Pathology, Clinical Features, and Natural History of Low-Grade B-Cell Lymphomas With Primary Osseous Presentation.

Primary bone lymphomas are rare, and most reported cases represent diffuse large B-cell lymphoma. Low-grade B-cell lymphomas presenting with primary osseous involvement (PB-LGBCLs) are far less common, and their clinicopathologic features and clinical course remain poorly defined. We retrospectively analyzed 32 PB-LGBCLs diagnosed at a single tertiary cancer center. Cases were included when the disease presented with bone-only involvement, with or without contiguous soft tissue extension, and without lymph node, spleen, or noncontiguous extranodal disease at initial diagnosis. Staging marrow and low-level peripheral blood involvement were recorded but were not used as exclusion criteria. The cohort included follicular lymphoma (FL; 22/32, 69%), marginal zone lymphoma (MZL; 6/32, 19%), lymphoplasmacytic lymphoma (LPL; 2/32, 6%), and chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL; 2/32, 6%). Median age was 64 years (range: 39 to 87), and 25/32 patients (78%) had multifocal skeletal disease, most often involving the pelvis and spine. Primary bone FL frequently showed features typical of conventional FL, including germinal center B-cell phenotype, frequent BCL2 rearrangement, and recurrent FL-associated mutations. Non-FL subtypes showed clinicopathologic features and molecular findings compatible with their systemic counterparts. Among 27 patients with more than 6 months of follow-up, disease progression or transformation occurred in 12 patients. Three FLs transformed to diffuse large B-cell lymphoma. No deaths occurred. PB-LGBCLs are rare with FL as the most common subtype and show heterogeneous biology and clinical behavior. Recognition of this presentation is important to avoid misclassification as aggressive lymphoma, nonhematolymphoid bone neoplasm, or reactive condition.