Atypical teratoid/rhabdoid tumors may show morphological and immunohistochemical features seen in choroid plexus tumors.

Atypical teratoid/rhabdoid tumors (AT/RT) are aggressive embryonal tumors having a poor prognosis and are associated with mutations in the tumor suppressor gene hSNF5/SMARCB1/INI1. Differential diagnosis includes choroid plexus carcinoma which has occasionally been attributed as showing an inactivation of INI1/SMARCB1 nuclear staining in immunohistochemistry. However, these findings have been challenged by others. We therefore examined eight AT/RTs from six patients by immunohistochemistry for membranous expression of the inward rectifier potassium channel Kir7.1, which was in the central nervous system so far considered specific for choroid plexus tumors and normal choroid plexus epithelium. Two AT/RT cases exhibited membranous staining of Kir7.1, indicating a plexus epithelial differentiation of these tumors. The implications of these results on tumor diagnosis are discussed.