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

Objectives. 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 MYC gene amplification. Methods. 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 MYC genes. Results. 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 (P = .02), rarely had ample rhabdoid cells (P = .05), and had a negative impact on overall survival (P = .04). The rhabdoid cells was a conspicuous component of posterior fossa tumors compared with the supratentorial ones (P = .06). The supratentorial tumors (P = .06), absence of rhabdoid cells (P = .06), and the presence of immunological divergent differentiation (P = .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 (P = .02). CMYC (n = 42) amplification was seen in 1 case and the NMYC (n = 42) amplification in none. Conclusion. AT/RT can occur in the noninfantile age group, at nonconventional sites and frequently overexpress cyclin D1. The MYC alterations are almost nonexistent in AT/RT.