Primary Sonic Hedgehog-activated dorsal brainstem medulloblastoma and ipsilateral cerebellar atrophy in an adult.

Medulloblastoma (MB) that arises outside the cerebellum from cells of the dorsal brainstem is rare. The most common subtype of MB in the dorsal brainstem is the Wingless (WNT) subtype that contains activating mutations in the WNT pathway effector CTNNB1. Ipsilateral cerebellar hemi-atrophy with a brainstem tumour is a finding that is usually documented with brainstem gangliogliomas as a possible specific imaging sign. We present a case of a 23-year-old female with progressive headache, imbalance on walking, double vision and difficulty in swallowing for a year. Magnetic resonance imaging demonstrated a mass with prominent restricted diffusion on the dorsal surface of the lower brainstem with ipsilateral cerebellar atrophy. The final histopathological diagnosis was a Sonic Hedgehog (SHH)-activated and TP53 wild-type primary lower dorsal brainstem MB. Primary SHH-activated TP53 wild-type dorsal brainstem MB is extremely rare, and as far as we know, the association of the tumour with ipsilateral cerebellar hemi-atrophy in an adult has never been reported. MB should be included in the differential diagnosis of focal dorsal brainstem tumours, even in the presence of ipsilateral cerebellar hemi-atrophy.