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T-follicular helper (TFH) markers are expressed in the microenvironnement of marginal zone B-cell lymphoma (MZL), and in lymphomas arising from TFH-cells, sometimes making the differential diagnosis difficult. In the skin, the "TFH-spectrum" is poorly defined, going from primary cutaneous lymphoproliferative disorder with small/medium CD4+ T-cells (SMLPD) to cutaneous localizations of systemic angioimmunoblastic T-cell lymphoma (cAITL), and may pass through intermediate forms (primary cutaneous T-follicular helper derived lymphoma, not otherwise specified (PCTFHL,NOS)). We retrospectively analyzed 20 MZL, 13 SMLPD, 5 PCTFHL, and 11 cAITL clinically, histologically, and molecularly, to define tools to differentiate them. Characteristics that might favor the diagnosis of MZL over SMLPD are: multiple skin nodules (p < 0.001), nodular architecture (p < 0.01), residual germinal centers with follicular dendritic cell network (p < 0.001), monotypic plasma cells (p < 0.001), and few staining with PD1 (p = 0.016) or CXCL13 (p = 0.03). PCTFHL and cAITL presented as multiple (p < 0.01) lesions, in older patients (p < 0.01), with systemic symptoms and/or biological alterations (p < 0.01). Immunophenotypic loss of T-cell markers (p < 0.001), BCL6 (p = 0.023) and/or CD10 staining (p = 0.08), and a higher proliferative index (≥ 30%, p = 0.039) favoured these diagnoses over SMLPD. Pathogenic variants were observed by genomic sequencing in 47% of MZL (TNFAIP3 (32%), EP300 (21%), NOTCH2 (16%), KMT2D (16%), CARD11 (10.5%)), 8% of SMLPD (TET2), 40% of PCTFHL (SOCS1 (20%), ARID1A (20%)) and 64% of cAITL (TET2 (63.6%), RHOA (36.4%), NOTCH1 (9%)). This study characterizes the various clinical and histological features between cutaneous lymphomas expressing TFH markers and highlights the value of the interest of screening for genomic mutations in difficult cases., (© 2023. The Author(s).)

Background: Primary cutaneous peripheral T-cell lymphomas with a T-follicular helper phenotype (pcTFH-PTCL) are poorly characterized, and often compared to, but not corresponding with, mycosis fungoides (MF), Sézary syndrome, primary cutaneous CD4 + lymphoproliferative disorder, and skin manifestations of angioimmunoblastic T-cell lymphomas (AITL)., Objectives: We describe the clinicopathological features of pcTFH-PTCL in this original series of 23 patients, and also characterize these cases molecularly., Methods: Clinical and histopathological data of the selected patients were reviewed. Patient biopsy samples were also analysed by targeted next-generation sequencing., Results: All patients (15 men, eight women; median age 66 years) presented with skin lesions, without systemic disease. Most were stage T3b, with nodular (n = 16), papular (n = 6) or plaque (atypical for MF, n = 1) lesions. Three (13%) developed systemic disease and died of lymphoma. Nine (39%) patients received more than one line of chemotherapy. Histologically, the lymphomas were CD4 + T-cell proliferations, usually dense and located in the deep dermis (n = 14, 61%), with the expression of at least two TFH markers (CD10, CXCL13, PD1, ICOS, BCL6), including three markers in 16 cases (70%). They were associated with a variable proportion of B cells. Eight patients were diagnosed with an associated B-cell lymphoproliferative disorder (LPD) on biopsy, including Epstein-Barr virus (EBV)-positive diffuse large B-cell lymphoma (n = 3), EBV + LPD (n = 1) and monotypic plasma cell LPD (n = 4). Targeted sequencing showed four patients to have a mutated TET2-RHOAG17V association (as frequently seen in AITL) and another a TET2/DNMT3A/PLCG1/SETD2 mutational profile. The latter patient, one with a TET2-RHOA association, and one with no detected mutations, developed systemic disease and died. Five other patients showed isolated mutations in TET2 (n = 1), PLCG1 (n = 2), SETD2 (n = 1) or STAT5B (n = 1)., Conclusions: Patients with pcTFH-PTCL have pathological and genetic features that overlap with those of systemic lymphoma of TFH derivation. Clinically, most remained confined to the skin, with only three patients showing systemic spread and death. Whether pcTFH-PTCL should be integrated as a new subgroup of TFH lymphomas in future classifications is still a matter of debate. What is already known about this topic? There is a group of cutaneous lymphomas that express T-follicular helper (TFH) markers that do not appear to correspond to existing World Health Organization diagnostic entities. These include mycosis fungoides, Sézary syndrome, or primary cutaneous CD4 + small/medium-sized T-cell lymphoproliferative disorder or cutaneous extensions of systemic peripheral T-cell lymphomas (PTCL) with TFH phenotype. What does this study add? This is the first large original series of patients with a diagnosis of primary cutaneous PTCL with a TFH phenotype (pcTFH-PTCL) to be molecularly characterized. pcTFH-PTCL may be a standalone group of cutaneous lymphomas with clinicopathological and molecular characteristics that overlap with those of systemic TFH lymphomas, such as angioimmunoblastic T-cell lymphoma, and does not belong to known diagnostic groups of cutaneous lymphoma. This has an impact on the treatment and follow-up of patients; the clinical behaviour needs to be better clarified in further studies to tailor patient management., (© 2022 The Authors. British Journal of Dermatology published by John Wiley & Sons Ltd on behalf of British Association of Dermatologists.)

Mycosis fungoides (MF) and primary cutaneous CD30-positive T-cell lymphoproliferative disorders (CD30LPD) are the most frequent primary cutaneous T-cell lymphomas. Our objective was to study pan-T-cell antigens and PD-1 expression in a large cohort of MF and CD30LPD with a special interest in antigen losses as a diagnostic tool. We retrospectively reviewed 160 consecutive samples from 153 patients over a 3 year period, including 104 with MF and 49 with CD30LPD. As controls, benign inflammatory dermatoses (BID, n=19) were studied. A semi-quantitative evaluation of CD2, CD3, CD4, CD5, CD7, CD8 expression was performed. PD-1 and double stainings (CD3+CD7 and PD-1+CD7) were performed in a subset of MF cases. CD8+ MF was frequent (23%) and CD7 was the most frequently lost antigen in both MF (45%) and CD30LPD (86%), while no significant T-cell antigen loss was observed in BID. CD7 loss was less frequent in folliculotropic MF (p<0.001). PD-1 was variably expressed in MF with no differences with BID. The CD3+/CD7- and PD-1+/CD7- neoplastic lymphocytes were highlighted by the use of chromogenic double staining experiments in MF with a CD7 loss identified with single staining. Multiple pan T-cell antigen losses were mostly seen in CD30LPD with CD2 being the most frequently preserved marker (90%). While PD-1 does not discriminate between MF and BID, CD7 is frequently lost in MF infiltrates as well as other pan-T-cell antigens in CD30LPD, which can be used as routine markers for diagnosis. We recommend the use of CD7 in addition to CD3, CD4 and CD8 as a minimal immunohistochemical panel for MF assessment, and the use of double stainings for CD3 and CD7 in difficult cases., (Copyright © 2022 Royal College of Pathologists of Australasia. Published by Elsevier B.V. All rights reserved.)

Among dermatologic adverse events induced by immune checkpoint inhibitors (ICI), bullous life-threatening reactions are rare. To better define the clinical and histological features, treatment, and prognosis of ICI-related severe blistering cutaneous eruptions. This retrospective case series was conducted between 2014/05/15 and 2021/04/15 by the dermatology departments of four international registries involved in drug reactions. Inclusion criteria were age ≥18 years old, skin eruption with blisters with detachment covering ≥1% body surface area and at least one mucous membrane involved, available pictures, and ICI as suspect drug. Autoimmune bullous disorders were excluded. Each participant medical team gave his own diagnosis conclusion: epidermal necrolysis (EN), severe lichenoid dermatosis (LD), or unclassified dermatosis (UD). After a standardized review of pictures, cases were reclassified by four experts in EN or LD/UD. Skin biopsies were blindly reviewed. Thirty-two patients were included. Median time to onset was 52 days (3-420 days). Cases were originally diagnosed as EN in 21 cases and LD/UD in 11 cases. After review by experts, 10/21 EN were reclassified as LD/UD. The following manifestations were more frequent or severe in EN: fever, purpuric macules, blisters, ocular involvement, and maximal detachment. Most patients were treated with topical with or without systemic corticosteroids. Eight patients (25%) died in the acute phase. The culprit ICI was not resumed in 92% of cases. In three patients, another ICI was given with a good tolerance. Histology did not reveal significant differences between groups. Severe blistering cutaneous drug reactions induced by ICI are often overdiagnosed as EN. Consensus for management is pending., (Copyright © 2022 Wolters Kluwer Health, Inc. All rights reserved.)

Purpose: To prospectively assess the impact of expert pathological review of skin adnexal carcinoma diagnosis in France., Methods: From 2014 to 2019, 2573 samples from patients with newly diagnosed or suspected skin adnexal carcinomas were reviewed prospectively by expert pathologists through the national CARADERM (CAncers RAres DERMatologiques) network. Changes in diagnosis between referral and expert review were analysed regarding their potential impact on patient care or prognosis., Results: The samples comprised 2205 newly diagnosed adnexal carcinomas, 129 benign adnexal tumours, 136 basal cell carcinomas, 74 squamous cell carcinomas, six cutaneous metastases and 13 other malignancies. There were 930 (42%) sweat gland carcinomas, of which porocarcinoma (261; 11.8%), microcystic adnexal carcinoma (125; 5.7%) and hidradenocarcinoma (109; 4.9%) were the most frequent subtypes; 778 (35%) hair follicle carcinomas, 238 (11%) sebaceous carcinomas and 212 (10%) extramammary Paget diseases/mammary-like anogenital gland adenocarcinomas. A diagnostic change between referral and expert review occurred in 503 (21.3%) patients, significantly higher for cases sent with a provisional diagnosis seeking an expert second opinion (45.7%) than for cases sent with a formal diagnosis (2.8%) (p < .0001). Sweat gland carcinomas were more prone to diagnostic discrepancies than other tumours (p < .0001), including 1.8% of patients with sweat gland carcinoma subtype misclassification with predicted clinical impact. Changes between benign and malignant conditions occurred in 117 samples (5% of patients)., Conclusion: The study provides a unique description of the distribution of skin adnexal carcinomas and highlights the importance of expert review for these rare cancers. Optimal clinical management was impacted in a significant proportion of patients., Competing Interests: Conflict of interest statement The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Background: Lymphomatoid papulosis (LyP) type D (LyP D) and type E (LyP E) have recently been described in small series of cases or isolated case reports., Aim: To further describe the clinical and histological features of LyP D and E based on a retrospective multicentre study., Methods: The clinical and histopathological features of 29 patients with an initial diagnosis of LyP D or LyP E were retrospectively assessed using standardized forms., Results: After exclusion of 5 cases, 24 patients (14 LyP D, 10 LyP E) were enrolled in the study. The median follow-up was 2.5 years (range 1 month to 13 years). LyP D was characterized by multiple recurrent self-regressing small papules that developed central erosion or necrosis, whereas LyP E presented as papulonodular lesions that rapidly evolved into necrotic eschar-like lesions > 10 mm in size. Epidermal changes were more frequent in LyP D, whereas dermal infiltrates were deeper in LyP E. Anaplastic cytology was rare and the DUSP22 rearrangement was never observed. Two patients (8%) had an associated cutaneous lymphoma., Conclusion: LyP D and E have distinct clinical findings and may be associated with other cutaneous lymphomas., (© 2021 British Association of Dermatologists.)

Background: Primary cutaneous lymphomas (PCLs) are a heterogeneous group of T-cell (CTCL) and B-cell (CBCL) malignancies. Little is known about their epidemiology at initial presentation in Europe and about potential changes over time., Objectives: The aim of this retrospective study was to analyse the frequency of PCLs in the French Cutaneous Lymphoma Registry (GFELC) and to describe the demography of patients., Methods: Patients with a centrally validated diagnosis of primary PCL, diagnosed between 2005 and 2019, were included., Results: The calculated incidence was unprecedently high at 1·06 per 100 000 person-years. The number of included patients increased yearly. Most PCL subtypes were more frequent in male patients, diagnosed at a median age of 60 years. The relative frequency of rare CTCL remained stable, the proportion of classical mycosis fungoides (MF) decreased, and the frequency of its variants (e.g. folliculotropic MF) increased. Similar patterns were observed for CBCL; for example, the proportion of marginal-zone CBCL increased over time., Conclusions: Changes in PCL frequencies may be explained by the emergence of new diagnostic criteria and better description of the entities in the most recent PCL classification. Moreover, we propose that an algorithm should be developed to confirm the diagnosis of PCL by central validation of the cases., (© 2020 British Association of Dermatologists.)

Malignant Triton tumour (MTT) is a subtype of malignant peripheral nerve sheaths tumour (MPNST) with exclusive heterologous rhabdomyosarcomatous contingent. MTT is rare and of poor prognosis. This entity illustrates the great heterogeneity of MPNST, the diagnosis of which is difficult in the absence of a specific marker, especially in sporadic forms. Although MTT preferentially develop in patients with type 1 neurofibromatosis, sporadic cases may occur. We herein present a case of MTT of the left arm, occurring in a 74-year-old patient, without clinical context of NF1. The fast-growing tumour reached 9.2cm of greater dimension at the time of surgical excision. Histology showed a spindle cell sarcoma with rhabdoid cell areas expressing myogenin. In the absence of neural crest markers expression, the diagnosis of MPNST was based on a significant loss of expression of the histone 3 tri-methylated lysine 27, a classical although not specific epigenetic mark for this sarcoma group, and on the identification of the heterologous rhabdomyosarcomatous contingent, previously described in the context of MTT., (Copyright © 2021 Elsevier Masson SAS. All rights reserved.)

After a first diagnosis proposition, management of cutaneous lymphomas requires a systematic review by an expert pathologist and each case is presented to a multidisciplinary meeting in the setting of the French Study Group of Cutaneous Lymphomas to propose an adequate treatment. A retrospective study of the 2760 cutaneous lymphoproliferations retrieved between 2010 and 2011 were analyzed and demonstrated the interest of diagnostic algorithms we built with the group. The objective of our study was to compare two cohorts from 2010-2011 and 2015-2017 regarding the proportion of cases sent for validation or expertise, the concordance and mismatch rates and potential diagnostic issues using our diagnostic algorithms. Between 2015-2017, 5640 skin lymphoproliferation cases were examined. It appeared that Pathologists were more confident and effective in finding the right diagnosis. Indeed, the rate of concordant diagnosis increased from 57% to 67%. Moreover, in comparison with the 2010-2011 concordant cases sent for expertise, 73.5% of concordant cases were sent for validation in 2015-2017. 14% of cases remained discordant, mainly sent for expertise. Furthermore, half of questionable cases (26.3%) were resolved after expertise, and 12.1% cases remained unsolved. These priority cases are important to be presented at multidisciplinary meeting. The analysis of discordant and doubtful cases unveiled recurrent diagnostic problems for which we proposed appropriate diagnostic algorithms including large B cell lymphomas, CD4+ T cell lymphoproliferations, epidermotropic CD8+ T-cell lymphoproliferations and the differential diagnosis of mycosis fongoïdes/Sezary syndrome versus inflammatory dermatitis., (Copyright © 2021 Elsevier Masson SAS. All rights reserved.)

The treatment of advanced-stage cutaneous T-cell lymphoma (CTCL) remains an unmet medical need. Mogamulizumab, anti-KIR3DL2, and brentuximab vedotin (BV), an anti-CD30 antibody-drug conjugate (ADC) coupled with monomethyl-auristatin-E (MMAE), provided encouraging results, but new targeted therapies are needed. Inducible T-cell costimulator (ICOS), a T-cell costimulatory receptor, is a promising therapeutic target, not only because it is expressed by malignant T cells in CTCL but also because of its connection with the suppressive activity of regulatory T (Treg) cells. Immunohistochemical analysis revealed that ICOS was widely expressed by malignant cells in skin biopsy specimens from 52 patients with mycosis fungoides and Sézary syndrome (SS), as well as in involved node biopsy specimens from patients with SS. Furthermore, flow cytometry demonstrated its strong expression by circulating tumor cells in all our patients with SS. Percentages of ICOS+ Treg cells were significantly higher in patients with SS than in healthy donors. We then investigated the preclinical efficacy of anti-ICOS ADCs generated by coupling murine anti-ICOS monoclonal antibodies with MMAE and pyrrolobenzodiazepine. In 3 CTCL cell lines (Myla, MJ, and HUT78), we observed a significant dose-dependent decrease in cell viability in the presence of anti-ICOS ADCs. In addition, anti-ICOS-MMAE ADCs had an in vitro and in vivo efficacy superior to BV in a mouse xenograft model (MyLa). Finally, we assessed the efficacy of anti-ICOS ADCs in ICOS+ patient-derived xenografts from patients with SS and angioimmunoblastic T-cell lymphoma. Collectively, our findings provide the preliminary basis for a therapeutic trial., (© 2020 by The American Society of Hematology.)

Primary cutaneous CD4 small/medium T-cell lymphoproliferative disorder (PCSMLPD) is a recently recognized entity in the 2017 World Health Organization (WHO) classification. It belongs to the T-follicular helper (TFH) lymphoproliferations. The clinical, pathologic, and molecular features of this localized disease are underresearched. We conducted a retrospective multicentric study of 60 patients with a PCSMLPD that presented as a single cutaneous lesion. Clinical, pathologic, and targeted molecular analyses were performed. PCSMLPD presented mostly as a nodule (45%), located on the head and neck area (50%) in adults (mean age: 59 y [43.3 to 75.2]). All patients had an indolent disease course, either at initial staging or during follow-up (mean: 16.6 mo [1.3 to 31.9]). Spontaneous regression was reported in 31.9% of cases. The infiltrates were most often nodular and/or diffuse, expanding in the whole dermis (78%, Pattern 1), rather than subepidermal band-like in the superficial dermis (22%, Pattern 2). Epidermotropism, folliculotropism, and capillary hyperplasia were common. The expression of TFH lineage markers was more extensive in lesions with Pattern 2, but a substantial B-cell infiltrate was seen in both types of lesions. A clonal rearrangement of the TCR genes was identified in 68% of cases. One sample of the 13 tested revealed a mutation in the DNMT3A gene among the 9 genes studied (TET2, DNMT3A, IDH2, RHOA, SETD2, PLCG1, STAT3, STAT5B, and CD28). PCSMLPD follows a benign clinical course and can spontaneously regress after biopsy. Although PCSMLPD expresses TFH lineage markers, mutations usually found in angioimmunoblastic T-cell lymphomas are uncommon.

Aims: We applied the 2017 World Health Organization (WHO) classification criteria to categorise a series of 64 primary cutaneous large B-cell lymphomas (PCLBCLs), containing a majority (≥80%) of large cells and a proliferative rate of ≥40%, raising the problem of the differential diagnosis between PCLBCL, leg type (PCLBCL-LT) and primary cutaneous follicle centre lymphoma, large cell (PCFCL-LC). The aims were to determine the reproducibility and prognostic relevance of the 2017 WHO criteria., Methods and Results: Morphology and phenotype identified 32 PCLBCLs-LT and 25 PCFCLs-LC; seven cases (11%) remained unclassified. Morphology was less reproducible than immunophenotype. Pertinent markers for the differential diagnosis were MUM1, FOXP1, CD10, and IgM. bcl-2 and bcl-6 were expressed by both PCFCLs-LC and PCLBCLs-LT at substantial levels. Neither Ki67 expression nor p63 expression was of diagnostic value. MYD88 was found to be mutated only in PCLBCLs-LT (n = 22, 69%). According to Hans/Hans modified algorithms, 23 of 25 PCFCLs-LC had germinal centre (GC) status, and the 32 PCLBCLs-LT had non-GC status. Overall survival was poorer for PCLBCLs-LT than PCFCLs-LC (P = 0.0002). Non-GC cases had poorer overall survival than GC cases (P = 0.0007). In PCLBCLs-LT, MYC expression was associated with cutaneous relapses (P = 0.014). When GC/non-GC status was applied to unclassified cases, only a single case remained discordant., Conclusions: Our results support the 2017 WHO classification criteria for PCLBCL diagnosis. The Hans modified algorithm using CD10 and MUM1 distinguished PCFCLs-LC from PCLBCLs-LT with optimal diagnostic value without requiring bcl-6 immunolabelling (poorly reproducible). Rare unclassified cases may constitute a provisionally heterogeneous subgroup for which GC/non-GC status (relevant for prognosis) may guide therapeutic decisions., (© 2019 John Wiley & Sons Ltd.)

Objective: To present the current terminology and natural history of neurofibromatosis 1 (NF1) cutaneous neurofibromas (cNF)., Methods: NF1 experts from various research and clinical backgrounds reviewed the terms currently in use for cNF as well as the clinical, histologic, and radiographic features of these tumors using published and unpublished data., Results: Neurofibromas develop within nerves, soft tissue, and skin. The primary distinction between cNF and other neurofibromas is that cNF are limited to the skin whereas other neurofibromas may involve the skin, but are not limited to the skin. There are important cellular, molecular, histologic, and clinical features of cNF. Each of these factors is discussed in consideration of a clinicopathologic framework for cNF., Conclusion: The development of effective therapies for cNF requires formulation of diagnostic criteria that encompass the clinical and histologic features of these tumors. However, there are several areas of overlap between cNF and other neurofibromas that make distinctions between cutaneous and other neurofibromas more difficult, requiring careful deliberation with input across the multiple disciplines that encounter these tumors and ultimately, prospective validation. The ultimate goal of this work is to facilitate accurate diagnosis and meaningful therapeutics for cNF., (© 2018 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the American Academy of Neurology.)

Introduction: The distribution of extranodal NK/T-cell lymphoma (ENKTCL) is highly inhomogeneous throughout the world. In Sub-Saharan Africa, despite the precocity of Epstein-Barr virus (EBV) infection and its endemicity, ENKTCL remains exceptionally reported. The purpose of this study was to report the epidemiological, clinical, paraclinical, and evolutionary characteristics of ENKTCL at the Aristide LeDantec University Hospital in Dakar, Senegal., Methodology: A 5-year retrospective review of all patients with histopathological, immunohistochemical, and in situ hybridization proven cutaneous lymphomas RESULTS: We collected seven cases corresponding to a frequency of 1.4 cases per year. ENKTCL accounted for 10.5% of all cutaneous lymphomas, ranking second after T-cell lymphomas. Men were predominantly affected (M : F ratio of 6), and the mean age was 38.5 years ± 4.06. The mean time before consultation was 7.3 months. The lymphomas affected primarily the nasal cavity in five cases and the skin in two cases. At admission, six patients had nasal mucosa involvement, which was isolated in three cases, associated with cutaneous lesions in three cases and lymph node involvement in three cases. CD56 was positive in only one case, and Eber transcribed RNA of EBV was expressed by in situ hybridization in all patients., Discussion: To our knowledge, we have reported the first and largest series of ENKTCL in Sub-Saharan Africa. Our study shows an intermediate prevalence between that reported from Asia, Latin America, and the West. It was also noted a young age of patients, a prolonged diagnostic delay, a frequent negativity of CD56 marker, and a very poor prognosis of the disease in our region., (© 2018 The International Society of Dermatology.)

Skin biopsies of 41 angioimmunoblastic T-cell lymphoma patients were retrospectively analyzed for the expression of follicular helper T-cell (TFH) markers, Epstein-Barr virus (EBV), and the presence of RHOA (p.G17V) and IDH2 (p.R172K/S) mutations using allele-specific polymerase chain reaction. We categorized cases into 4 distinctive patterns: (1) low-density lymphocytic perivascular infiltrates (n=11), (2) dense perivascular infiltrates with atypical cells and occasional inflammatory cells (n=13), (3) diffuse infiltrates reminiscent of angioimmunoblastic T-cell lymphoma (n=4), or (4) other aspects (n=13). Two EBV and 2 plasmacytoid lymphoproliferative disorders were seen. We observed variable expression of TFH markers (CD10 [50%], BCLB6 [84%], PD1 [94%], CXCL13 [84%], and ICOS [97.5%]), and EBV B-blasts (26%). A TFH phenotype was identified in 82% and 73%, respectively, of cases with the most challenging patterns 1 and 2. TFH markers and EBV can thus help for diagnosis and are detected in samples with low-density infiltrates. We found RHOA G17V and IDH2 R172K/S mutations in the skin in 14/18 (78%) and 3/16 (19%) cases, respectively. The RHOA G17V mutation was identified in a proportion of biopsies with patterns 1 and 2, which represent a diagnostic challenge. The RHOA G17V mutation was detected both in the skin and lymph node (LN) biopsies in 7/9 (64%) cases, and in only the skin or the LN of 1 sample each. The frequency of RHOA G17V mutation was similar to that reported in LNs. It may represent a sensitive diagnostic marker in the skin, helpful in cases with low-density infiltrates.

Background: Granulomatous slack skin (GSS) is an extremely rare subtype of T-cell lymphoma, a variant of mycosis fungoides (MF). Herein, we describe the first reported case of GSS associated with metastatic testicular seminoma., Patients and Methods: A 28-year-old male patient presented with circumscribed erythematous loose skin masses, especially in the body folds and which had been relapsing for 4years. Skin biopsy showed a loss of elastic fibers and an atypical granulomatous T-cell infiltrate with epidermotropism, enabling a diagnosis of GSS to be made. A biopsy of a retroperitoneal lymphadenopathy showed testicular seminoma metastasis., Discussion: Patients suffering from GSS have a statistically higher risk of developing a second primary cancer, especially Hodgkin's lymphoma. The association found between GSS and a lymphoproliferative malignancy requires long-term follow-up and determines the patient's prognosis., Conclusion: It is not possible to prove a formal link between GSS and testicular seminoma. However, this case illustrates the value of screening for a second cancer, particularly where extra-cutaneous lesions appear during GSS treatment. Lymph node biopsy should be performed routinely in the event of GSS with possible lymph node involvement., (Copyright © 2017 Elsevier Masson SAS. All rights reserved.)

The classification of cutaneous follicular lymphoma (CFL) into primary cutaneous follicle center lymphoma (PCFCL) or secondary cutaneous follicular lymphoma (SCFL) is challenging. SCFL is suspected when tumor cells express BCL2 protein, reflecting a BCL2 translocation. However, BCL2 expression is difficult to assess in CFLs because of numerous BCL2+ reactive T cells. To investigate these issues and to further characterize PCFCL, we studied a series of 25 CFLs without any extracutaneous disease at diagnosis, selected on the basis of BCL2 protein expression using 2 BCL2 antibodies (clones 124 and E17) and BOB1/BCL2 double immunostaining. All cases were studied using interphase fluorescence in situ hybridization with BCL2, BCL6, IGH, IGK, IGL breakapart, IGH-BCL2 fusion, and 1p36/1q25 dual-color probes. Nineteen CFLs were BCL2 positive, and 6 were negative. After a medium follow-up of 24 (6 to 96) months, 5 cases were reclassified as SCFL and were excluded from a part of our analyses. Among BCL2+ PCFCLs, 60% (9/15) demonstrated a BCL2 break. BCL2-break-positive cases had a tendency to occur in the head and neck and showed the classical phenotype of nodal follicular lymphoma (CD10+, BCL6+, BCL2+, STMN+) compared with BCL2-break-negative PCFCLs. Del 1p36 was observed in 1 PCFCL. No significant clinical differences were observed between BCL2+ or BCL2- PCFCL. In conclusion, we show that a subset of PCFCLs harbor similar genetic alterations, as observed in nodal follicular lymphomas, including BCL2 breaks and 1p36 deletion. As BCL2 protein expression is usually associated with the presence of a BCL2 translocation, fluorescence in situ hybridization should be performed to confirm this hypothesis.

Background: Monoclonal T-cell receptor (TCR) rearrangement is detected in 57-75% of early-stage mycosis fungoides (MF) at diagnosis. A retrospective study showed molecular residual disease (MRD) in 31% of patients in complete clinical remission (CR) after 1 year of treatment., Objectives: To confirm the frequency of MRD at 1 year and to determine its prognostic value for further relapse., Methods: Patients with T1-, T2- or T4-stage MF were prospectively included in this multicentre study. At diagnosis, clinical lesions and healthy skin were biopsied. After 1 year of topical treatment, previously involved skin of patients in CR was biopsied for histology and analysis of TCR-γ gene rearrangement. The results were compared with the clinical status each year for 4 years., Results: We included 214 patients, 133 at T1, 78 at T2 and three at T4 stage. At diagnosis, 126 of 204 cases (61·8%) showed TCR clonality in lesional skin. After 1 year, 83 of 178 patients (46·6%) still being followed up were in CR and 13 of 63 (21%) showed MRD. At 4 years, 55 of 109 patients (50·5%) still being followed up were in CR and 44 of 109 (40·4%) were in T1 stage. MRD did not affect clinical status at 4 years (CR vs. T1/T2, P = 1·0; positive predictive value 36·4%; negative predictive value 67·6%)., Conclusions: T-cell clonality at diagnosis and MRD at 1 year are not prognostic factors of clinical status at 4 years., (© 2015 British Association of Dermatologists.)

Background: Patients with erythrodermic disease are a diagnostic challenge regarding the clinical and histological differential diagnosis., Objectives: To evaluate histopathological and immunohistochemical diagnostic markers for Sézary syndrome., Methods: Ninety-seven erythrodermic cases [Sézary syndrome (SS), n = 57; erythrodermic inflammatory dermatoses (EIDs), n = 40] were collected by the EORTC Cutaneous Lymphoma Task Force histopathology group. Evaluation criteria were (i) epidermal and dermal changes; (ii) morphology of the infiltrate; (iii) immunohistochemical analysis of marker loss (CD2, CD3, CD4, CD5 and CD7); (iv) bystander infiltrate by staining for CD8, FOXP3 and CD25; and (v) expression of Ki-67, CD30, PD-1 and MUM-1., Results: The workshop panel made a correct diagnosis of SS in 51% of cases (cutaneous T-cell lymphoma 81%) and of EID in 80% without clinical or laboratory data. Histology revealed a significantly increased degree of epidermotropism (P < 0.001) and more intraepidermal atypical lymphocytes (P = 0.0014) in SS biopsies compared with EID. Pautrier microabscesses were seen only in SS (23%) and not in EID (P = 0.0012). SS showed significantly more dermal cerebriform and blastic lymphocytes than EID. Immunohistochemistry revealed a significant loss of CD7 expression (< 50%) in 33 of 51 (65%) cases of SS compared with two of 35 (6%) EID (P < 0.001). The lymphocytic infiltrate in SS skin samples was found significantly to express PD-1 (P = 0.0053), MUM-1 (P = 0.0017) and Ki-67 (P < 0.001), and showed less infiltration of CD8(+) lymphocytes (P < 0.001). A multivariate analysis identified CD7 loss, increased numbers of small cerebriform lymphocytes, low numbers of CD8(+) lymphocytes and increased proliferation (Ki-67(+) lymphocytes) as the strongest indicators for the diagnosis of SS., Conclusions: A number of different histological and immunophenotypical criteria are required to differentiate between SS and EIDs., (© 2015 British Association of Dermatologists.)

Background: Sézary syndrome is a cutaneous T-cell lymphoma characterized by erythroderma and leukemic involvement., Objective: We sought to define the clinical, biologic, and histopathologic features of Sézary syndrome without erythroderma., Methods: Features of patients with Sézary syndrome and normal-appearing skin or stage-T1 patches, fulfilling Sézary syndrome hematologic criteria and with histologically documented disease in normal-appearing skin were collected. Expression of Sézary syndrome molecular biomarkers in peripheral blood and skin lymphocytes were studied., Results: Five women and 1 man (median age: 71 years) were all referred for generalized pruritus. Four had no specific lesions; 2 had T1-stage patches. Histologic examination of normal-appearing skin from all patients showed lesions compatible with Sézary syndrome. Peripheral blood lymphocytes from 3 of 4 patients tested strongly expressed PLS3, Twist-1, and KIR3DL2. All normal-appearing skin biopsy specimens expressed programmed death-1. Median follow-up was 9 years. Although no patient developed erythroderma, tumors, or abnormal lymph nodes, specific skin lesions appeared in all patients during follow-up. Only 1 death, unrelated to Sézary syndrome, occurred., Limitations: Retrospective design and small sample size are limitations., Conclusion: Sézary syndrome without erythroderma is a rare entity that may have a better prognosis than classic Sézary syndrome., (Copyright © 2014 American Academy of Dermatology, Inc. Published by Elsevier Inc. All rights reserved.)

Variations in the clinical and histological presentation of cutaneous T-cell lymphoma (CTCL) can hamper diagnosis. We report two cases of a novel presentation of CTCL characterized by an aberrant immunophenotype with complete loss of pan T-cell antigens including T-cell receptor β chain and showing the clinical and histopathological appearance of erythrodermic and plaque-stage mycosis fungoides., (© 2014 British Association of Dermatologists.)

Introduction: Taking as a base our retrospective study of 2760 cases of cutaneous lymphoproliferations from the LYMPHOPATH and GFELC networks, we analyzed the doubtful and discordant cases between non-expert and expert pathologists, and the interest of clinicopathological confrontation., Material and Methods: We defined the main diagnostic difficulties presented by cutaneous lymphoproliferations. We then designed and tested the algorithms on 20 random cases with 20 pathologists, in order to be used by any pathologist (not necessarily specialised in dermatopathology)., Results: The problematic differential diagnoses most frequently encountered are the following: MF or reactive dermatose; lymphoma without any other precision or reactive infiltrate; small B cell lymphoproliferation: lymphoma or reactive infiltrate; phenotyping of large B cell lymphoproliferation. We also analyzed less common problematic differential diagnoses, on the grounds that they are over- or under- diagnosed. Our test had a 72% success rate among the 20 randomly tested cases. The use of several algorithms for the same case is possible., Discussion: Our study shows that an expert second-opinion is of interest in the area of cutaneous lymphoproliferations. A second opinion is useful for distinguishing a small B cell lymphoma from a HLR, and for defining a final diagnosis when the first pathologist doubts between lymphoma and reactive infiltrate. However, we demonstrate that for the problem MF or reactive dermatose, an initial clinicopathological confrontation produces more results than a second-opinion pathology review., Conclusion: This is the first study of cutaneous lymphoproliferations that, without excluding reactionary infiltrates, concentrates on doubtful and discordant diagnoses between non expert and expert pathologists, and which has produced tested diagnostic algorithms., (Copyright © 2015 Elsevier Masson SAS. All rights reserved.)

Background: Extranodal NK/T-cell lymphoma (ENKTL) is a rare form of non-Hodgkin's lymphoma and carries a poor prognosis. Depending on the primary sites of anatomical involvement, it is subcategorized into nasal or extra-nasal ENKTL. Cutaneous involvement is the second localization reported for these lymphomas., Patients and Methods: A woman was admitted for erythematous infiltrative patches on the breasts having an ulcerative course. Cutaneous histopathology showed a dense, diffuse infiltrate of atypical lymphocytes. Immunohistochemistry revealed expression of specific markers for NK-cells and of cytotoxic molecules (TIA-1, granzyme B and perforin), lack of expression of T-cell markers (except positivity of cytoplasmic CD3 and CD2), and the presence of EBV-DNA in lymphoma cells. Positron emission tomography-computed tomography revealed sub- and supra-diaphragmatic multi-organ involvement (kidneys, breasts, stomach, duodenum, lungs, pleural cavity, uterus, bones). No bone marrow infiltration was noted. PCR (polymerase chain reaction) showed high circulating levels of EBV-DNA in peripheral blood. A systemic nasal-type ENKTL was diagnosed. A chemotherapy regimen including high-dose methotrexate, oxaliplatin, gemcitabine, L-asparaginase and dexamethasone was started. Despite good initial therapeutic response, the outcome was rapidly fatal with bone marrow involvement and multi-organ failure., Discussion: Major cutaneous manifestations of ENKTL comprise erythematous infiltrative patches mimicking panniculitis or cellulitis and evolving towards ulceration or necrosis. Subcutaneous nodules may also be noted. Late diagnosis at an advanced stage accounts for the poorer prognosis in extra-nasal ENKTL. In the advanced stages, treatment is based on a chemotherapy regimen including L-asparaginase, possibly followed by autologous or allogeneic hematopoietic stem cell transplantation., (Copyright © 2014 Elsevier Masson SAS. All rights reserved.)

Background: Mycosis fungoides (MF) and pseudo-MF (or MF simulant) can be associated with B-cell malignancies, but distinction between a true neoplasm and a reactive process may be difficult., Objectives: To report seven patients with B-cell malignancy and folliculotropic MF or pseudo-MF and emphasize on criteria allowing distinction between the two conditions., Methods: We retrospectively and prospectively included seven patients with B-cell malignancy who presented skin lesions histologically consisting in a folliculotropic T-cell infiltrate and reviewed the literature on the topic., Results: Four men and three women had a chronic lymphocytic leukaemia (n = 6) or a MALT-type lymphoma (n = 1). Five patients had localized papules, and two had patches and plaques. Histological examination showed in all cases a diffuse dermal T-cell infiltrate with folliculotropic involvement and follicular mucinosis associated with clusters of the B-cell lymphoma, without significant expression of follicular helper T-cell markers. T-cell rearrangement studies showed a polyclonal pattern in the patients with papules and a monoclonal pattern in the cases of patches and plaques. Papular lesions had an indolent evolution, whereas patches and plaques persisted or worsened into transformed MF., Conclusion: Folliculotropic T-cell infiltrates associated with B-cell malignancies can be either a true folliculotropic MF or a pseudo-MF. The distinction between both conditions cannot rely only on the histopathological aspect, but needs both a clinical pathological correlation and the search for a dominant T-cell clone. Whether the neoplastic T and B cells derive from a common ancestor or the T-cell proliferation is promoted by the underlying B-cell lymphoma remains unsolved, but interaction between B and T cell in the skin does not appear to be dependent on a TFH differentiation of the T-cell infiltrate., (© 2014 European Academy of Dermatology and Venereology.)

CD160 is a GPI-anchored Ig-like receptor identified by the BY55 mAb on human circulating CD56dim+ NK cells and TCRγδ lymphocytes. In addition, while most intestinal T lymphocytes express it, only a minor circulating CD4+ or CD8+ T lymphocyte subset is CD160+. Here we describe a population of CD4+ CD160+ human blood T lymphocytes of circulating cutaneous T cells. These rare T lymphocytes represent 2.1 ± 1.9% of the circulating CD3+ CD4+ T cells, coexpress CD8αα, CD244, and perforin but lack CD28 expression, a phenotype corresponding to effector memory cytotoxic T-lymphocytes. Functional studies further confirmed their cytotoxic potential. These cells lack αEβ7 integrin and CCR7 expression but do express skin-addressing molecules CLA, and CCR4. In normal human skin, CD4+ CD160+ cells represent 34.6 ± 14.7% of the CD4+ T lymphocytes extracted by collagenase treatment. These T cells coexpress CLA (81 ± 13.6%), CCR4 (62.3 ± 15.9%), and some CD8αα (19.6 ± 13%) or CCR7 (24.4 ± 11.7%) expression. Cutaneous T-cell lymphoma cells express the natural killer receptor KIR3DL2 (CD158k) used as a tumor marker. Not only we confirmed the expression of this marker in the blood and/or skin of mycosis fungoides patients but we also show for the first time CD158k expression (often associated with CD160) on cutaneous CD4+ T cells from healthy individuals (25.3 ± 15%). Therefore, CD4+ CD160+ T cells expressing CD158k might represent specialized cutaneous lymphocytes devoted to immune surveillance, from which could originate cutaneous T-cell lymphomas such as mycosis fungoides., (© 2014 International Society for Advancement of Cytometry.)

We report the case of a two patients who presented with a solitary, asymptomatic, angiomatoid nodule on the right thigh. Histopathological finding showed a poorly circumscribed lesion, located in the dermis. The morphological aspect strongly suggested the diagnosis of atypical fibrous histiocytoma (AFH), but surprisingly, the neoplastic cells were diffusely CD30+, with a membrane staining devoid of paranuclear dot. The lesions were tested for p80/ALK1 expression. Surprisingly, we found a diffuse cytoplasmic positivity. Interestingly, using break-apart fluorescent in situ hybridization (FISH), we evidenced an ALK rearrangement in nearly 50% of the neoplastic cells. The expression of CD30 and ALK1 with ALK gene rearrangement raised the possibility of three diagnoses: a primary cutaneous anaplastic large cell lymphoma (ALCL), a cutaneous inflammatory myofibroblastic tumor (IMT), an AFH of the skin associated with ALK gene rearrangement and CD30 positivity. The three hypotheses were discussed and finally, although p80/ALK1 expression and cytogenetic abnormalities in fibrous histiocytoma (FH) are not yet reported to the best of our knowledge, we favored the diagnosis of AFH., (© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Blastic plasmacytoid dendritic cell neoplasm is a rare clinicopathologic entity, characterized by strong skin tropism and a poor prognosis. The diagnosis is generally made by skin biopsy with appropriate immunohistochemical studies. To identify potential biological prognostic factors for blastic plasmacytoid dendritic cell neoplasm, we performed an extended clinico-immunohistochemical study on a series of 91 well-documented cases collected since 1995 by the French Study Group on Cutaneous Lymphomas. Skin biopsies were analyzed using a panel of 12 immunohistochemical markers (CD4, CD56, CD123, CD303, TCL1, CD68, CD2, CD7, TdT, Ki-67, S100, and MX-1). The results were correlated with survival. The 5 most characteristic markers of this entity (CD4, CD56, CD123, CD303, and TCL1) were expressed simultaneously in only 46% of patients. However, when 4 markers were expressed the diagnosis could still be reliably made without resorting to any additional stains. Expression of TdT and/or S100 correlated with varying degrees of maturation. Statistical survival analyses showed that CD303 expression and high proliferative index (Ki-67) were significantly associated with longer survival.

Mycosis fungoides (MF), the most common primitive cutaneous T-cell lymphoma, can undergo transformation in about 10% of cases. Transformed mycosis fungoides (T-MF) is often associated with the appearance of a CD20 component. The aim of this study was to analyze whether such cells are reactive or lymphomatous and to evaluate their prognostic impact. Among 311 T-MFs from the French Cutaneous Lymphoma Study Group registry, we studied 148 cases. CD20 was expressed in 88 cases (59%). The proportion of CD20 cells among T-MF lesions was <10% for 54 cases (38%), 10% to 49% for 71 cases (81%), and >50% for 17 cases (19%). We focused the study on 23 cases that contained >50% CD20 cells. To evaluate the nature of the CD20 component, we used immunohistochemistry (2 anti-CD20 antibodies, L26 and 7D1 clones, and 2 other anti-B-cell antigen antibodies, CD22 and PAX5) and a double-stain immunofluorescence technique (anti-CD20 and anti-CD3 antibodies). The clonality of B cells was studied by polymerase chain reaction. Three profiles were observed. In 15 of the 23 cases, the CD20 cells were reactive. In 6 cases, CD20 protein was aberrantly expressed in T-MF lesions. Lastly, there were 2 composite lymphomas (T-MF infiltrate with a B-cell follicular lymphoma). In view of this series, we propose a simple algorithm to help pathologists evaluate the nature of the CD20 component associated with T-MF. Although statistically not significant, there was a trend toward a worse prognosis in the presence of >50% CD20 cells and of a nodular perifollicular pattern of this component.

Angioimmunoblastic T-cell lymphoma (AITL) is frequently associated with skin lesions, but epidermotropic cutaneous involvement has never been described. A 37-year-old man presented with erythematous and pruriginous plaques, clinically suggestive of mycosis fungoides, distributed all over the body, 3 weeks after the last line of a polychemotherapy, given for an AITL diagnosed 1 year earlier on a lymph node biopsy. Skin biopsy showed an epidermotropic CD4(+) T-cell lymphoma, so that a diagnosis of mycosis fungoides was first proposed. Further investigations showed that atypical lymphocytes strongly expressed CD10 and markers of follicular helper T cells (T(FH) ) including PD1, BCL-6 and CXCL13. The diagnosis of an unusual epidermotropic cutaneous localization of the AITL was finally made, supported by the presence of the same T-cell clone in the initial lymph node biopsy and the skin. We therefore recommend performing markers of T(FH) cells in patients with unusual epidermotropic cutaneous T-cell lymphomas, particularly if they have any clinical features suggestive of AITL., (Copyright © 2012 John Wiley & Sons A/S.)

Chronic myelomonocytic leukemia (CMML) is a rare clonal hematopoietic disorder that can also involve the skin. The histopathology of these skin lesions is not clearly defined, and few data are available in the literature. To better understand tumoral skin involvements in CMML we carried out an extensive, retrospective clinicopathologic study of 42 cases selected from the database of the French Study Group of Cutaneous Lymphomas. On the basis of clinical data, morphology, and phenotype we identified 4 clinicopathologic profiles representing 4 distinct groups. The first group comprised myelomonocytic cell tumors (n=18), exhibiting a proliferation of granulocytic or monocytic blast cells, which were CD68 and/or MPO positive but negative for dendritic cell markers. The second group comprised mature plasmacytoid dendritic cell tumors (n=16), denoted by a proliferation of mature plasmacytoid dendritic cells, which were CD123, TCL1, and CD303 positive but CD56, CD1a, and S100 negative. The third group comprised blastic plasmacytoid dendritic cell tumors (n=4), characterized by a proliferation of monomorphous medium-sized blast cells, which were CD4, CD56, CD123, TCL1 positive but CD1a and S100 negative. The fourth group consisted of a putatively novel category of tumor that we named blastic indeterminate dendritic cell tumors (n=4), distinguished by a proliferation of large blast cells that not only exhibited monocytic markers but also the dendritic markers CD1a and S100. These 4 groups showed distinctive outcomes. Finally, we showed, by fluorescence in situ hybridization analysis, a clonal link between bone marrow disease and skin lesions in 4 patients. Herein, we have described a novel scheme for pathologists and physicians to handle specific lesions in CMML, which correspond to a spectrum of myelomonocytic and dendritic cell proliferations with different outcomes. A minimal panel of immunohistochemical markers including CD68, CD1a, S100, Langerin, and CD123 is necessary to make the correct classification in this spectrum of cutaneous CMML tumors, in which dendritic cell lineage plays an important role.

Background: Giant skin tumours are defined as greater than 10 cm in size and are frequently a consequence of neglect. We report three cases of giant skin tumours and discuss possible factors associated with delayed diagnosis and treatment., Observations: Two men and one woman, aged 52, 62 and 78 years, presented with giant skin tumours. One was mentally retarded, one had a borderline personality and another was presenting a major depressive disorder. Pain, smell, bleeding and/or loss of function forced the patients to seek medical care. Tumour sizes were 11 to 30 cm and were present for between six months and 20 years. All patients had hidden their lesion from their doctor and families. In one case, a sebaceous carcinoma of the right shoulder required amputation of the right upper limb. In a second, palliative surgery and medical care was given for a metastatic porocarcinoma. The last patient received neoadjuvant chemotherapy for an undifferentiated carcinoma., Discussion: Neglected skin tumours continue to be encountered even in 2011. Consultation was delayed due to fear of the diagnosis and/or lack of corresponding physicians, resulting in worsening of the prognosis. A larger scale study could help us to identify the factors associated with neglected giant tumours and to improve their management., (Copyright © 2012. Published by Elsevier Masson SAS.)

Syringomatous carcinoma is a rare cutaneous neoplasm, most frequently situated on the face and scalp and histologically characterised by an infiltrative pattern of basaloid or squamous cells, a desmoplastic stromal reaction and keratin filled cysts. We report the case of a 76-year-old woman who presented an ulcerative interscapular lesion measuring 3x4cm. After resection, the histological examinations of the specimens have identified a basal cell carcinoma. However, a local recurrence was observed 18 months later; histopathological findings showed a syringomatous pattern and neoplastic epithelial cells arranged in interconnecting cords with microcystic areas. Nests, cords, and tubules of the tumour extended into the dermis and into the adjacent muscle. Sclerosis of stroma around the cords was present. Tumour cells were not connected to the epidermis. The immunohistochemical analysis showed positivity for anti-CK7, AE1/AE3 and negativity for anti CEA and anti CK20. These histological and immunohistochemical analyses were consistent with the diagnosis of syringomatous eccrine carcinoma. Syringomatous carcinoma is an extremely invasive tumor, locally destructive and slowly growing adnexal tumour, derived from eccrine sweat glands. It is often mistaken, both clinically and microscopically, for other benign and malignant entities. The tumour recurrence is high due to extensive perineural invasion, but regional or distant metastases are rare. The local aggressive nature of the tumour and the high recurrence rate may necessitate mutilating procedures. Optimal treatment consists of a complete microscopically controlled surgical excision with clear surgical margins.

A retrospective analysis of 173 skin biopsy specimens of myeloid leukemia cutis (MLC) was performed to determine histologic and immunophenotypic criteria that could distinguish the varied myeloid disorders from one another. For the study, 11 relevant histologic items were scored and 12 antigens were studied (CD68 [KP1], CD163, CD14, CD4, myeloperoxidase [MPO], CD33, CD117, CD34, CD56, MIB-1, CD303, and CD123). Underlying myeloid disorders were essentially acute myeloid leukemias (65.3%), chronic myelomonocytic leukemias (11.0%), and refractory anemia (10.4%). Skin lesions were de novo in 7.5%, concurrent in 26.6%, and subsequent in 60.7%. Several morphologic characteristics (density, size of tumor cells, inflammatory background) were statistically useful in distinguishing between varied myeloid disorders. De novo MLCs displayed a specific morphologic profile. Association of CD68, CD33, and MPO could diagnose 100% of the cases of MLC. However, the immunohistochemical panel could not distinguish between the varied underlying myeloid disorders, with the exception that CD123 was particularly powerful in recognizing chronic myelomonocytic leukemia and also permitted reclassification of 4 cases as blastic plasmacytoid dendritic cell neoplasm.