Primary lymphomatous presentation of hairy cell leukemia as osteolytic vertebral lesions: a case report

Hairy cell leukemia (HCL) is a rare and indolent mature B cell neoplasm. Most patients with HCL have excellent response to purine analogs and BRAF inhibitors. Therefore, it is clinically important to identify and differentiate HCL from other B cell neoplasms. HCL is predominantly found in the peripheral blood, bone marrow, or spleen. HCL presenting initially or solely in unusual sites is extremely rare (approximately ten or fewer). We hereby report a unique case of HCL presenting as vertebral osteolytic and epidural mass lesions without bone marrow, spleen, or peripheral blood involvement. The patient was a 53-year-old male with acute and chronic radicular pain down the posterior aspect of bilateral thighs. Imaging study with MRI demonstrated a L4 posterior vertebral osteolytic mass lesion, extending into the spinal canal, and a second right sacral S1 lesion extending into the sacral iliac joint. These two noncontiguous lesions were thought to be consistent with metastatic cancer or sarcoma, and the specimen was submitted to non-hematopoietic pathologist for diagnosis without flow cytometry. Initial immunohistochemical stains were ordered for solid tumor markers, which were all negative. As part of the workup, immunostain for BRAF V600E (VE1) was ordered for melanoma, and CD138 and cyclin D1 for myeloma, which returned positive and suggested the possibility of hairy cell leukemia. Additional workup for hairy cell leukemia confirmed the diagnosis. HCL typically presents with peripheral blood, bone marrow, or spleen involvement, while mass lesions in other sites are rare. High index of suspicion is essential for the correct diagnosis of HCL in mass lesions at unusual sites. In the absence of initial hematopathological workup and flow cytometry, HCL presenting as solid mass in unusual locations is misdiagnosed most likely as marginal zone B cell lymphoma (mucosa-associated lymphoid tissue (MALT) lymphoma). The expression of cyclin D1, TRAP, annexin A1, and BRAF V600E mutation would confirm the diagnosis of HCL. The main differentials include MALT lymphoma, mantle cell lymphoma (MCL), and hairy cell leukemia variant (HCLv). This case report will be interesting for the clinicians and pathologists alike and broaden the spectrum of clinical presentation of lymphomatous HCL.