Female, 30 years old, with fetal hydrocephalus on ultrasound. The midline structures of the brain are replaced by a very heterogeneous, mixed cystic and solid mass which measures up to 9.8 x 6.0 cm in the axial plane, and 8.3 cm in the sagittal plane. The lesion contains areas of susceptibility and mild patchy T1 hyperintensity which may reflect the present of blood product, calcification, or both. The point of origin of this lesion cannot be determined, but it is quite clearly a midline lesion.The normal midline structures of the brain are not visualized and have either been encompassed by the tumor, or have failed to develop. Surrounding the tumor, there is a thin mantle of cerebral cortex on both sides which tapers posteriorly. Fluid filled spaces in the occipital regions and the posterior fossa may represent dilated components of the ventricular system. In any event, the normal ventricular system is not visualized and may be dilated beyond recognition or just severely deformed. The cerebellum is very small.Prominent fluid is also present external to the thinned cerebral cortex, perhaps in the subdural space. The head is markedly enlarged and the calvarium is thin. The soft tissues of the scalp are thickened and presumably edematous or fluid-containing as well.
A large mixed cystic and solid midline tumor is present with numerous associated abnormalities including lack of normal midline structures, severe thinning and deformation of the cortical mantle, and marked abnormal intracranial fluid which is probably ventricular, extra-ventricular and subdural in location. Statistically, the most likely etiology for a tumor with these characteristics is teratoma. Craniopharyngioma would be a less common entity but one which could present with similar imaging findings. Other histologies remain possible as well.