Key Points Within MDS/MPN, the WHO 2008 criteria for aCML identify a subgroup of patients with aggressive clinical features distinct from MDS/MPN-U. The MDS/MPN-U category is heterogeneous, and patient risk can be further stratified by a number of clinicopathological parameters.

Summary An increased white blood cell count, or leukocytosis, is a common laboratory finding. Appropriate specimen evaluation depends on which lineages are increased and the morphologic findings on peripheral blood smear review to guide further testing. The presence of blasts is concerning for acute leukemia and may require bone marrow biopsy. Lymphocytosis may be morphologically divided into polymorphic and monomorphic populations. Polymorphic lymphocytosis is most consistent with a reactive process, while monomorphic populations are concerning for lymphoproliferative neoplasm. The differential can be further narrowed based on morphologic findings. Myeloid leukocytosis can occur in a number of reactive conditions as well as myeloid malignancies. The types of cells present and morphology can help to guide additional workup. This study provides guidance for the appropriate evaluation and further workup of leukocytosis.

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The 2008 WHO classification scheme of hematolymphoid neoplasms recognizes a category of myeloid and lymphoid neoplasms (MLNs) with eosinophilia and abnormalities of PDGFRA, PDGFRB, or FGFR1.1 The postulated cell of origin for PDGFRA or FGFR1-rearranged diseases is a pluripotent progenitor capable of giving rise to myeloid neoplasms (myeloproliferative neoplasms and acute leukemias) and to lymphoblastic leukemia/lymphoma. Historically, PDGFRB translocations have not been associated with malignancies of the lymphoid lineage. Myeloid neoplasms with abnormalities of PDGFRB have been described to have hematologic features of chronic myelomonocytic leukemia (CMML), sometimes with eosinophilia, or as various other myeloproliferative and/or myelodysplastic neoplasms.2 In recognition of this distinction, the classification scheme designates specific categories of "MLNs" with PDGFRA and FGFR1 rearrangement, but omits the word "lymphoid" from the PDGFRBassociated entity.1 To call attention to the rare occurrence of lymphoid and mixed MLNs with abnormalities of PDGFRB, we present the clinical and pathological details of 2 cases. Case 1: MLN, eosinophilia, and RABEP1-PDGFRB fusion.A 64-year old man with splenomegaly and diffuse lymphadenopathy (supraclavicular, axillary, mediastinal, paraaortic, retroperitoneal, pelvic) was diagnosed with T-lymphoblastic lymphoma (T-LBL) on a left cervical lymph node (LN) biopsy. A complete blood count (CBC) showed anemia, thrombocytopenia and mild eosinophilia, but no blasts (Table 1). A staging bone marrow (BM) biopsy was abnormal, demonstrating features of a myeloid neoplasm with mixed myeloproliferative/myelodysplastic features but was not involved by T-LBL (Figure 1). He was treated with a vincristine/prednisone-based induction protocol for T-LBL. Following identification of a t(5;17)(q33;p13) in BM and confirmation of PDGFRB rearrangement in the LN and BM samples, imatinib 400 mg/day was added, but stopped after 18 days due to drug intolerance. Based on prior literature, fluorescence in situ hybridization (FISH) studies were subsequently performed that confirmed RABEP1 as the partner gene (Figure 1).