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Poorly differentiated chordoma with whole‐genome doubling evolving from a SMARCB1‐deficient conventional chordoma: A case report

Evolution of poorly differentiated chordoma from conventional chordoma has not been previously reported. We encountered a case of a poorly differentiated chordoma with evidence of whole‐genome doubling arising from a SMARCB1‐deficient conventional chordoma. The tumor presented as a destructive sacral mass in a 43‐year‐old man and was comprised of a highly cellular poorly differentiated chordoma with small, morphologically distinct nodules of conventional chordoma accounting for <5% of the total tumor volume. Immunohistochemistry (IHC) revealed both components were strongly reactive for brachyury and lacked normal staining for INI1. Single nucleotide polymorphism (SNP) array analysis identified multiple genomic imbalances in the conventional component, including deletions of 1p, 3p, and 22q (involving SMARCB1) and loss of chromosomes 5 and 15, while the poorly differentiated component exhibited the same aberrations at a more profound level with additional loss of chromosome 4, low level focal deletion of 17p (involving TP53), and tetraploidy. Homozygous deletion of SMARCB1 was present in both components. Fluorescence in situ hybridization (FISH) analysis confirmed the relevant deletions in both components as well as genome doubling in the poorly differentiated tumor. This case suggests that SMARCB1 loss is an early event in rare conventional chordomas that could potentially evolve into poorly differentiated chordoma through additional genomic aberrations such as genome doubling. Further studies with additional patients will be needed to determine if genome doubling is a consistent pathway for evolution of poorly differentiated chordoma.