Orbital Sarcoma with <italic>BCOR</italic> Genetic Alterations in the Pediatric Age Group.

Abstract<bold>Introduction:</bold> Pediatric orbital tumors encompass a wide spectrum of neoplasms, many of which are malignant small round cell tumors with overlapping histology. Sarcomas with <italic>BCOR</italic> genetic alterations are undifferentiated round cell sarcomas (URCS) characterized by <italic>BCOR</italic> rearrangements or internal tandem duplications, having distinct clinical features. Being previously unrecognized in the orbit, they have potential for misdiagnosis. <bold>Patients:</bold> We describe two cases of orbital sarcomas with <italic>BCOR</italic> genetic alterations. <bold>Results:</bold> Both girls, 8 and 16 months of age, respectively, presented with progressive proptosis. Both tumors showed sheets of round to ovoid cells with monomorphic nuclei and frequent mitoses. Delicate branching capillaries and myxoid stroma were absent. Diffuse BCOR, cyclin D1, and SATB2 immunopositivity was present. <bold>Conclusion:</bold> Orbital sarcomas with BCOR genetic alterations are extremely rare. Pathologists should have high index of suspicion for novel genetically defined entities in the differential diagnosis of pediatric orbital URCS and perform appropriate ancillary tests for accurate diagnosis. [ABSTRACT FROM AUTHOR]