evaluation and complete excision provide favorable outcomes.