An unusual morphological variant of follicular lymphoma. Report of two cases

A rare face of follicular lymphoma: reverse variant of follicular lymphoma

Background

Reverse Variant of Follicular Lymphoma (RVFL) is one of the rare morphological variants of FL, characterized by dark staining small centrocytes in the center and pale staining large centroblasts at the periphery of the neoplastic follicles. Only rare cases of RVFL have been described to date. The histological appearance of this little known variant of FL may be misinterpreted if pathologists are unaware of its existence. The main purpose of this study is to draw pathologists’ attention to such an uncommon growth pattern of FL so that this variant can be correctly recognized and the clinical significance further studied in the future.

Methods

Four cases of FL with unusual morphologic features were evaluated for the expression pattern of CD20, CD10, BCL6, BCL2, CD21, CD23, CD3, CD5, Cyclin D1, IgD and Ki67 by immunohistochemistry. Fluorescence in situ hybridization (FISH) with break-apart probes was performed to detect BCL2 gene rearrangement.

Results

All four cases showed distinctive morphologic pattern of RVFL; in addition, each also exemplified unique morphological features. Immunohistochemical stains confirmed the cells in both the central areas and the peripheral cuffs had the same immunophenotypic profiles, contrasting to the FL with marginal zone differentiation in which only the center of the nodules showed expression of CD10. FISH demonstrated BCL2 gene rearrangement in all cases.

Conclusion

The growth pattern of this rare FL variant may mimic FL with marginal-zone differentiation and other entities including but not limited to marginal zone lymphoma (MZL), progressive transformation of germinal centers (PTGC) and nodular lymphocyte predominant Hodgkin lymphoma (NLPHL). Pathologists should be familiar with this unusual morphological variant to avoid diagnostic pitfalls.

Follicular lymphoma subgrouping by fluorescence in situ hybridization analysis

The frequency of t(14;18) in follicular lymphoma (FL) in Japan has been reported to be low compared to North America and other European countries. Recently, it has also been reported that FL lacks t(14;18), mainly among histological grade 3b, and occasionally has a rearranged Bcl-6 gene. It is not known whether a difference in histology or immunostaining pattern exists between FL with and without t(14;18). We performed interphase fluorescence in situ hybridization (FISH) analysis to detect Bcl-2/IgH, Bcl-6 gene rearrangement, Bcl-2 gene amplification, and the cyclinD1/IgH gene in formalin-fixed paraffin embedded specimens from our FL archives. The correlation between morphological features, histological grades, immunohistochemical findings, and cytogenetical aberrations was studied. In total, we found that 28 of 47 cases (59.6%) had t(14;18). Bcl-6 gene rearrangement and extra Bcl-2 gene signals were found in five and two cases, respectively. Only one had cyclinD1/IgH fusion. Ten of 12 grade 1, nine of 17 grade 2, and 0 of two grade 3 cases had fusion signals, respectively. None of the above abnormalities were detected in 12 of 47 cases (25.5%). Our data confirmed a high frequency of t(14;18) in FL in grade 1, but a lower incidence among grade 2, that could be attributed to the lower incidence of the translocation in FL in Japan. Immunostaining of both Bcl-2 and CD10 was highly predictable for the presence of t(14;18); the positive predictive value was 75%, suggesting the usefulness of the staining.