Dedifferentiation of B-lymphoblastic leukemia/lymphoma with t(9;22) BCR::ABL1 to an undifferentiated neoplasm with strong keratin expression in a patient receiving blinatumomab

Objectives: Lineage switch in patients with B-ALL treated with blinatumomab has been previously reported. We present a case of blinatumomab-treated B-ALL with t(9;22)(q34.1;q11.2) (Ph + B-ALL) relapsing as a keratin-positive undifferentiated neoplasm with lack of B-cell, T-cell, or definitive myeloid antigens and persistent BCR::ABL1. Results: A 47-year-old male with Ph + B-ALL (p190 fusion transcript) experienced early relapse and received multi-agent therapy including blinatumomab and achieved complete remission. Eleven months after initiation of treatment, he presented with a chest wall mass. Biopsy showed an immature- appearing neoplasm without lineage-specific differentiation, but with expression of keratin and retention of the BCR::ABL1. Subsequently, bone marrow relapse with keratin positivity and BCR::ABL1 was identified. The findings were most compatible with dedifferentiation and aberrant keratin expression of Ph + B-ALL. Conclusions: We present, to our knowledge, the first reported case of an acute leukemia which relapsed as a dedifferentiated neoplasm and the second reported case of lymphoblastic leukemia with aberrant keratin expression in a patient with Ph + B-ALL and treatment including blinatumomab.