follicular lymphoma follicular lymphoma yasmin

Yasmin

1. 55 years old female, presented with axillary lump

2. Diagnosis Follicular lymphoma: WHO 4: grade 1-2 WHO 5: Classical ICC: Grade 1-2

3. In-Situ Follicular B-Cell Neoplasm Essential and desirable diagnostic criteria Essential : - Variable numbers of B-cells within germinal centres staining intensely for BCL2. - Maintained lymph node or extranodal lymphoid tissue architecture, and lacking features of classic FL . Desirable : - Strong CD10 expression in the BCL2-positive B-cells within the follicles.

4. In-situ follicular B-cell neoplasm (H&E ×100). This photomicrograph illustrates strong aberrant reactivity for BCL2 in two germinal centers showing a slightly stronger staining than the surrounding marginal zone B- and T-cells. The overall nodal architecture is well preserved.

5. In-Situ Follicular B-Cell Neoplasm The new term of In-situ follicular B-cell-neoplasm (ISFN) in WHO-HAEM5 is equivalent to in-situ follicular neoplasia in WHO-HAEM4R and the ICC proposal . ISFN is defined as a partial or complete colonization of rare or some reactive germinal centers (GCs) by follicular (CD10+) B-cells that carry the IGH::BCL2 fusion and display strong expression of BCL2. It is rare, as a rule fortuitous, finding in lymph nodes or lymphatic tissues that have been removed for other reasons. In larger series, it has been described in 2% of lymph nodes . In the vast majority of cases, no subsequent FL will develop; however, in some patients, staging procedures reveal concurrent established FL or other types of lymphoma such as mantle cell lymphoma or chronic lymphocytic leukemia . Only a small number of patients will ever develop FL or other types of lymphoma subsequent to the diagnosis of ISFN.

6. Histologically, the condition may be suspected owing to the monotonous appearance of a few or a number of GCs that are mainly harboring centrocytes . However, these changes are easily overlooked and ISFN is usually recognized only upon BCL2 staining revealing the partial or complete colonization of GCs by strongly BCL2 positive B cells . As a rule, CD10 expression—in contrast with reactive follicles—is also unusually strong.

7. Conceptually, ISFN maybe be viewed as the first recognizable tissue manifestation of circulating t(14;18)-positive FL-like B-cells that already have acquired additional mutations. The condition is invariably t(14;18) positive and clonal relationship has been shown between circulating IGH::BCL2 fusion-positive cells and ISFN in the same patient. In addition, mutations in CREBBP, KMT2D und TNFRSF14, deletions in 1p36, and acquisition of N-glycosylation sites have been reported. Cases of FL that are preceded by an ISFN have acquired additional mutations to those already present in the ISFN such as in EZH2 . Up to now, no t(14;18)-negative ISFN has been reported.

8. A schematic overview on the evolution of follicular lymphoma. The t(14;18) arose in pre- Bcells during failures in VDJ joining and leads to the generation of long-lived FL-like B-cells. Via several re-entries into germinal centers, these t(14;18)-positive B-cells acquire additional mutations enabling them to be founders of ISFN or manifest lymphoma. Orange cells and green stars represent T-cells and follicular dendritic cells, respectively. Under physiological conditions germinal center B-cells undergo apoptosis, while this process is inhibited in FL (marked by X).

9. Classic FL This is a typical example of classic FL (FL grade 1/2 according to WHO-HAEM4R) with a predominance of centrocytes (red arrows) and only a few interspersed centroblasts (black arrows). (B) In this example, the number of large, transformed cells (centroblasts, black arrows) In this example, the number of large, transformed cells (centroblasts, black arrows) is distinctly higher (>15 centroblasts per high-power-field (FL grade 3A according to WHO-HEAM4R), however, centrocytes (red arrows) are still present.

10. Classic follicular lymphoma Essential and desirable diagnostic criteria Essential : B-cell lymphoma composed of varying proportions of centrocytes (CC) and/or centroblasts (CB)/large transformed cells, with the dominance of CC in the overwhelming majority of cases. Immunophenotype compatible with germinal center B-cell origin with positivity to markers such as CD10, BCL6, MEF2B, GCET1, GCET2 or LMO2 Desirable : At least partly follicular growth pattern BCL2

11. Follicular lymphoma with unusual cytological features (H&E ×400). In this example of a neoplasm growing in follicular structures, there is a predominance of small to medium-sized blastoid cells with round nuclei, finely dispersed chromatin and in part small nucleoli. Note the increased number of mitotic figures. This case had a proliferation index of 80%.

12. Low magnification shows a preserved subcapsular rim of preserved reactive lymphatic tissue to the left and an ill-defined lymphomatous infiltration to the right (H&E 40).

13. Higher magnification reveals an entirely diffuse growth pattern (H&E 100).

14. There is a clearly discernible follicular growth pattern (H&E ×40)

15. On high magnification, the neoplastic follicles are composed of large, transformed cells (centroblasts) exclusively, without an admixture of (typical) centrocytes (H&E 400).

16. Prerequisite: Subtyping applies only to lymphomas already identified as FL. The reliability of subtyping is highly dependent on the size/ representativity of the biopsy and should be performed on surgical biopsies for optimal accuracy. 2. Prerequisite: specific clinicopathological variants of FL must be excluded (e.g. PTFL, DTFL, TFL) 3. At least 1 follicle is required on the section (identified either morphologically or using CD21/CD23) to use the algorithm . See text for grading small biopsies with no follicular component and classic FL cytology , 4. See text for centrocyte (CC) definition. CCs must be differentiated from reactive T-cells using good quality sections. Immunostainings Including CD3 and CD20 may be helpful . “Mixture” implies a significant number of unequivocal CCs. 5. The lack of typical CC component renders MUM1 testing mandatory, at least by immunohistochemistry ( and subsequent FISH in case of positivity) to exclude IRF4-DLCL 6. Uncommon cytology includes – ( i ) predominant component of small to medium sized cells with immature or blastoid chromatin (and/or obvious nucleoli) – (ii) predominant component of large cleaved cells (so called “large centrocytes ”) i . e. large cells with irregular nuclei and immature or blastoid chromatin. 7. The term “Follicular large B-cell lymphoma“ (FLBCL) is justified if follicles are composed almost entirely of large cells (CB and/or immunoblasts). The definite diagnosis of FLBCL requires – ( i ) IHC testing for Ki67 and MUM1 (often highly expressed ), as well as FISH testing (frequent BCL6R and/or MYCR and/or lack of BCL2R) - (ii) exclusion of concurrent DLBCL by careful sampling. 8. Optional descriptive features include: - ( i ) Morphology : e. g. “with large centrocytes “ - (ii) Phenotype/proliferation: e.g. “CD10 negative “, “ with high proliferative index“ –(iii) Genetics: e.g. “with/without BCL2 (or BCL6) translocation , and/or 1p36 deletion“; “double hit profile” 9. Counting in 10 neoplastic follicles, expressed per high-power (40x magnification, 0.159 mm2) microscopic field (HPF), with at least 10 HPFs within different follicles evaluated). It is recommended to present the result using a threshold of 90 CB/ 1 mm2 (equivalent to 15 CB/ 0.159 mm2). Follicles should be representative, not those with the most numerous large cells

17. Pediatric-type follicular lymphoma (PTFL) (H&E 100). In PTFL, there is an intriguing proliferation of large follicles, often lacking mantle zones, and that are large and expansile , often with a “ serpiginous ” growth pattern.

18. Pediatric-type follicular lymphoma (PTFL) Essential and desirable diagnostic criteria Essential - Paediatric and young adult age group (usually age <40 years, most 2-25 years) - Localized nodal disease - Purely follicular growth with marked architectural distortion and germinal center marker expression - Predominance of intermediate to large-sized ‘ blastoid’cells and high proliferation fraction - Absence of diffuse proliferation of large cells meeting criteria for DLBCL - Evidence of B-cell monoclonality by immunophenotyping or genetics - Absence of BCL2 , BCL6 , and MYC rearrangements - Absence of strong, uniform MUM1 expression and/or absence of IRF4 rearrangement Desirable - Markedly expansile follicles - Mutations in MAP2K1 and TNFRSF14

19. Duodenal-type follicular lymphoma (H&E ×40). Low magnification shows large, atypical nodules/follicles occupying the mucosa and extending into the upper part of the submucosa in this duodenal resection specimen. There is an absence of polarization of the follicles that are composed nearly exclusively of small centrocytes .

20. Duodenal-type follicular lymphoma Essential and desirable diagnostic criteria Essential : Germinal centre B-cell lymphoma with tumour cells confined predominantly to the mucosa of the intestine, and characterized by follicles composed predominantly of centrocytes , and with positivity for germinal center markers and BCL2. Desirable : Exclusion of secondary involvement

21. Primary cutaneous follicle center lymphoma (H&E 20). This example shows atypical follicular structures occupying the dermal portions of the skin underneath an intact epidermis.

22. Primary cutaneous follicle center lymphoma Essential and desirable diagnostic criteria Essential: - Follicular and/or diffuse proliferation of centrocytes and admixed centroblasts (diffuse lymphomas comprising exclusively centroblasts / immunoblasts are excluded) - B cells with co-expression of germinal center markers (BCL6 and/or CD10 or other germinal centre markers) - No extracutaneous involvement by lymphoma Desirable: - Localization to head or trunk - Evidence of B-cell monoclonality - Absent or weak BCL2 expression (usually) - Lack of MUM1 expression - Lack of BCL2 rearrangement (usually)