# CNS Tumor with BCOR/BCORL1 Fusion: A Rare Tumor Entity

## TL;DR

A rare CNS tumor involving BCOR/BCORL1 fusion is described, with a new case showing successful treatment using radiation and temozolomide.

## Contribution

The paper presents a new case of a rare CNS tumor with BCOR/BCORL1 fusion and highlights a novel treatment approach.

## Key findings

- A 37-year-old woman with a BCOR/BCORL1 fusion tumor was successfully treated with radiation and temozolomide.
- EMA showed focal strong dot-like perinuclear immunoreactivity, a feature not previously reported in these tumors.
- The case supports the use of radiation and temozolomide for tumors in the BCOR/BCORL1 fusion methylation class without detectable fusion.

## Abstract

Central nervous system (CNS) tumor with BCL6 corepressor gene BCOR/BCORL1 fusion is an extremely rare tumor entity, with fewer than 40 cases reported. These tumors are distinct from the WHO 2021-defined CNS tumor with BCOR internal tandem duplication. Even rarer are CNS tumors that match to the methylation class of CNS tumors with BCOR/BCORL1 fusion, but lack fusions and instead harbor truncating small nucleotide variants in BCOR. To our knowledge, only two other cases of this scenario have been previously reported. Due to their scarcity and morphological features that mimic oligodendrogliomas and ependymomas, the diagnosis of CNS tumor with BCOR/BCORL1 fusion can be challenging, and misdiagnoses are not uncommon. Histologic findings of Olig2 positivity with focal to absent GFAP warrant further evaluation for this tumor entity. Moreover, no standard of care therapy exists for these tumors, making treatment selection difficult. We present a case of a 37-year-old woman with a midline CNS tumor with BCOR/BCORL1 fusion, harboring a pathogenic BCOR c.626del (p.S209Cfs*7) (Exon 4) variant, who was successfully treated with definitive radiation therapy and adjuvant temozolomide. Notably, EMA showed focal strong dot-like perinuclear immunoreactivity, which has not been previously reported in these tumors. This case adds to the limited but growing body of evidence supporting the use of radiation and temozolomide in treating tumors matching the methylation class of CNS tumors with BCOR/BCORL1 fusion without a detectable fusion.