The workshop jointly organized by the EAHP and the SH in 2008 in Bordeaux focused on the difficult topic of gray-zone lymphomas. In the 2008 WHO classification, these borderline categories were added mainly to delineate the borderlands between BL and DLBCL and DLBCL and HL. As emerged from the discussions during the workshop—among the panel members, the auditorium, and the authors of the WHO classification—several issues were considered important for the proper use of these borderline categories. The main two issues were that all information—cytomorphologic, biologic, and clinical—should be considered to arrive at an accurate diagnosis, and second, whenever possible, distinct entities should be diagnosed. It was agreed, however, that this was not possible in all cases, thus justifying the creation of borderline diagnoses (unclassifiable), looked upon as practical categories awaiting for more data to become available and not as discrete entities.
Session 1 was devoted to the possible overlap between BL and DLBCL. Specifically, this overlap is caused by similarities in morphology, phenotype, and genetic aberrations. During the session, it became clear that the “double-hit” cases (a combination of a MYC breakpoint with mostly BCL2 breakpoints and other recurrent chromosomal breakpoints), often with distinct morphological features of BL should fall in a novel category of “B cell lymphoma, unclassifiable, with features intermediate between DLBCL and BL.” In fact, there was some discussion whether or not B cell lymphomas, irrespective of morphology, should be moved to this novel category, but at the end it was felt that this change would be premature. The second most difficult category that remains in the gray zone consists of lymphomas with morphological features of both DLBCL and BL, but with a phenotype that is consistent with BL, in particular since up to 40% of such cases may have a MYC breakpoint. Many of such cases might fall in this novel category; however, it was felt that cases with a typical morphology of DLBCL should be kept in the DLBCL category, in spite of the fact that DLBCL patients with a MYC breakpoint will have a relatively poor prognosis.
Session 2 was devoted to mediastinal gray-zone lymphoma and other lymphomas with atypical immunophenotype that caused differential diagnostic problems, often between CHL and NHL. During the discussion, it was stressed that deviation of a single marker or criterion, e.g., strong and uniform CD20 expression in otherwise typical HL, was not sufficient for placing a case in the category of “B cell lymphoma, unclassifiable, with features intermediate between DLBCL and CHL,” but that a hybrid phenotype not being in line with current morphologic and/or biologic concepts would well be. Examples of those features could be a significant mononuclear large B cell component not compatible with HL or uniformly strong expression of pan B cell markers along with the expression of one or more of the transcription factors BOB.1, OCT2, and PU.1 in a mediastinal tumor with morphological features of CHL. This stresses the need of doing a complete phenotype (B cell markers and transcription markers) in cases with morphological features of CHL with strong and uniform CD20 expression.
Among the tumors with unusual phenotypes presented during this session were DLBCL with cyclin D1 overexpression and cases with aberrant expression of T-cell-associated markers. Also, in this group, several cases of T cell lymphomas with CD30 and/or CD15 expression including ALCL cases were shown.
Session 3 dealt with the differential diagnosis between NLPHL and THRLBCL, and the gray zones between CHL and DLBCL, mainly THRLBCL in lymph nodes. The main issue addressed during the workshop was to define criteria to reliably distinguish these entities. Within the first topic, it was considered important that NLPHL can manifest in various progression stages, being characterized by diffuse areas and by progressive loss of the typical small B cell background. While it was agreed that transition of NLPHL into THRLBCL might exist, the proposal of the WHO classification to name areas reminiscent of THRLBCL in an otherwise typical NLPHL background “THRLBCL-like areas” was generally considered a practical guideline. It implies that one single nodule of typical NLPHL in a THRLBCL background is sufficient to exclude de novo THRLBCL. Other important issues highlighted during the workshop were the reactivity of the LP cells for IgD in the progressed cases or the rare expression of CD15. In addition, three bona fide NLPHL showed the presence of EBV genomes in their nuclei, a feature considered unusual for NLPHL and rather characteristic of CHL.
Several cases submitted to the workshop exemplified the difficulties in accurately separating cases of CHL from THRLBCL due to overlapping clinical, morphological, and/or immunophenotypic features. In general, CHL is negative or weakly positive for B cell antigens and PAX5 and does show loss of transcription factors expression; however, deceptively similar cases may be placed in the THRLBCL category, when CD15 and/or CD30 are negative and CD20 and/or CD79a are strongly and uniformly positive. Similar to the mediastinal gray zone, cases with CHL and DLBCL boundary features also exist in lymph nodes. The cases presented during the workshop showed an amazing variety and overlapping immunohistochemical features, thus giving clear evidence for the need of a borderline category. In addition, the frequent association of these cases with EBV also discloses overlapping features with (THRBCL-like) EBV-associated DLBCL of the elderly in young patients.
Session 4 dealt with the heterogeneous lymphoma categories of EBV-positive lymphomas, lymphomas occurring in HIV+ individuals, and PTLD-related B cell lymphoproliferations. This session exemplify how meticulous diagnostic assessment of the cases is needed in arriving at the correct diagnosis. Clinical background information, and the exact knowledge of the sites involved, is absolutely essential in providing a correct diagnosis. EBV-associated lymphoproliferations such as posttransplant lymphoproliferative disorders, body-cavitary-based or extracavitary PEL or HIV-associated or lymphoproliferations following iatrogenic measurements cannot be diagnosed without proper clinical information. On the other hand, on purely morphological grounds, these cases might have been put into a different category. With respect to EBV association, the positivity of a DLBCL for CD30 should encourage performing EBER ISH or LMP-1 immunohistochemistry.
In the workshop, EBV+ DLBCL-E showed regular CD79a expression and frequent CD20 positivity, usually with strong PAX5, BOB.1, and OCT-2 staining, thus setting it apart from EBV-associated CHL. Among lymphomas occurring in HIV+ individuals, plasmablastic lymphomas and BL were the two categories represented. Two cases were consistent with the diagnosis of extracavitary PEL and, most interestingly, were diagnosed in immunocompetent hosts, a finding rarely encountered in elderly individuals from areas with high prevalence for HHV8 infection like the Mediterranean. In keeping with what has been stated above, the PTLD cases referred to the workshop often presented with hybrid morphological and immunophenotypic features intermediate between CHL and DLBCL; since two of the PTLD monomorphic DLBCL were EBV negative, clinical information is essential in arriving at the correct diagnosis.
Examples of lymphomatoid granulomatosis, EBV-associated lymphoproliferative disorders in the course of immunosuppressive hematological malignancies, cases of DLBCL associated with chronic inflammation, and other virus-associated lymphoproliferations completed the spectrum of diseases seen in session 4. For the latter category, two instructive cases were presented, both with an initial suspicion of malignant lymphoma but turning out to represent virus-associated reactive lymphoproliferations, one associated with EBV and one with HHV6. These difficult cases again underline the risk of misinterpreting such reactive disorders as DLBCL, if the clinical and laboratory data are unknown and/or a limited panel of antibodies is applied.