Atypical Teratoid/Rhabdoid Tumor: Revisiting Histomorphology and Immunohistochemistry With Analysis of Cyclin D1 Overexpression and Amplification.

. Atypical teratoid/rhabdoid tumor (AT/RT) is a rare malignant pediatric brain tumor, characterized by inactivation of INI1/hSNF5 gene and loss of its protein. We studied the histomorphological and immunohistochemical spectrum of this tumor including cyclin D1 expression and gene amplification. . Cases with INI1 loss by immunohistochemistry (IHC; from 2005 to 2018) were retrieved, reviewed, and evaluated for cyclin D1 expression by additional IHC and fluorescence in situ hybridization for genes. . A total of 66 cases were identified. Age ranged from 1 to 20 years (≤3 years, 44 cases; >3 years, 22). Male to female ratio was 1.7:1. Tumor locations were as follows: posterior fossa: 30; supratentorial: 31; spinal: 5. AT/RT in patient ≤3 years was frequently located in the posterior fossa, composed of primitive embryonal morphology ( = .02), rarely had ample rhabdoid cells ( = .05), and had a negative impact on overall survival ( = .04). The rhabdoid cells was a conspicuous component of posterior fossa tumors compared with the supratentorial ones ( = .06). The supratentorial tumors ( = .06), absence of rhabdoid cells ( = .06), and the presence of immunological divergent differentiation ( = .11) had a comparatively better outcome. Cyclin D1 overexpression (n = 46) was noted in 32 cases and was frequently seen in the posterior fossa tumors ( = .02). (n = 42) amplification was seen in 1 case and the (n = 42) amplification in none. . AT/RT can occur in the noninfantile age group, at nonconventional sites and frequently overexpress cyclin D1. The alterations are almost nonexistent in AT/RT.