Primary cutaneous peripheral T-cell lymphomas with a T-follicular helper phenotype: an integrative clinical, pathological and molecular case series study.

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.

Acute and fatal cephalosporin-induced autoimmune haemolytic anaemia.

We report the case of an 82-year old male patient admitted in our medical intensive care unit for diffuse skin lesions, 3 days after the onset of ceftriaxone for bilateral pneumonia without microbiological documentation. The patient concomitantly exhibited diffuse skin lesions compatible with livedo and neurological and haemodynamic failure. Biological analysis revealed acute haemolytic anaemia. Warming of patient, red blood-cells transfusion and high-doses corticosteroids were initiated and ceftriaxone was stopped. Despite these therapeutics, the patient exhibited multiple organ failure and died. The main suspected triggering factor of this acute and fatal haemolytic anaemia was ceftriaxone administration considering: (i) the delay between cephalosporin administration and symptoms; (ii) the worsening of livedo and acrocyanosis a few hours after meningeal ceftriaxone doses; and (iii) fatal evolution. Cephalosporin-induced autoimmune haemolytic anaemia is a rare and serious cause of livedo that should be suspected in patients exhibiting livedo and acute haemolytic anaemia within hours/days following cephalosporin administration.

Cutaneous Involvement in Waldenström's Macroglobulinaemia.

Cutaneous involvement in Waldenström's macroglobulinaemia (WM) has been poorly characterized. To describe this involvement, a retrospective study of 19 patients with WM and cutaneous involvement of tumour B cells was performed. Twelve patients (group 1) had lymphoplasmacytic, non-transformed cutaneous proliferation, while in 7 cases (group 2) cutaneous involvement corresponded to histological transformation. In group 1, skin involvement was inaugural in 6 cases. The lesions were infiltrated plaques (83%), papules (25%) and tumours (42%). Four patients had a similar clinical picture (purplish, bilateral and symmetrical infiltration on the face). MYD88 L265P mutation was detected in the skin biopsy in all 6 cases tested. The 3-year specific survival rate was 88%. In group 2, cutaneous transformation occurred during the follow-up of the WM (71%). Lesions presented as ulcerated tumours (86%) of the trunk (57%) and lower limbs (57%). The 3-year specific survival rate was 22%. Skin involvement in WM has distinctive characteristics (e.g. clinical, histological, immunohistochemical, MYD88 L265P mutation).

Primary cutaneous large B-cell lymphomas: relevance of the 2017 World Health Organization classification: clinicopathological and molecular analyses of 64 cases.

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.

Erythema Nodosum Leprosum-Like Lesions Are a Histopathologic Pattern in Whipple's Disease and a Sign of the Immune Reconstitution Inflammatory Syndrome: A Case Series and Review of the Literature.

Inflammatory and subcutaneous nodules can arise in treated and untreated cases of Whipple disease (WD). The inflammatory immune reconstitution syndrome describes paradoxical clinical inflammatory worsening of a preexisting condition because of a return of immune function. Clinicopathologic examination of 4 patients with WD who presented with erythema nodosum leprosum (ENL)-like lesions and the findings of a systematic review of this phenomenon revealed that ENL-like lesions occurred in predominantly middle-aged male patients who suffered from WD, mostly on the legs. Patients showed a nonvasculitic, mostly septal panniculits with neutrophils, macrophages, and lymphocytes. Numerous bacteria-laden periodic acid-Schiff + macrophages and free bacilli were detected in the dermis, as well as subcutaneous septae and adipose lobules. These lesions occurred in both untreated and treated patients as part of inflammatory immune reconstitution syndrome. In conclusion, ENL-like lesions represent a characteristic histopathologic pattern associated with WD, which can occur in different contexts whenever there is a change in the immunological status of the patient. This change can be triggered by antimicrobial treatment, immunomodulatory and immunosuppressant therapy, or occur spontaneously, rarely.

The guanine exchange factor SWAP70 mediates vGPCR-induced endothelial plasticity.

BACKGROUND: The viral G protein-coupled receptor (vGPCR) is proposed to act as one of the predominant mediators of Kaposi's sarcoma (KS), a human herpes virus 8 (HHV8)-elicited disease. The actions of vGPCR manifest pathogenesis, in part, through increased permeability of endothelial cells. Endothelial cell-cell junctions have indeed emerged as an instrumental target involved in the vasculature defects observed within the tumor microenvironment. The pathway leading to adherens junction destabilization has been shown to involve the activation of the small GTPase Rac, in the context of either latent infection or the sole expression of vGPCR. However, the precise molecular mechanisms governed by vGPCR in vascular leakage require further elucidation. FINDINGS: Guanine exchange factors (GEFs) function as critical molecular switches that control the activation of small GTPases. We therefore screened the effects of 80 siRNAs targeting GEFs on vGPCR-driven endothelial permeability and identified switch-associated protein 70 (SWAP70) as necessary for its elevating effects. Pull-down experiments further showed that Rac activation by vGPCR was dependent on SWAP70. Examination of tissues and cells from HHV8-positive patients revealed that SWAP70 was ubiquitously expressed. Furthermore, SWAP70 was found to be crucial for vGPCR-driven endothelial tube formation and endothelial sprouting in vitro. CONCLUSIONS: SWAP70 appears to act as a molecular intermediate between vGPCR and endothelial activation. Because of the important role of vGPCR-mediated endothelial plasticity in KS pathogenesis, inhibition of SWAP70 function could be of interest for blocking vGPCR-driven activities in HHV8-defined diseases.

Sarcoidosis in Patients Treated with Vemurafenib for Metastatic Melanoma: A Paradoxical Autoimmune Activation.

BACKGROUND: Vemurafenib, a BRAF inhibitor, is a first-line treatment for inoperable melanoma. Sarcoidosis has never been reported in patients on vemurafenib. OBJECTIVES: We describe 5 cases of sarcoidosis in patients treated with vemurafenib. METHODS: Seventy patients receiving vemurafenib for a BRAF-mutated inoperable stage III or IV melanoma were treated in our centre. RESULTS: Five patients (7.1%) developed sarcoidosis or a sarcoid-like reaction on vemurafenib; 4 patients had cutaneous signs and 3 had extracutaneous disorders (bilateral hilar lymph nodes, uveitis). Histological analysis of skin lesions revealed epithelioid granulomas without necrosis, consistent with sarcoidosis. Angiotensin-converting enzyme levels were high in 2 patients. Cutaneous and ophthalmological lesions rapidly disappeared on topical corticosteroid treatment without the cessation of vemurafenib treatment. Complete remission of melanoma was observed in 3 patients and partial remission was observed in another. CONCLUSION: BRAF inhibitors probably have immune system-enhancing effects and should therefore be recognized as potential inducers of sarcoidosis.

[Cutaneous lymphoproliferations: proposal for the use of diagnostic algorithms based on 2760 cases of cutaneous lymphoproliferations taken from the INCa networks (LYMPHOPATH and GFELC) over a two-year period]. / Lymphoproliférations cutanées : proposition d'algorithmes diagnostiques à partir de l'expérience sur 2ans des réseaux INCa (LYMPHOPATH et GFELC) sur 2760 lymphoproliférations cutanées.

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.

Assessment of the interplay between blood and skin vascular abnormalities in adult purpura fulminans.

RATIONALE: Purpura fulminans in adults is a rare but devastating disease. Its pathophysiology is not well known. OBJECTIVES: To understand the pathophysiology of skin lesions in purpura fulminans, the interplay between circulating blood and vascular alterations was assessed. METHODS: Prospective multicenter study in four intensive care units. Patients with severe sepsis without skin lesions were recruited as control subjects. MEASUREMENTS AND MAIN RESULTS: Twenty patients with severe sepsis and purpura fulminans were recruited for blood sampling, and skin biopsy was performed in deceased patients. High severity of disease and mortality rates (80%) was observed. Skin biopsies in purpura fulminans lesions revealed thrombosis and extensive vascular damage: vascular congestion and dilation, endothelial necrosis, alteration of markers of endothelial integrity (CD31) and of the protein C pathway receptors (endothelial protein C receptor, thrombomodulin). Elevated plasminogen activating inhibitor-1 mRNA was also observed. Comparison with control patients showed that these lesions were specific to purpura fulminans. By contrast, no difference was observed for blood hemostasis parameters, including soluble thrombomodulin, activated protein C, and disseminated intravascular coagulation markers. Bacterial presence at the vascular wall was observed specifically in areas of vascular damage in eight of nine patients tested (including patients with Streptococcus pneumoniae, Neisseria meningitidis, Escherichia coli, and Pseudomonas aeruginosa infection). CONCLUSIONS: Thrombi and extensive vascular damage with multifaceted prothrombotic local imbalance are characteristics of purpura fulminans. A "vascular wall infection" hypothesis, responsible for endothelial damage and subsequent skin lesions, can be put forward.

Trichodysplasia spinulosa associated with lupus.

Trichodysplasia spinulosa (TS) is a unique clinical and histological entity described in immunosuppressed patients. The recent discovery of genomic DNA from a new Polyomavirus named trichodysplasia spinulosa-associated Polyomavirus in TS lesions and good clinical response to cidofovir strengthens the hypothesis of a viral etiology for the disease. The authors report a case of TS associated with lupus erythematosus in a 26-year-old woman with no history of transplant, hemopathy, or cyclosporine treatment. The patient developed a progressive worsening eruption composed of confluent papules and spiky filiform excrescences concentrated in the midfacial area. Pathological features were characterized by aberrant distended and abnormally maturated hair follicles with sheets of eosinophilic cells containing large purple granules, and the presence of trichodysplasia spinulosa-associated Polyomavirus DNA was confirmed by polymerase chain reaction. This case is the first description in a nontransplanted lupus patient without underlying hemopathy.

Re-evaluation of the concept of basaloid follicular hamartoma associated with naevoid basal cell carcinoma syndrome: a morphological, immunohistochemical and molecular study.

Naevoid basal cell carcinoma syndrome (NBCCS) is a rare genodermatosis caused by germline mutations in genes of the Sonic Hedgehog (SHH) pathway and is characterised by early onset of multiple basal cell carcinomas (BCCs). Although skin tumours with follicular differentiation, notably basaloid follicular hamartoma (BFH), have been reported in NBCCS, their relations with BCC are poorly defined. In this context, the aim of this study was to clarify morphological, immunohistochemical â€‹and molecular features of BFH arising in a context of NBCCS. A total of 140 skin tumours from NBCCS and 140 control BCC tumours were reviewed, blinded to clinical data and classified as BCC or BFH. The morphological characteristics of these two groups were then compared. Twenty cases were submitted for immunohistochemical and molecular analysis. Thirty-three tumours among the exploratory cohort were classified as BFH and were exclusively detected in NBCCS patients. Histopathological criteria that were significantly different from BCC were as follows: a small size (<1.5 mm), connection to a hair follicle, arborescent organoid architecture, lack of cytological atypia and infundibulocystic differentiation. Immunohistochemical analysis confirmed activation of the SHH pathway in these lesions. Targeted next-generation sequencing suggested that MYCN and GLI2/3 amplifications and TP53 mutations might be involved in progression of these follicular tumours to BCC. Our study confirms the high prevalence of BFH, representing up to 24% of skin tumours in NBCCS and potentially being BCC precursors.

Subacute cutaneous lupus erythematosus induced and exacerbated by proton pump inhibitors.

BACKGROUND: Subacute cutaneous lupus erythematosus (SCLE) can be induced by numerous drugs. We report 3 cases of SCLE induced by proton pump inhibitors (PPIs). OBJECTIVE: To highlight a rare cutaneous side effect induced by a frequently prescribed drug such as a PPI. CASE REPORTS: Case 1 was a 30-year-old man who developed multiple annular plaques over the trunk and lower limbs 1 month after the initiation of pantoprazole. Antinuclear antibodies (ANA) were positive with anti-Ro/SSA and anti-La/SSB antibodies, and histology confirmed the diagnosis. Clinical improvement was achieved 8 weeks after the discontinuation of pantoprazole and the introduction of a treatment combining topical steroids and hydroxychloroquine. Lesions relapsed when pantoprazole was accidentally rechallenged. The second case was a 31-year-old woman, 28 weeks pregnant, who presented erythematous annular plaques over the trunk 7 weeks after starting esomeprazole. ANA and anti-Ro/SSA antibodies were positive, and the histology was compatible with SCLE. Fetal ultrasound was normal. She was treated with topical and oral steroids and hydroxychloroquine. Clinical improvement was achieved 4 weeks after the discontinuation of esomeprazole. The third case was a 57-year-old woman with systemic erythematosus lupus presenting annular and psoriasiform lesions on the trunk for 15 months. She was treated successively with hydroxychloroquine, azathioprine, mycophenolate mofetil and methotrexate with prednisone. A review of her drug history revealed the introduction of omeprazole a few weeks before the first appearance of skin lesions and omeprazole was contraindicated. CONCLUSION: SCLE should systematically be suspected in case of eruption after the introduction of PPI. The risk of fetal cardiac complications is important in pregnant women.

Acneiform Eruption Following Elexacaftor-Tezacaftor-Ivacaftor Treatment in Patients With Cystic Fibrosis.

Importance: A new treatment for cystic fibrosis combining 3 CFTR modulators-elexacaftor (ELX), tezacaftor (TEZ), and ivacaftor (IVA)-has recently been approved for cystic fibrosis treatment. The cutaneous adverse effects following treatment with this combination are poorly described in the literature. Objective: To describe the clinicopathological features and treatment response of ELX-TEZ-IVA-associated acneiform eruptions in patients with cystic fibrosis. Design, Setting, and Participants: This case series study was conducted in the Dermatology Department of Cochin Hospital, Paris, France, from July 2021 to June 2022 in collaboration with the Cochin Reference Center for Cystic Fibrosis. Referred patients were examined by senior dermatologists. All patients with cystic fibrosis treated with ELX-TEZ-IVA and referred for an acneiform rash were included. Exposures: Treatment with ELX-TEZ-IVA. Main Outcomes and Measures: Onset of acneiform rash, type of lesions, and degree of severity, as well as treatments initiated and response, were evaluated. When performed, skin biopsies were reviewed. Results: This study included 16 patients (11 women [68.7%]) with a median (range) age of 27 (22-38) years. Six patients (37.5%) developed new-onset acneiform rashes, whereas 10 patients (62.5%) had a relapse (5 patients) or worsening (5 patients) of previous acne. The median (range) onset of acneiform rash was 45 (15-150) days. At inclusion, 11 patients (68.7%) had facial hyperseborrhea, 15 patients (93.7%) had noninflammatory lesions, and 14 (87.5%) had inflammatory lesions of seborrheic regions. Four patients (25.0%) had severe acne with deep inflammatory lesions and pitted scars. A specific pathological pattern of necrotizing infundibular crystalline folliculitis was observed in 4 patients. Topical acne treatments, antibiotics, and isotretinoin were used successfully in these patients, resulting in partial or complete remission in 12 patients (85.7% of patients reevaluated). Conclusions and Relevance: This case series study found that acneiform eruption is an adverse event associated with ELX-TEZ-IVA treatment in patients with cystic fibrosis. Most patients developed mild lesions. However, isotretinoin treatment may be necessary in some patients. The mechanism of ELX-TEZ-IVA-associated acneiform eruption is currently unknown, but the observation of necrotizing infundibular crystalline folliculitis in biopsied patients may guide further exploration.

Chronic meningococcemia cutaneous lesions involve meningococcal perivascular invasion through the remodeling of endothelial barriers.

Chronic meningococcemia is a form of sepsis with frequent polymorphous skin lesions. Both in vivo and in vitro data suggest that, in these lesions, meningococci gain access from the capillary lumen to the peripheral extravascular compartment, in the absence of vascular dislocation, through a paraendothelial route.

Distinct merkel cell polyomavirus molecular features in tumour and non tumour specimens from patients with merkel cell carcinoma.

Merkel Cell Polyomavirus (MCPyV) is associated with Merkel Cell carcinoma (MCC), a rare, aggressive skin cancer with neuroendocrine features. The causal role of MCPyV is highly suggested by monoclonal integration of its genome and expression of the viral large T (LT) antigen in MCC cells. We investigated and characterized MCPyV molecular features in MCC, respiratory, urine and blood samples from 33 patients by quantitative PCR, sequencing and detection of integrated viral DNA. We examined associations between either MCPyV viral load in primary MCC or MCPyV DNAemia and survival. Results were interpreted with respect to the viral molecular signature in each compartment. Patients with MCC containing more than 1 viral genome copy per cell had a longer period in complete remission than patients with less than 1 copy per cell (34 vs 10 months, P = 0.037). Peripheral blood mononuclear cells (PBMC) contained MCPyV more frequently in patients sampled with disease than in patients in complete remission (60% vs 11%, P = 0.00083). Moreover, the detection of MCPyV in at least one PBMC sample during follow-up was associated with a shorter overall survival (P = 0.003). Sequencing of viral DNA from MCC and non MCC samples characterized common single nucleotide polymorphisms defining 8 patient specific strains. However, specific molecular signatures truncating MCPyV LT were observed in 8/12 MCC cases but not in respiratory and urinary samples from 15 patients. New integration sites were identified in 4 MCC cases. Finally, mutated-integrated forms of MCPyV were detected in PBMC of two patients with disseminated MCC disease, indicating circulation of metastatic cells. We conclude that MCPyV molecular features in primary MCC tumour and PBMC may help to predict the course of the disease.

Impact of expert pathology review in skin adnexal carcinoma diagnosis: Analysis of 2573 patients from the French CARADERM network.

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.