Peripheral Eosinophilia camouflaging anaplastic large cell lymphoma

Eosinophilia is a nonspecific laboratory finding, often noted incidentally during routine blood analysis. When persistent, eosinophilia can herald an underlying parasitic infection, drug reaction or less commonly, a neoplastic process. Anaplastic large cell lymphoma (ALCL) and tissue eosinophilia has been described; however, such cases have not displayed marked leukocytosis with eosinophilia. This article reports a patient presenting with marked leukocytosis with profound peripheral eosinophilia initially thought to be related to a chronic myeloproliferative disorder, likely chronic eosinophilic leukemia. After further diagnostic evaluation, ALCL was noted in the bone marrow, masked by the myeloid hyperplasia and eosinophilia. This case emphasizes the importance of a full diagnostic workup for T-cell malignancies, including ALCL rather than focusing on the far less common eosinophilia-associated myeloid malignancies in the clinicopathologic setting of marked eosinophilia. Moreover, bone marrow involvement by ALCL is exceedingly rare and when noted, presents as one or more localized lytic lesions. This is the first reported case of ALCL primarily involving bone marrow without radiographic evidence of lytic bone lesions.