ATYPICAL TERATOID/RHABDOID TUMOR (AT/RT)

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Definition

AT/RT is a highly malignant embryonal tumor of childhood composed of rhabdoid cells within a cellular background resembling a classical primitive neuroectomdermal tumor, epithelial tissue and neoplastic mesenchyme.. AT/RT corresponds to WHO Grade IV.

Epidemiology

AT/RT is a rare tumor of childhood, accounting for about 2% of all primary CNS tumors in children less than 18 years old.

Localization

AT/RT may arise in the posterior fossa (52%), supratentorial cerebrum (39%), pineal region (5%), or spine (2%). These lesions may occasionally be multi-focal. Cerebrospinal metastases are evident in nearly one-third of patients at the time of initial presentation.

Clinical presentation

Clinical presentation is variable, depending on the location of the tumor and age of the patient. Infants and young patients tend to present with evidence of increased intracranial pressure, while focal neurological deficits are more common in older children.

Axial post-contrast T1 MR demonstrating tumor encasing the Vertebro-basilar junction.

Imaging

Similar to Medulloblastoma, AT/RT is an avidly enhancing tumor. Due to the high Nuclear:Cytoplasmic ratio, it demonstrates hyperdensity on CT. Hypercellular lesions are bright on DWI, but isointense on ADC maps (aka "false-restriction"), similar to epidermoids. When AT/RT occurs in the posterior fossa, it tends to center around the CPA cistern, unlike the classic "fourth ventricular" appearance of medulloblastoma. Unlike ependymoma, which grows out of the Foramen of Lushka, AT/RT appears to 'erupt' from the lateral ponto-medullary junction.

Intraoperative view demonstrating tumor encasing the Vertebro-basilar junction.

Macroscopy

AT/RT appears as a greyish-purple tumor on gross specimen, similar to medulloblastoma. However, it tends to envelope cranial nerves, and the vertebro-basilar system as it grows outside the brainstem.

10x H&E Stain

10x Vimentin Stain

10x Smooth Muscle Antigen (SMA) Stain

10x Epithelial Membrane Antigen (EMA) Stain

Histopathology

AT/RT contains rhabdoid cells within a variable cellular background resembling a classical primitive neuroectomdermal tumor, epithelial tissue and neoplastic mesenchyme. The typical rhabdoid cell is medium-sized, round to oval, with an eccentric nucleus with prominent nucleoli. The cytoplasm is filled with eosinophilic material which occasionally coalesces to form a dense collection resembling an inclusion body. The neuroectomdermal component may form Homer-Wright or Flexner-Wintersteiner rosettes. The mesenchymal component may resemble a frank sarcoma.

AT/RT cells are characteristically Vimentin+, EMA+, SMA+.

Molecular genetics

Nearly all AT/RTs display monosomy or deletion of chromosome 22 with loss of the hSNF5/INI1 gene on 22q11.2.

Prognosis

The majority of patients diagnoses with AT/RT die within one year despite aggressive surgical and medical management.