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SAMHD1 is a deoxynucleoside triphosphate triphosphohydrolase (dNTPase) that restricts viral replication in infected cells and limits the sensitivity to cytarabine by hydrolysing its active metabolite, as recently shown in acute myeloid leukemia. Cytarabine is an essential component in the Nordic mantle cell lymphoma protocols (MCL2 and MCL3) for induction and high-dose chemotherapy treatment before autologous stem cell transplantation for younger patients with mantle cell lymphoma (MCL). We here investigated the expression of SAMHD1 in a population-based cohort of MCL (N = 150). SAMHD1 was highly variably expressed in MCL (range, 0.4% to 100% of positive tumor cells). Cases with blastoid/pleomorphic morphology had higher SAMHD1 expression (P = 0.028) and SAMHD1 was also correlated to tumor cell proliferation (P = 0.016). SAMHD1 expression showed moderate correlation to the expression of the transcriptional regulator SOX11 (P = 0.036) but genetic silencing of SOX11 and SAMHD1 by siRNA in MCL cell lines did not suggest mutual regulation. We hypothesized that expression of SAMHD1 could predict short time to progression in patients treated with Cytarabine as part of high-dose chemotherapy. Despite the correlation with known biological adverse prognostic factors, neither low or high SAMHD1 expression correlated to PFS or OS in patients treated according to the Nordic MCL2 or MCL3 protocols (N = 158).

Plasmacytic differentiation may occur in almost all small B cell lymphomas (SBLs), although it varies from being uniformly present (as in lymphoplasmacytic lymphoma (LPL)) to very uncommon (as in mantle cell lymphomas (MCLs)). The discovery of MYD88 L265P mutations in the vast majority of LPLs has had a major impact on the study of these lymphomas. Review of the cases contributed to the 2014 European Association for Haematopathology/Society for Hematopathology slide workshop illustrated how mutational testing has helped refine the diagnostic criteria for LPL, emphasizing the importance of identifying a clonal monotonous lymphoplasmacytic population and highlighting how LPL can still be diagnosed with extensive nodal architectural effacement, very subtle plasmacytic differentiation, follicular colonization, or uncommon phenotypes such as CD5 or CD10 expression. MYD88 L265P mutations were found in 11/11 LPL cases versus only 2 of 28 other SBLs included in its differential diagnosis. Mutational testing also helped to exclude other cases that would have been considered LPL in the past. The workshop also highlighted how plasmacytic differentiation can occur in chronic lymphocytic leukemia/small lymphocytic lymphoma, follicular lymphoma, SOX11 negative MCL, and particularly in marginal zone lymphomas, all of which can cause diagnostic confusion with LPL. The cases also highlighted the difficulty in distinguishing lymphomas with marked plasmacytic differentiation from plasma cell neoplasms. Some SBLs with plasmacytic differentiation can be associated with amyloid, other immunoglobulin deposition, or crystal-storing histiocytosis, which may obscure the underlying neoplasm. Finally, although generally indolent, LPL may transform, with the workshop cases suggesting a role for TP53 abnormalities.

Follicular lymphoma (FL) is a lymphoma composed of germinal center B cells, i.e., centroblasts and centrocytes, that almost always show at least a focal follicular growth pattern. Most cases have a characteristic CD5-, CD10+, BCL6+, and BCL2+ immunophenotype, and 85 % of cases exhibit the hallmark translocation t(14;18)(q32;q21) involving BCL2 and IGH. Although the typical clinicopathological findings of FL are well recognized, cases with unusual clinical, morphologic, immunophenotypic, and genetic features may pose problems in diagnosis and nomenclature. In the slide workshop organized by the European Association for Haematopathology (EAHP) and the Society for Hematopathology (SH) held in Istanbul, Turkey, unusual variants of FL were discussed based on the submitted cases, including early lesions, localized extranodal presentation, uncommon immunophenotype, rare genetic alterations, diffuse variant, and marginal zone differentiation. Interesting features such as blastoid morphology and unusual progression forms were presented, aiming to understand the genetic basis of transformation. In this report, novel findings and diagnostic challenges emerging from the submitted cases will be highlighted, and new terminologies for some of these lesions are proposed.

Mantle cell lymphoma (MCL) is a distinctive lymphoma type generally characterized by the presence of CCND1 translocation and overexpression of cyclin D1. MCL usually presents with advanced stage and rapid clinical progression. The diagnosis is in most instances uncomplicated but cases with variant morphologies or immunophenotypes, especially cyclin D1-negative cases, may cause diagnostic difficulties. During the mantle cell lymphoma (MCL) session at the European Association of Haematopathology/Society for Hematopathology workshop 2014 held in Istanbul, Turkey, submitted cases illustrated interesting features such as unusual morphology or immunophenotypes. In several submitted cases of cyclin D1-positive MCL, CCND1 rearrangement could not be detected by t(11;14)(q13;q32) dual-color dual-fusion FISH but was suggested by CCND1 break-apart probes, and advantages and disadvantages of different FISH probes were highlighted. Three cyclin D1-negative MCL cases were submitted. These were identified by SOX11 immunohistochemistry and found to carry CCND2 translocations and/or to express high levels of cyclin D2 mRNA. Features associated with aggressive clinical course were presented including high expression of p53 protein and MYC aberrations. The need to integrate histological, immunophenotypic, genetic, and clinical data to arrive at the correct diagnosis was emphasized.

Indolent lymphomas in the pediatric population were discussed during the 2014 European Association for Haematopathology/Society of Hematopathology workshop in Istanbul, Turkey. This session was focused on pediatric-type follicular lymphoma (FL), and its differential diagnosis with the newly recognized entity of IRF4/MUM1+ lymphomas mainly involving Waldeyer's ring. The differential diagnosis between t(14;18) negative FL grade 1/2 and pediatric-type FL in adults was highlighted. The overlapping pathological and clinical features between FL and nodal marginal zone lymphoma (NMZL) in children and young adults were recognized and morphologic and immunophenotypical criteria helpful for the differential diagnosis were presented. Both pediatric-type FL and NMZL are indolent processes that should be distinguished from atypical lymphoid hyperplasia of the tonsils and lymph nodes. The demonstration of a B cell monoclonal population by molecular studies is strongly recommended for the diagnosis. Recognition of these indolent variants to avoid overtreatment was emphasized. Whereas most indolent lymphomas in the pediatric population show characteristic clinical, pathologic, and genetic features that differ from the adult counterpart, other rare indolent lymphoid tumors such as chronic lymphocytic leukemia (CLL) have similar characteristics. In this report, novel findings, areas of special interest, and diagnostic challenges emerging from the cases submitted to the workshop will be discussed.