Rhabdoid tumour was originally described in kidney as a rhabdomyosarcomatoid variant of Wilm’s tumour; however it soon became clear that the two were different entities. Subsequently, rhabdoid tumours were reported in a variety of extrarenal sites including the brain, skin, liver, thymus and orbit. The most common location for extrarenal rhabdoid tumor is the central nervous system. In the brain, these tumors may be composed entirely of rhabdoid cells, or they may present as a mixture of rhabdoid tumour with PNET, mesenchymal, and/or epithelial elements, an entity referred to as atypical teratoid/rhabdoid tumour (AT-RT). The exact cell of origin for AT-RT of the kidney or extrarenal tissues is not known, however, these are believed to arise from a primitive cell with the capacity to diverge along multiple differentiation pathways.
Central nervous system AT-RT is a very rare, fast-growing tumor of the brain and is considered one of the hostile and lethal malignancies in pediatric-oncology. It typically occurs in children less than two to three years of age. It is 50% more common among boys than girls. AT-RT may arise anywhere within the nervous system, the majority (>50%) arises in the posterior fossa, and the remainder in the supratentorial compartment (suprasellar or temporal region). Other sites of occurrence include the pineal region(5%) and the spinal cord(2%). AT-RT present as a bulky invasive infratentorial mass with a markedly heterogeneous appearance on CT or MR-imaging. It usually demonstrates an off-midline location in the posterior fossa. It is most commonly cerebellar in location and characteristically projects into the cerebellopontine angle cistern. The tumour consists of both solid and cystic component and often contains multiple foci of calcifications and haemorrhage. It is associated with multiple eccentric cysts, which may demonstrate peripheral rim enhancement. On intravenous contrast administration the solid component of the mass shows a markedly inhomogeneous contrast enhancement. A contrast study is also required to rule out CSF spread. Leptomeningeal spread if present carries a poor prognosis. The closest differential of AT-RT is a medulloblastoma, which occurs in older children with a peak incidence at 6-years of age. Medulloblastoma classically present as a midline infratentorial mass, which arises from the cerebellar vermis and grows into the fourth ventricle. Compression and displacement of the fourth ventricle by the tumour is very unusual in medulloblastoma. They are solid tumours rarely associated with cysts. In contrast AT-RT arises from the cerebellar hemisphere and grows into the adjacent cisternal spaces, instead of filling the fourth ventricle. Other posterior fossa tumours, which can rarely be confused with AT-RT include ependymoma and pilocytic astrocytoma. However both these tumours occur in older children. Unlike AT-RT, ependymoma rarely shows a cystic component. Pilocytic astrocytoma is a well-circumscribed, non-invasive cystic tumour, which characteristically demonstrates an enhancing mural nodule.
To conclude, although there are no characteristic neuroimaging findings of AT-RT, its possibility should be included in the differential diagnosis of a bulky, off-midline, heterogeneous infra-tentorial mass with calcification and eccentric cysts occurring in a child less than 2 year of age.