CNS Low-grade Diffusely Infiltrative Tumors With INI1 Deficiency, Possessing a High Propensity to Progress to Secondary INI1-deficient Rhabdoid Tumors

Atypical teratoid/rhabdoid tumors (AT/RTs) are highly malignant tumors of the central nervous system that predominantly occur in infants, and are characterized by the presence of rhabdoid cells and inactivation of INI1 or (rarely) BRG1. Most AT/RT are identified as primary tumors; however, rare AT/RT or INI1-deficient RTs arising from other primary tumors have been reported. Here, we report 3 cases of hitherto unclassifiable low-grade tumors with loss of INI1 nuclear expression, for which we propose the designation of central nervous system low-grade diffusely infiltrative tumors with INI1 deficiency (CNS LGDIT-INI1), 2 of which progressed to secondary RT. All 3 CNS LGDIT-INI1 exhibited a similar histology: diffusely distributed small tumor cells with round to oval or irregular nuclei and scant cytoplasm were admixed with degenerative neurons and large reactive astrocytes in an edematous, myxoid, or collagenous background. Mitotic figures were absent. Immunohistochemistry revealed that the tumor cells in all 3 CNS LGDIT-INI1 and 2 RT were negative for INI1. Genetically, total or partial homozygous deletions of the INI1 gene were detected in all CNS LGDIT-INI1 and RT excluding 1 CNS LGDIT-INI1 without sufficient DNA quality and quantity. Despite the loss of INI1 expression, these low-grade lesions were clearly distinguishable from AT/RT by their low proliferative activity, diffusely infiltrative growth pattern, and lack of rhabdoid cells and polyphenotypic immunoreactivity. In conclusion, CNS LGDIT-INI1 may represent a rare group of tumors that are clinically indolent but have a high propensity to progress to RT.

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.