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There is a cystic lesion of extra-axial origin with unclear borders, located in the posterior fossa, just below the cerebellar vermis and the floor of the 4th ventricle, extending to the cisterna magna and foramen magnum. This mass displays a signal similar to cerebrospinal fluid on all imaging sequences, specifically low on T1, FLAIR, and high on T2W, GRE, with no restriction of diffusion (low on DWI and ADC).
The mass exerts a mass effect on adjacent structures, causing compression of the brainstem and cerebellar hemispheres.