Atypical teratoid/rhabdoid tumors (AT/RT) are uncommon WHO grade 4 tumors, 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.
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Epidemiology
AT/RTs typically present in very young children (median age is less than 2-3 years 2,13).
Pathology
Microscopic features
Rhabdoid cells are the hallmark of AT/RT, but only comprise a fraction of the tumor and are not required for the diagnosis provided genetic of DNA-methylation changes are compatible (see below) 13.
Other portions of the tumor are indistinguishable on imaging and histology from a medulloblastoma or embryonal tumor with multilayered rosettes.
Genetics
The vast majority (~95%) of AT/RTs have inactivation of both copies of SMARCB1 (also known as INI1, BAF47 or hSNF5). In a small minority of cases (< 5%) it is SMARCA4 (also known as BRG1) that is inactivated 13.
Additionally, AT/RTs can be divided into three subgroups according to DNA-methylation profiling: AT/RT-SHH, AT/RT-TYR, and AT/RT-MYC 13.
To make the diagnosis, either SMARCB1 or SMARCA4 mutations or compatible DNA-methylation profiling need to identified 13.
Immunophenotype
Rhabdoid cells usually demonstrate:
Radiographic features
Atypical teratoid/rhabdoid tumors are usually large and very heterogeneous masses. They may be difficult to distinguish from an embryonal tumor with multilayered rosettes by imaging.
Location
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infratentorial: ~50%
cerebellum (most common)
brainstem
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supratentorial
cerebral hemispheres
pineal gland region (see pineal mass differential diagnosis)
septum pellucidum
hypothalamus
CT
often isodense to grey matter
may demonstrate heterogeneous enhancement
calcification is common
may show associated obstructive hydrocephalus
MRI
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
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MR spectroscopy
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DWI
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 1980s. Prior to this, these tumors were likely misdiagnosed as primitive neuroectodermal tumor/medulloblastoma, as they are relatively similar in microscopic appearance 12. Early reports variably used the term malignant rhabdoid tumor.
Differential diagnosis
Imaging differential considerations include:
embryonal tumor with multilayered rosettes particularly for supratentorial AT/RTs
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tends to occur in an older age group 6
rarely involve cerebellopontine angle
hemorrhage is much less frequently seen than in AT/RTs 6
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