Working Diagnosis:
Hairy Cell Leukemia

Treatment:
The patient was immediately started on chemotherapy with five days of cladribine followed by weekly maintenance with rituximab for 8 weeks.

Following chemotherapy, the patient started physical and occupational therapy to strengthen the right elbow, rotator cuff, and scapular stabilizers.

Outcome:
Four months after his initial diagnosis of hairy cell leukemia, the patient was confirmed to be in remission by a repeat bone marrow biopsy and negative findings in both CT abdomen and pelvis and PET scans. The patient was able to return to work without major functional deficits or pain. Three months after the negative biopsy, the patient was independent in all activities without pain. He returned to work as a manager.

Author's Comments:
With recurrent symptoms, it was imperative to pursue advanced imaging. With atypical imaging findings that do not align with the history, it was paramount to expand the differential diagnosis to include less common diagnoses.

Editor's Comments:
As the author stated, it is important to expand the differential diagnosis and consider additional imaging when patients come to the clinic with an atypical presentation or when symptoms worsen or fail to respond to treatment. Hairy Cell Leukemia (HCL) is a rare type of chronic B-cell malignancy. HCL only involves the skeletal system in 3% of cases. When there is bone involvement, patients will typically present with lytic lesions in the femoral head or neck. It is rare for patients to present with musculoskeletal pain in the absence of systemic symptoms though HCL can present with isolated skeletal disease. MRI is the most sensitive imaging modality for examining these lytic lesions. Patients should be referred to oncology for further workup and treatment, which can include radiation and systemic treatments (2).

References:
1. Paillassa, J., and Troussard, X. (2020). Biology and treatment of hairy cell leukemia. Current Treatment Options in Oncology, 21(6), 44–0. https://doi.org/10.1007/s11864-020-00732-0.
2. Robak, P., Jesionek-Kupnicka, D., Kupnicki, P., Polliack, A., and Robak, T. (2020). Bone lesions in hairy cell leukemia: Diagnosis and treatment. European Journal of Haematology, 105(6), 682–691. https://doi.org/10.1111/ejh.13505.

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