Primary Bone Diffuse Large B-cell Lymphoma (DLBCL) in a Young Adult: A Case Report.

Primary bone diffuse large B-cell lymphoma (DLBCL) is a rare subtype of extranodal non-Hodgkin lymphoma that predominantly involves long bones, particularly the metaphyseal regions. We report the case of a 23-year-old man presenting with painful swelling of the right knee. Imaging revealed an osteolytic lesion of the distal right femur with cortical destruction and soft tissue extension. Staging contrast-enhanced computed tomography of the chest, abdomen, and pelvis showed no nodal or visceral involvement, consistent with localized (limited-stage) disease. Histopathological examination of the biopsy specimen demonstrated diffuse sheets of large atypical lymphoid cells, and immunohistochemical analysis confirmed DLBCL with a non-germinal center B-cell phenotype. The patient received six cycles of R-CHOP chemotherapy (rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone), resulting in rapid clinical improvement and complete metabolic remission on post-treatment positron emission tomography, consistent with a complete response. This case underscores the importance of including lymphoma in the differential diagnosis of destructive bone lesions in young adults and highlights the favorable prognosis of localized primary bone DLBCL with appropriate chemoimmunotherapy.