Case history
A 64-year-old, previously healthy man presented with a history of cervical and axillary lymphadenopathy of unknown duration. He did not endorse night sweats, weight loss, or fever. Radiologic examination (CT chest and MRI abdomen) revealed numerous enlarged mediastinal, peritracheal, periaortic, periportal and retroperitoneal lymph nodes. He underwent excisional biopsy of a 3.5 cm axillary lymph node.
Microscopic Description
Histologic examination of the node revealed distortion of nodal architecture by a proliferation of neoplastic-appearing follicles. Follicles were distinct from one another, and closely packed. In areas the follicles were present back-to-back. Follicular centers were comprised of mostly small, cleaved centrocytes and showed no obvious zonation. There was loss of tingible body macrophages.
Immunophenotyping
Immunohistochemical analysis revealed CD20-positive B cells in a follicular pattern. The germinal centers revealed an underlying follicular dendritic meshwork highlighted by staining for CD21. Interestingly, while the germinal centers demonstrated immunopositivity for BCL-6, there was minimal to absent CD10 staining on follicular B cells. Analysis of BCL-2 staining revealed only few cells to be positive within the follicular centers, consistent with resident follicular helper T cells (Th cells). Equivalent numbers of CD3 and CD5 positive T cells were noted in the interfollicular zones. The Ki-67 proliferation index was estimated at 15-20% within follicular centers. Flow cytometric phenotyping demonstrated a lambda light chain restricted clonal B-cell population expressing CD20, CD19 and, FMC7. These neoplastic B-cells were negative for CD5 and CD10 expression.
Diagnosis
The morphologic features were consistent with Follicular Lymphoma; however the phenotype (BCL-2 negativity in follicular centers) was unusual for this diagnosis. Fluorescence in situ hybridization (FISH) was negative for an IgH/BCL-2 fusion; however, a BCL-6 rearrangement at the 3q27 locus was detected in 70% of the cells. Taken together, a diagnosis of Follicular Lymphoma with a BCL-6 rearrangement was given.
Discussion
Follicular lymphoma (FL) is a germinal center derived B-cell neoplasm. The majority of cases exhibit the pathognomonic translocation t(14; 18)(q32; q21). This translocation leads to overexpression of the anti-apoptotic BCL-2 protein, which can be detected by immunohistochemistry on germinal center B cells. Lymphoma cells are usually positive for germinal center origin markers BCL-6 and CD10 and do not co-express CD5. As exhibited in this case, FL can exhibit biologic heterogeneity and may not express these typical markers. The follicular proliferation with absence of germinal center zonation and tingible body macrophages as seen in this case represents classic morphology of follicular lymphoma but aberrant phenotypic markers [and absence of t(14;18)] may be a pitfall in this diagnosis.
FL with lacking of CD10 expression, BCL-2 expression, and t(14;18) translocation and harboring only BCL-6 positivity with 3q27 rearrangement is rare. Only few such cases have been reported in the literature. Published data reveals that the hallmark t(14;18) translocation is absent in about 10-15% of FL. The majority of these cases are negative for BCL-2 expression, and 9-14% of them demonstrate BCL-6 rearrangement (3q27 locus). While BCL-6 rearrangement can be present in both the usual t(14;18) harboring FL, and also in cases without t(14;18), the latter is rare. Interestingly, studies have shown BCL-6 rearrangements to be more frequent in in BCL-2 rearrangement negative FL – which is evidence of the anti-apoptotic role of non-rearranged BCL-6 in certain microenvironments.
One third of t(14;18) negative FL are also reported to have rare or negative expression of CD10. Morphologically, this subtype has been shown to have significantly larger follicles than their t(14;18)-positive counterparts, but the distinction may not be obvious in all cases. Some of these cases are shown to have a component of monocytoid B cells. This findings can be problematic in differentiating these FL cases from marginal zone lymphoma (MZL) that can also harbor BCL-6 rearrangements and lack t(14;18), CD10 and BCL-2 positivity. Absence of prominent marginal zone proliferation, BCL-6 protein expression and characteristic genetic alterations present in MZL, such as trisomies 3, 7, and 18 can help differentiating MZL from t(14;18)-negative FL.
This case highlights the importance of morphologic evaluation of a excisional biopsy tissue, and FISH studies to help identify the rare t(14;18) negative FL. While the reported cases are few, there is no published difference in prognosis or survival when compared to t(14;18)-positive FL. As such, it is not clear whether the follicular lymphoma grading scheme applies to t(14;18)-negative FL; however, no significant grading difficulties or differences have been reported.
References
- Jardin F, Gaulard P, Buchonnet G, et al. Follicular lymphoma without t(14;18) and with BCL-6 rearrangement: a lymphoma subtype with distinct pathological, molecular and clinical characteristics. Leukemia. 2002;16:2309–2317
2. Leich E, Salaverria I, Bea S, et al. Follicular lymphomas with and without translocation t(14;18) differ in gene expression profiles and genetic alterations. Blood. 2009;114(4):826-834.
–Aadil Ahmed, MD is a 3rd-year anatomic and clinical pathology resident at Loyola University Medical Center. Follow Dr. Ahmed on Twitter @prion87.
-Kamran M. Mirza, MD PhD is an Assistant Professor of Pathology and Medical Director of Molecular Pathology at Loyola University Medical Center. He was a top 5 honoree in ASCP’s Forty Under 40 2017. Follow Dr. Mirza on twitter @kmirza.
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