Relapsed acute promyelocytic leukemia lacks "classic" leukemic promyelocyte morphology and can create diagnostic challenges

Objectives: Although current therapies for acute promyelocytic leukemia (APL), such as all-trans retinoic acid and arsenic trioxide, usually result in remission, some patients relapse. Early recognition of relapse is critical for prompt intervention. In this study, we systematically reviewed morphologic, immunophenotypic, and cytogenetic findings in paired diagnostic and relapsed APL cases and describe and quantify the changes in blast morphology at relapse. Methods: By electronic database search, we identified eight paired diagnostic and relapsed APL cases for which peripheral blood or bone marrow smears were available for review. For two cases, diagnostic material was available for relapse after hematopoietic cell transplantation. Results: Neoplastic hypergranular or microgranular promyelocytes with indented or bivalve nuclei predominated at diagnosis in all patients. Most patients had undifferentiated blasts at relapse and/or hypergranular blast equivalents with round to oval nuclei. Classic acute promyelocytic leukemia cells with bivalve nuclei and bundles of cytoplasmic Auer rods were easily identifiable in fewer than half of cases at diagnosis and rare to absent in all relapsed cases. Conclusions: Morphologic features of relapsed APL overlap with other types of acute myeloid leukemia, creating diagnostic challenges, especially if no history is available when relapsing patients seek treatment for care.