Primitive myxoid mesenchymal tumor of infancy (PMMTI) is a rare fibroblastic-myofibroblastic malignancy that typically arises in the first year of life. Alaggio et al1 originally described PMMTI as being characterized by primitive spindle, polygonal, and round cells in a myxoid background with positive vimentin staining immunohistochemically, but negative for smooth muscle actin, muscle specific actin, desmin, myogenin, and S100 expression. Recently, internal tandem duplication (ITD) of BCOR has been detected in PMMTI,2 a molecular alteration identical to the hallmark feature found in most clear cell sarcoma of the kidney tumors (CCSK).3–6 The clinical course of PMMTI is often indolent, with multiple recurrences reported in some patients. Treatment strategies have focused on surgical resection alone when feasible. No chemotherapy regimen has been established as a reliably effective standard of care for unresectable PMMTI tumors. Importantly, most patients with PMMTI with clinical follow-up reported to date (n=19) were described as alive with disease (n=10), dead of disease (n=1), or dead with disease (n=1) at most recent follow-up. Hence, the morbidity and mortality of this rare tumor type in young children is potentially immense.2,7 This report describes an infant with a recurrent PMMTI with confirmed BCOR ITD that responded completely to multiagent chemotherapy, similar to regimens used successfully for CCSK.
Dr. Ali has disclosed that he is an employee of and has equity in Foundation Medicine. The remaining authors have disclosed that they have no financial interests, arrangements, affiliations, or commercial interests with the manufacturers of any products discussed in this article or their competitors.
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