Hairy cell leukemia presenting as localized skeletal involvement

We report a 45-year-old man who presented with localized skeletal involvement as the initial manifestation of hairy cell leukemia (HCL) without abnormal peripheral blood counts, splenomegaly or posterior iliac crest bone marrow involvement. The patient presented with pain in the left thigh. A plain radiograph was normal, but a magnetic resonance imaging (MRI) of this region showed a marrow-based lesion occupying the left femur neck, left proximal femur and both greater trochanters. Histological and immunophenotypic examination revealed a focal infiltrate of HCL. Skeletal involvement by HCL without co-existing bone marrow involvement should be included in the differential diagnosis of bone marrow lesions where metastatic tumor is the foremost consideration.