Atypical teratoid/rhabdoid tumor
Atypical teratoid/rhabdoid tumors (AT/RT) are an uncommon WHO grade IV tumor, which in the vast majority of cases occurs in young children less than two years of age. It most frequently presents as a posterior fossa mass. AT/RT often resembles medulloblastoma by imaging and even H&E microscopy, and the diagnosis requires cytogenetic analysis of the tissue.
They present in young children (median age is less than 2-3 years ), whereas medulloblastomas typically occur in mid-childhood (median age 6 years).
Rhabdoid cells are the hallmark of AT/RT, but only comprise a fraction of the tumor. Other portions of the tumor are indistinguishable on imaging and histology from a medulloblastoma or embryonal tumor with multilayered rosettes.
According to 2016 WHO classification scheme, a diagnosis of AT/RT requires confirmation of specific genetic aberration (loss of INI1 tumor suppressor gene on chromosome 22 or BRG1 gene); otherwise, a descriptive diagnosis of CNS embryonal tumor with rhabdoid features is used .
Atypical teratoid/rhabdoid tumors are usually large and very heterogeneous masses. They may be difficult to distinguish from a PNET by imaging.
- infratentorial: ~50%
- cerebellum (most common)
- cerebral hemispheres
- pineal gland region (see pineal mass differential diagnosis)
- septum pellucidum
- often isodense to grey matter
- may demonstrate heterogeneous enhancement
- calcification is common
- may show associated obstructive hydrocephalus
Can show necrosis, multiple foci of cyst formation and sometimes hemorrhage:
- T1: iso- to slightly hyperintense to grey matter (hemorrhagic areas can be more hyperintense)
- T2: generally hyperintense (hemorrhagic areas can be hypointense)
- T1 C+ (Gd): heterogeneous enhancement
- MR spectroscopy
- almost all restrict diffusion
Leptomeningeal seeding has been described in up to 15-30% of cases and so post-contrast imaging of the entire neuroaxis should be considered in suspected AT/RTs.
Treatment and prognosis
Surgery with debulking can be offered in some cases. Tumors can demonstrate leptomeningeal dissemination. Clinically AT/RTs have a much poorer prognosis than medulloblastomas, with little if any response to chemotherapy and death usually occurring within a year of diagnosis.
History and etymology
Primary CNS rhabdoid tumor was first identified as a unique entity in the mid/late 1980's. Prior to this, these tumors were likely misdiagnosed as primitive neuroectodermal tumor/medulloblastoma, as they are relatively similar in microscopic appearance . Early reports variably used the term malignant rhabdoid tumor.
Imaging differential considerations include: