Low-Grade Follicular Lymphoma with Highly Unusual Flow Cytometric, FISH, Molecular, and Classical Cytogenetic Results: A Case Report

We present a low-grade follicular lymphoma case with highly unusual ancillary
study results. The patient is a 50-year-old female with a 4.3 x 3.8 x 2.2 cm left inguinal
mass, from which fine needle aspiration and excisional biopsy specimens were obtained.
H&E sections showed effacement of the lymph node architecture by closely packed
follicles comprised of mainly centrocytes and without tingible body macrophages.
Immunohistochemical stains revealed the follicle cells positive for CD20, PAX5, BCL6,
CD10 (weak), and BCL2 (weak), while CD3 highlighted small T lymphocytes mainly in
the interfollicular areas.


Introduction
Follicular Lymphoma (FL) is the second most common Non-Hodgkin Lymphoma (NHL) with the highest incidence in Western countries [1,2]. The diagnosis of FL is commonly based on the morphology as well as immunophenotyping with immunohistochemical staining and flow cytometry, and sometimes molecular studies. Follicular and diffuse patterns are recognized with a varying proportion of each that may be seen in a lymph node.
The most predominant pattern is follicular with closely packed follicles that efface the nodal architecture. The FL cells usually express Surface Immunoglobulin (sIg), B cell antigens, BCL2, BCL6, and CD10, and are negative for CD5 and CD43. Genetically, FL is usually characterized by the translocation t (14;18) (q32; q21), especially in low-grade FL. Clonality detection by PCR analysis of immunoglobulin heavy and light chains may provide further support for the diagnosis. Of interest, the demonstration of sIg light chain restriction for monoclonality is not always achieved in malignant B cell NHL. Although it is rare, sIg-negative NHL has been reported and lack of sIg light chain expression may help diagnose peripheral B cell lymphoma [3,4] We report a case of a lymph node involved by low-grade FL without surface or cytoplasmic light chain expression, negative Ig heavy chain clonal rearrangement by PCR, negative IgH/BCL2 translocation by FISH, and an abnormal karyotype of 46,XX,-12,+mar [6].  Flow cytometry demonstrated an identical result to that of the previous fine needle aspiration. Morphologically, numerous follicles were closely packed, lacked mantle zones, and showed a back-to-back pattern at low power ( Figure 1A), consistent with follicular pattern of FL. CD23 highlighted the follicular dendritic cell meshwork throughout ( Figure 1B). At high power, mainly centrocytes and few centroblasts were distributed in the follicles and tingible body macrophages were absent ( Figure 1C).

Case report
Immunohistochemical stains showed the follicle cells positive for CD20, BCL6, CD10 (weak), and BCL2 (weak) (Figures 1D-1K), while CD3 highlighted small T lymphocytes mainly in the interfollicular areas ( Figure 1H) and sparsely in the follicular areas ( Figure 1I).  translocation was not detected in this case by FISH even though BCL2 expression was identified by immunohistochemistry. The translocation t(14;18)(q32;q21) is a genetic hallmark of FL, but its presence in FL depends on the technique used. Even with the most sensitive FISH assay, it has been reported that BCL2 translocation can be detected in 64% to 100% of FL [6,7].
The overexpression of BCL2 protein can be present or absent in FL without t (14;18). In a study of 49 FL cases without t (14;18), BCL2 protein overexpression was present in 33% of the cases [8].
In another study involving 164 FL cases, 17 cases were identified to be without t (14;18), among which 6 cases were positive for BCL2 at the protein level [9]. Interestingly, FL with and without t (14;18) showed distinct genetic features, gene expression and hampering PCR primer annealing. It was found in a study that only 86% of 109 definitive FL cases demonstrated clonality at the IGH locus, while 100% of these cases showed clonality when IGH locus PCR assessment was combined with IGK and/or IGL PCR [11].
Furthermore, immunoglobulin heavy chain rearrangement was not detected among 29% of the sIg negative B cell lymphoma cases [3].
Also, this case demonstrated a cytogenetic abnormality of 46, XX, -12, +mar [6]. This case represents the first known reported case with this constellation of findings.

Conclusion
The case presented here is that of a low-grade FL with the following highly unusual ancillary testing results: lack of surface or cytoplasmic immunoglobulin light chain expression, negative immunoglobulin heavy chain rearrangement by PCR, and negative t (14;18) by FISH. While the H&E is typical for a low-grade FL with a follicular growth pattern, it is highly unusual for all the studies to be negative, coupled with the abnormal cytogenetic finding of 46, XX, -12, +mar [6].