LAUSR

OBJECTIVES Rare cases of clonally related histiocytic sarcoma (HS) following B-lymphoblastic leukemia/lymphoma (B-ALL/LBL) have been reported to date. METHODS We present a patient with HS, which appeared as a breast mass 12 months after the initial diagnosis of B-ALL. RESULTS Both HS and the B-ALL shared IGH-MYC and IGK gene rearrangements. Next-generation sequencing and whole-exome sequencing (WES) studies detected 35 common mutations, as well as mutations unique to B-ALL (16) and HS (15), including BRAF D594G. The patient achieved complete remission of B-ALL, but HS failed to respond to many cycles of intensive chemotherapy regimens. A partial response was achieved with sorafenib, a BRAF-targeted therapy. CONCLUSIONS To our knowledge, this is the first study to demonstrate by WES that clonally related B-ALL and HS arise through divergent evolution from a common precursor. We present our findings together with a discussion of the previously reported cases of HS in patients with B-ALL.