This is a case of a rapidly progressive posterior fossa tumor diagnosed on day 1 in a term newborn, which narrows down the list of possible diagnoses by a long way. Molecular genetics on pathology were typical of an atypical teratoid rhabdoid tumor.

As always in tumors, the location and age at diagnosis are key features. Posterior fossa tumors in children usually lead to possible diagnoses:

• pilocytic astrocytoma
• ependymoma
• medulloblastoma

AT/RT stands for atypical teratoid rhabdoid tumor. It mimics medulloblastoma with its round shape and midline location. However, medulloblastoma are usually found in older children around 10 years of age, with high cellularity, DWI restriction, and is rarely hemorrhagic and heterogeneous. On the other hand, AT/RT is almost exclusively found in newborns and neonates up to 2 years of age and is frequently heterogeneous. Tumor progression is rapid, as expected from embryonal malignant tumors, and the prognosis is poor. This tumor is associated with rhabdoid tumor predisposition syndrome, a genetic autosomal dominant condition which is in most cases de novo.