A Rare Case of Acute Promyelocytic Leukemia With Focal Bone Marrow Involvement Presenting as a Paraspinal Myeloid Sarcoma

Acute promyelocytic leukemia (APL) accounts for 5–8% of acute myeloid leukemias (AMLs). Myeloid sarcomas (MS) are rare extramedullary tumors consisting of immature myeloid cells. These tumors may develop de novo or concurrently with AML and may precede or coincide with the occurrence of AML. The occurrence of MS in an APL patient is a rare clinical event. In these cases, it usually occurs as a sign of relapse. Focal bone marrow involvement by APL is also rare; the majority of cases of APL demonstrate a diffusely hypercellular bone marrow throughout the axial skeleton due to expansion of neoplastic blasts/promyelocytes. We describe a 23-year-old female who presented with gait problems, including multiple falling episodes, and lower extremity weakness. An MRI demonstrated a paraspinal soft tissue mass. Histopathologic analysis demonstrated a myeloid sarcoma expressing CD45, myeloperoxidase, CD117, BCL2, focal weak CD34, and weak CD99 and negative for CD30, TdT, CD20, CD3, and CD10. The bone marrow aspirate demonstrated blasts/promyelocytes accounting for approximately 26% of the total bone marrow aspirate cellularity. The bone marrow biopsy and clot section demonstrated normocellularity with a few blast/promyelocyte foci on the clot section accounting for approximately 5% of total bone marrow cellularity. These foci expressed MPO and CD117 and were negative for CD34. Flow cytometry demonstrated blasts accounting for approximately 31% of all CD45+ events expressing MPO, CD33, CD13, CD117, CD64, CD2, CD38, and CD71 and negative for CD34, HLA-DR, CD11b, CD14, CD15, CD16, CD19, CD22, CD10, CD56, CD61, and surface and cytoplasmic CD3. Fluorescence in situ hybridization (FISH) analysis was positive for the t(15;17)(q22;q12) promyelocytic leukemia/retinoic acid receptor alpha (PML/RARA) translocation. The patient was induced with arsenic trioxide and subsequently underwent consolidation chemotherapy with daunorubicin and all-trans-retinoic acid. Her post-chemotherapy bone marrow biopsy was negative for residual disease. Flow cytometry and FISH analysis were also negative. This is a rare case of APL involving both the initial presentation as myeloid sarcoma and focal bone marrow involvement.