Atypical teratoid/rhabdoid tumor

Atypical teratoid/rhabdoid tumours (AT/RT) are an uncommon WHO grade IV tumour, 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 ²), 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 tumour. Other portions of the tumour are indistinguishable on imaging and histology from a medulloblastoma or embryonal tumour with multilayered rosettes.

According to 2016 WHO classification scheme, a diagnosis of AT/RT requires confirmation of specific genetic aberration (loss of INI1 tumour suppressor gene on chromosome 22 or BRG1 gene); otherwise, a descriptive diagnosis of CNS embryonal tumour with rhabdoid features is used ^8,10.

Immunophenotype

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

Radiographic features

Atypical teratoid/rhabdoid tumourtumours 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
  • 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 haemorrhage:

• T1: iso- to slightly hyperintense to grey matter (haemorrhagic areas can be more hyperintense)
• T2: generally hyperintense (haemorrhagic 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. Tumours 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 tumour was first identified as a unique entity in the mid/late 1980's. Prior to this, these tumours were likely misdiagnosed as primitive neuroectodermal tumour/medulloblastoma, as they are relatively similar in microscopic appearance ¹². Early reports variably used the term malignant rhabdoid tumour.

Differential diagnosis

Imaging differential considerations include:

• supratentorial CNS PNET: particularly for supratentorial ATRTs
• medulloblastoma
  • tends to occur in an older age group ⁶
  • rarely involve cerebellopontine angle
  • haemorrhage is much less frequently seen than in AT/RTs ⁶
• intracranial teratoma