Brachyury expression in intracranial SMARCB1-deficient tumors: important points for distinguishing poorly differentiated chordoma from atypical teratoid/rhabdoid tumor

Loss of SMARCB1 protein expression has recently been identified in a variety of tumor types such as poorly differentiated chordoma (PCh) and malignant rhabdoid tumor (MRT) including atypical teratoid/rhabdoid tumor (AT/RT). PCh is characterized by poorly differentiated epithelioid tumor cells, sheet arrangement, and coexpression of nonepithelial and epithelial markers. Rhabdoid cells are sometimes present. Therefore, the differentiation of these tumors is often difficult. Brachyury is a transcription factor within the T-box family typically expressed in notochord tissue and chordomas. Some studies have reported high specificity and sensitivity of brachyury expression in chordomas. In the present study, we analyzed immunohistochemical brachyury expression in SMARCB1-deficient tumors and discuss important clinicopathological and diagnostic points, especially in cases of intracranial SMARCB1-deficient tumors with brachyury expression. Brachyury and cytokeratin immunoexpression status was examined in 42 formalin-fixed paraffin-embedded SMARCB1-deficient tumor specimens (PCh, 6 cases; extra-central nervous system [CNS] MRT, 26 cases; AT/RT, 10 cases) and 25 cases of conventional chordoma (CCh). All cases of PCh and CCh showed diffuse immunopositivities for cytokeratin 8, pan-cytokeratin, and brachyury. Brachyury immunoexpression was present in 2 extra-CNS MRT (8%) and 5 AT/RT (50%) cases, but immunopositivity was focal not diffuse. Indeed, in almost all cases of AT/RT (cytokeratin 8, 7/10 cases; pan-cytokeratin, 7/10 cases) and extra-CNS MRT (cytokeratin 8, 23/26 cases; pan-cytokeratin, 25/26 cases), fewer than 50% of cells showed immunoreactivity. Although the histological and clinical features of PCh resemble those of AT/RT, semiquantitative evaluations of the degree of brachyury and cytokeratin immunoexpressivity may help to distinguish PCh from AT/RT.