Splenic B-Cell Lymphomas/Leukemias

Follicular lymphoma and marginal zone lymphoma: how many diseases?

Follicular lymphoma (FL) and marginal zone lymphoma (MZL) are indolent mature B-cell neoplasms with variable clinical presentation and distinct histopathologic features. Recent advances in the biology and molecular characteristics of these lymphomas have further expanded our understanding of the heterogeneous nature of these lymphomas, with increasing recognition of specific disease entities within the broader categories of FL and MZL. Here, we discuss the conclusions of the 2022 International Consensus Classification of Mature Lymphoid Neoplasms (2022 ICC) dealing with FL, and review differences with the proposed WHO 5th Edition classification. We review issues related to grading and alternative forms of FL especially those lacking the genetic hallmark of FL, the t(14;18) chromosomal alteration. Among them, t(14;18)-negative CD23⁺ follicle center lymphoma has been proposed by the 2022 ICC as a provisional entity. Other follicle center-derived lymphomas such as pediatric-type follicular lymphoma, testicular follicular lymphoma, primary cutaneous follicle center lymphoma, and large B-cell lymphoma with IRF4 rearrangement are considered distinct entities separate from conventional FL. Importantly, large B-cell lymphoma with IRF4 rearrangement introduced as a provisional entity in the WHO 2017 is upgraded to a definite entity in the 2022 ICC. We also discuss diagnostic strategies for recognition of MZLs including splenic MZL, extranodal MZL (MALT lymphoma), and primary nodal MZL. The importance of molecular studies in the distinction among marginal zone lymphoma subtypes is emphasized, as well as their value in the differential diagnosis with other B-cell lymphomas.

Immunohistochemistry for BRAF V600E in the Differential Diagnosis of Hairy Cell Leukemia vs Other Splenic B-Cell Lymphomas

OBJECTIVES

Recent reports have used immunohistochemistry (IHC) with a mutation-specific antibody to detect the BRAF V600E mutation, which is found in nearly all cases of hairy cell leukemia (HCL). To date, however, only a small number of non-HCL, splenic B-cell lymphomas have been examined by IHC.

METHODS

We analyzed 121 cases, including 26 HCLs, 52 non-HCL splenic lymphomas, 22 chronic lymphocytic leukemias/small lymphocytic lymphomas (CLLs/SLLs), and 21 plasma cell neoplasms (PCNs) for BRAF V600E expression by IHC. Molecular testing for BRAF V600E was performed in a subset of cases, using allele-specific polymerase chain reaction and/or Sanger sequencing.

RESULTS

Twenty-six (100%) of 26 HCL cases were positive by IHC vs one (1%) of 95 non-HCL cases. Positive staining was identified in one (2%) of 44 splenic marginal zone lymphomas (SMZLs), while each of 22 CLLs/SLLs, 21 PCNs, six unclassifiable splenic lymphomas, and two HCL variants were negative. IHC and molecular results were concordant in all cases examined (21 HCLs and 21 non-HCLs, including the BRAF+ SMZLs).

CONCLUSIONS

The detection of BRAF V600E by IHC is useful in the distinction of HCLs from other splenic-based lymphomas, although the identification of at least rare SMZLs containing this abnormality illustrates the continuing need for a multiparameter approach to diagnosis.