BCOR alterations are described in ultra-rare infantile soft tissue sarcomas including primitive myxoid mesenchymal tumor of infancy and undifferentiated round cell sarcoma (URCS). Previous reports often describe dismal outcomes. Thus, we undertook a retrospective, multi-institutional study of infants with BCOR -rearranged soft tissue sarcomas. Nine patients aged 6 weeks to 15 months were identified. One tumor carried a BCOR :: CCNB3 fusion, whereas 7 tumors harbored internal tandem duplication of BCOR , including 4 cases classified as primitive myxoid mesenchymal tumor of infancy, 1 case as URCS, and 2 cases characterized by a "hybrid morphology" in our evaluation. Four patients underwent upfront surgery with residual disease that progressed locally after a median of 2.5 months. Locoregional recurrences were observed in hybrid patients, and the URCS case recurred with brain metastases. Complete radiographic responses after chemotherapy were achieved in patients treated with vincristine/doxorubicin/cyclophosphamide alternating with ifosfamide/etoposide, vincristine/doxorubicin/cyclophosphamide alternating with cyclophosphamide/etoposide (regimen I), and ifosfamide/carboplatin/etoposide. Seven patients received radiotherapy. With a median of 23.5 months off therapy, 8 patients are with no evidence of disease. In our study, observation was inadequate for the management of untreated postsurgical residual disease. Tumors demonstrated chemosensitivity with anthracycline-based regimens and ifosfamide/carboplatin/etoposide. Radiotherapy was required to achieve durable response in most patients.
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