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.

Epidemiology

They present in young children (median age is less than 2-3 years ), whereas medulloblastomas typically occur in mid-childhood (median age 6 years).

Pathology

Microscopic features

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 .

Immunophenotype
  • EMA: positive
  • vimentin: positive
  • smooth muscle actin: positive

Radiographic features

Atypical teratoid/rhabdoid tumors are usually large and very heterogeneous masses. They may be difficult to distinguish from a PNET by imaging.

Location
  • infratentorial: ~50%
    • cerebellum (most common)
    • brainstem
  • supratentorial
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
  • MR spectroscopy
    • Cho: elevated
    • NAA: decreased
  • 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 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.

Differential diagnosis

Imaging differential considerations include: