Cutaneous Histiocytoma Research Articles

A Case of Benign Fibrous Histiocytoma of External Auditory Canal

Benign fibrous histiocytoma (BFH) is a benign tumor composed of fibroblasts and histiocytes that most frequently occur in upper and lower extremities. BFH is rarely found in the area of head and neck; especially so in the external auditory canal. The authors have encountered a 52-year old male patient with a chief complaint of otorrhea from the right ear. A round, well-circumscribed mass filled 80% of the right external auditory canal. Non-enhanced temporal bone CT and enhanced temporal bone MRI showed a 0.9×1.0 cm sized dermal or subdermal polypoid mass arising at the right anterosuperior external auditory canal. Under local anesthesia, the patient underwent a complete resection of the mass, and the pathological result of the mass showed BFH. We thus report a rare case of BFH of the external auditory canal. Key words: Benign fibrous histiocytoma ㆍ Cutaneous histiocytoma ㆍ Dermatofibroma

A Clinicohistopathological Analysis of Cutaneous Fibrous Histiocytomas of the Finger.

Background:Cutaneous fibrous histiocytoma (CFH) is a common, benign skin tumor predominantly occurring on the extremities or trunk. However, CFH on the finger is rare.Objective:This study was undertaken to examine the clinicohistopathological features of CFH of the finger.Materials and Methods:This is a retrospective study of 12 CFHs located on fingers in a tertiary hospital in Korea. All case slides were retrieved from saved files.Results:Ages of the CHF of the finger affected individuals ranged from 9 to 48 years with a male-to-female ratio of 1.4:1. Picker's nodule or wart was the most common clinical diagnosis. In only 2 out of the 12 cases was the pre-biopsy diagnosis of CFH ventured. Fibrocollagenous type was the most common histological type. Majority of the cases were mitotically inactive, exhibiting only 0–1 mitoses per high-power field and there was no recurrence. Tumor cells were uniformly CD34 negative.Conclusion:Because CFH can resemble malignancies including dermatofibrosarcoma protuberans, a lack of familiarity with the occurrence of CFH of the finger may lead to more aggressive treatment. Dermatologists should include CFH in their differential diagnosis of circumscribed nodules on the fingers to ensure proper management.

A Role for Angiogenesis in Canine Cutaneous Histiocytoma Regression: Insights into an Old Clinical Enigma.

Canine Cutaneous Histiocytoma (CCH) is a Langerhans' cells benign tumour that undergoes spontaneous regression. The aim of the present study was to investigate the role of angiogenesis, a key step for tumour development, in CCH regression. 50 CCH samples were classified into 4 histological groups according to a regression scale, and evaluated for expression of vascular endothelial factor-A (VEGF-A) and its receptor VEGFR-2 as well as microvessel density (MVD). Tumours during early stages of the regressive process had a lower MVD compared to later stages, while CCH tumoural cells showed a limited production of VEGF, but higher levels of VEGFR-2. On the contrary, tumours in advanced phases of regression showed a higher number of neovessels, probably associated with the inflammatory state and the healing process. Our results suggest that angiogenesis may be compromised at early stages of histiocytoma development and this may be a determinant of regression in this tumour.

Giant Fibrous Histiocytoma : Clinical and Dermoscopic Features

Benign fibrous histiocytoma is one of the most frequent benign neoplasms ,the diagnosis of cutaneous benign fibrous histiocytoma is generally easy ,however the atypical fibrous histiocytoma is an uncommon, poorly documented variant of cutaneous fibrous histiocytoma,may be difficult to identify, and the diagnosis only confirmed after exhaustive histopathological examination. Recently the dermoscopy has revealed a new dimension of dermoscopic features giant Fibrous Histiocytoma . Our purpose was to report the clinical and dermoscopic characteristics of giant Fibrous Histiocytoma and review the few cases of this variant of Fibrous Histiocytoma reported in the literature.

Aneurysmal Fibrous Histiocytoma: Clinicopathology Analysis of 30 Cases of a Rare Variant of Cutaneous Fibrohistiocytoma.

Aneurysmal fibrous histiocytoma is often clinically misdiagnosed. In this study, we put forward an insight on how to help diagnose this disease clinically. A retrospective chart review was performed on all patients diagnosed with aneurysmal fibrous histiocytoma from 2007 to 2017 in the Department of Dermatology, Union Hospital, China, and all clinical data were collected from the hospital archives. From a total of 418 patients diagnosed with cutaneous fibrous histiocytoma, only 30 patients were confirmed to have aneurysmal fibrous histiocytoma out of which only 2 patients were clinically diagnosed with aneurysmal fibrous histiocytoma. The remaining 28 patients were diagnosed with various types of vascular tumors although pathology classified them as having aneurysmal fibrous histiocytoma. Among the 30 patients, 9 were male and 21 were female. There were following age groups: 13-19 (mean 16, n=4), 20-29 (mean 26.25, n=8), 30-39 (mean 33, n=7), 40-49 (mean 44, n=4), 50-59 (mean 56.75, n=4), 60 and above (mean 61, n=3). Tumors were present on the head, neck, back, waist, hips and upper and lower extremities. After complete excision, there was no recurrence and no complications. Histologically, lesions showed the typical pseudoangiomatoid spaces without endothelial lining and infiltration of fibrohistiocytes in hemosiderotic pigmentation. It was suggested that although the prognosis of aneurysmal fibrous histiocytoma is good, accurate diagnosis is paramount to avoid clinical misdiagnosis and subsequent complications.

Expression of Ki67 in canine cutaneous histiocytoma

Expression of Ki67 in canine cutaneous histiocytoma

Anterior abdominal wall spindle cell tumour - atypical fibrous histiocytoma: a rare presentation

Atypical fibrous histiocytoma is a rare and a distinct variant of cutaneous fibrous histiocytoma which can be misdiagnosed as sarcoma. It is mainly composed of a mixture of fibroblastic and histiocytic cells, especially found in the skin (dermatofibroma), particularly in the limbs. It is quite uncommon and is difficult to distinguish from a malignant lesion. Due to the lack of clear cut predictive morphological patterns and due to the suspicion of malignancy, complete surgical excision is recommended. Provided that atypical fibrous histiocytoma is treated by complete excision, a benign outcome is to be expected in most cases. However, like the cellular and aneurysmal variants of fibrous histiocytoma, atypical fibrous histiocytoma shows a higher tendency to recur locally than ordinary fibrous histiocytoma and may rarely metastasize. Lesions with floridly atypical features represent potential pitfalls for overinterpretation as pleomorphic sarcoma, which would appear to be inappropriate in most cases. Due to its rarity and uncertainty, we report a case of atypical fibrous histiocytoma and discuss its presentation, nature, types and treatment with reference to a brief review of literature.

PCR-based clonality analysis of antigen receptor gene rearrangements in canine cutaneous plasmacytoma

Plasmacytomas are discrete, B cell-derived, round cell tumours that sometimes are difficult to distinguish from canine cutaneous histiocytomas or T cell lymphosarcomas (lymphomas). Diagnosis of plasmacytomas relies on morphological observations and immunohistochemistry for multiple myeloma oncogene-1 (MUM-1) and cluster of differentiation 3 (CD3). Clonality testing often is used as an adjunct diagnostic tool to examine lymphoproliferative diseases. In this study, the sensitivity of PCR-based clonality analysis of antigen receptor gene rearrangements in canine cutaneous plasmacytomas was determined. Formalin-fixed paraffin-embedded sections of 29 canine plasmacytomas, 23 diffuse large B cell lymphomas (DLBCLs) and 23 lymph nodes without lymphoma were used for clonality analysis. New oligonucleotide primers for the framework (FR)2 and FR3 regions of the immunoglobulin heavy chain (IGH) V gene subgroup 3 were designed and used with previously reported FR3 primers. Although plasma cells are of B cell lineage, the detected frequency of IGH clonality in plasmacytoma was 0–34.5% with the seven primers used, whereas in DLBCLs it was 8.7–78.3%. In 23 lymph nodes without lymphoma, IGH clonality was detected in only one case with two out of the seven primers used. Sequence analysis of PCR products from plasmacytomas revealed mismatches in the annealing region of the FR3 primers. The sensitivity of detecting IGH clonality in canine plasmacytomas was lower than in DLBCLs. The low detection rate of IGH clonality in canine plasmacytoma may be due to somatic hypermutation of the variable region.

Epithelioid fibrous histiocytoma: molecular characterization of ALK fusion partners in 23 cases

AbstractEpithelioid fibrous histiocytoma is a rare and distinctive cutaneous neoplasm. Most cases harbor ALK rearrangement and show ALK overexpression, which distinguish this neoplasm from conventional cutaneous fibrous histiocytoma and variants. SQSTM1 and VCL have previously been shown to partner with ALK in one case each of epithelioid fibrous histiocytoma. The purpose of this study was to examine a large cohort of epithelioid fibrous histiocytomas by next-generation sequencing to characterize the nature and prevalence of ALK fusion partners. A retrospective archival review was performed to identify cases of epithelioid fibrous histiocytoma (2012–2016). Immunohistochemistry was performed to confirm ALK expression. Targeted next-generation sequencing was applied on RNA extracted from formalin-fixed paraffin-embedded tissue to identify the fusion partners. Twenty-three cases fulfilled inclusion criteria. The mean patient age was 39 years (range, 8–74), there was no sex predilection, and >75% of cases involved the lower extremities. The most common gene fusions were SQSTM1-ALK (N=12; 52%) and VCL-ALK (N=7; 30%); the other four cases harbored novel fusion partners (DCTN1, ETV6, PPFIBP1, and SPECC1L). The pattern of ALK immunoreactivity was usually granular cytoplasmic (N=12; 52%) or granular cytoplasmic and nuclear (N=10; 43%); the case containing an ETV6 fusion partner showed nuclear staining alone. There was no apparent relationship between tumor morphology and the ALK fusion partner. In summary, SQSTM1 and VCL are the most common ALK fusion partners in epithelioid fibrous histiocytoma; DCTN1, ETV6, PPFIBP1, and SPECC1L represent rare fusion partners. The proteins encoded by these genes play diverse roles in scaffolding, cell adhesion, signaling, and transcription (among others) without clear commonalities. These findings expand the oncogenic promiscuity of many of these ALK fusion genes, which drive neoplasia in tumors of diverse lineages with widely varied clinical behavior. This is the first documented account of ETV6-ALK and SPECC1L-ALK translocations in neoplasms.

Immunohistochemical Labeling of Multiple Myeloma Oncogene 1/Interferon Regulatory Factor 4 (MUM1/IRF-4) in Canine Cutaneous Histiocytoma.

Multiple myeloma oncogene 1/interferon regulatory factor 4 (MUM1/IRF-4) immunohistochemistry (IHC) is mainly used for diagnostic confirmation of plasma cell tumors (PCTs) in dogs and cats. This article describes MUM1/IRF-4 IHC expression in 20 cases of canine cutaneous histiocytoma (CH) and compares it with 10 cutaneous or mucocutaneous PCTs and 5 cutaneous histiocytic sarcomas (HSs) submitted to the same IHC protocol. All histiocytomas had strong nuclear and variable cytoplasmic immunolabeling for MUM1/IRF-4, whereas all PCTs had strong nuclear and moderate cytoplasmic immunolabeling for MUM1/IRF-4. No MUM1/IRF-4 immunolabeling was detected in the HSs. Although not typically a diagnostic challenge, MUM1/IRF-4 expression may have to be used with caution or in conjunction with additional immunomarkers to differentiate among poorly differentiated round cell tumors, especially when a histiocytic or plasma cell origin is suspected.

Immunohistochemical Expression of CD31 (PECAM-1) in Nonendothelial Tumors of Dogs.

CD31 immunoreactivity has been reported in human nonendothelial tumors of both epithelial and mesenchymal origin. This study examined CD31 immunoreactivity of 347 formalin-fixed, paraffin-embedded normal, nonneoplastic, and neoplastic canine tissues. CD31 expression was considered positive if at least 10% of the cell population had membranous reactivity. Labeling with the CD31 antibody (clone JC/70A) was observed in 16 samples of normal organs (liver, kidney, lymph node), 6 of 6 specimens of hepatic nodular hyperplasia, 3 of 3 hepatic regenerative nodules, 1 of 4 anal sac carcinomas, 6 of 6 hemangiosarcomas, 18 of 20 hepatocellular carcinomas, 1 of 6 mammary carcinomas, 3 of 5 plasmacytomas, 18 of 53 renal cell carcinomas, and 1 of 5 cutaneous histiocytomas. CD31 expression did not correlate with case outcome in hepatocellular or renal cell carcinomas. Although distinguishing hemangiosarcoma from other neoplasms is typically straightforward, pathologists should be aware of potential cross-reactivity when relying on CD31 immunohistochemistry for diagnosis, particularly in small biopsy samples or when faced with an epithelioid or poorly differentiated vascular neoplasm.

Dermoscopic findings in an early malignant fibrous histiocytoma on the face.

Malignant fibrous histiocytoma (MFH), currently classified as undifferentiated pleomorphic sarcoma, is the most frequent soft tissue sarcoma in adulthood, but it is not as common as a primary skin tumor. MFH affects mostly the thighs and trunk, head and neck is an infrequent presentation in adults. MFH is often diagnosed in advanced stages, with a tendency to local recurrence and systemic metastasis. Since tumor thickness and size are identified as major prognostic factors, early recognition becomes crucial to improve prognosis. We present a case of a cutaneous malignant fibrous histiocytoma located on the face in which dermoscopy was useful in clinical management and definition.

Clinicopathologic analysis of four cases of desmoplastic melanocytic nevus

Objective To investigate pathological characteristics and differential diagnosis of desmoplastic melanocytic nevus. Methods Four cases of desmoplastic melanocytic nevus were analyzed based on the clinical manifestations and histological, immunohistochemical and fluorescence in situ hybridization (FISH) features. Results Of the 4 cases, 2 were male and 2 were female. Their age ranged from 19 to 30 years with the average age being 26.5 years. The skin lesions were located on the extremities in 3 cases, on the vulva in 1 case. Histologically, the lesions were bilaterally symmetrical intradermal nevus. Nevus cells appeared epithelioid and/or fusiform, some were clustered or scattered in the proliferative fibrous tissue. None of lymphocyte aggregation, necrosis or ulceration was observed. Immunohistochemical examination showed positive staining for S100 and Melan A in 3 cases, positive staining for P16 in 2 cases, Ki-67-labeling index less than 5% in all the 4 cases, and negative staining for factor XIII (FXIII) and CD34 in 2 cases. FISH assay showed no copy-number variations in gene loci 6p25 (RREB1) , 6q23 (MYB) , 6p11.1-q11.1 (Cep6) and 11q13 (CCND1) in desmoplastic melanocytic nevus. Conclusion Desmoplastic melanocytic nevus is a kind of histologically unique, benign melanocytic nevus, and immunohistochemical staining for Ki-67, S-100, Melan A and FⅩⅢand FISH assay on melanoma can be helpful for the differential diagnosis between cutaneous fibrous histiocytoma and melanoma. Key words: Nevus, pigmented; Pathologic processes; Skin manifestations; Immunohistochemistry; Desmoplastic melanocytic nevi; FISH

Conjunctival reactive epithelial hyperplasia in a black African patient- a case report

Background: Reactive hyperplasia can occur in any part of the body but of concern is occurrence adjacent to certain neoplasm such as cutaneous fibrous histiocytoma, granular cell tumor, spitz nevus and melanoma. Ocular reactive epithelial hyperplasia is not as common as ocular reactive lymphoid hyperplasia. This is the first reported case in our environment. The patient was concerned about his cosmetic appearance, comments of friends and fear of eye problem in future. We report the case of a 32 year old patient with painless, progressive swelling of the conjunctiva following stone injury to the eye while on a bike 5 years earlier. A traumatic conjunctival cyst to rule out melanoma was the initial diagnosis. We therefore, managed the case by excision biopsy and histology report was in keeping with conjunctival reactive epithelial hyperplasia. One year after removal, he was free of any swelling and had no complaints. Conclusion: Excision biopsy is effective and it will assist in ruling out neoplasm and taking care of the patient’s cosmetic problem.

Expression pattern of proliferating cell nuclear antigen (PCNA) in round cell tumors of dogs from Grenada, West Indies

A retrospective study was performed on archivedtumor tissues received by the Veterinary Pathology Diagnostic Laboratory, School of Veterinary Medicine, St. George's University, Grenada over an 11 year period (2001–2011). The expression pattern of proliferating cell nuclear antigen (PCNA) was evaluated in canine round cell tumors. A total of 40 tumors comprising of transmissible venereal tumor(TVT), canine cutaneous histiocytoma(CCH)and mast cell tumor(MCT) were evaluated for PCNA expression using immunohistochemistry technique. Moderate to strong immunoreactivity was detected in most of these tumors. The percentage of immunomarked cells for PCNA was 42.1, 57.1, and 57.1 for TVT, CCH, and MCT, respectively. A statistical significance (P<0.05) between the average mitotic counts and PCNA expression for TVT and CCH was noted. PCNA expression in the TVT cases ranged from 17.4% to 56.3% which suggests the presence of variable growth fraction in this tumor. The results showed that PCNA immunohistochemistry could be an important adjunct tool in determining the biological behavior of round cell tumours.

ALK rearrangement and overexpression in epithelioid fibrous histiocytoma

Epithelioid benign fibrous histiocytoma, also known as 'epithelioid cell histiocytoma,' has traditionally been considered a morphologic variant of cutaneous fibrous histiocytoma (dermatofibroma). In addition to its characteristic epithelioid cytomorphology, several phenotypic differences suggest that epithelioid fibrous histiocytoma may differ biologically from other variants. Recently, ALK rearrangement was described in two cases of epithelioid fibrous histiocytoma and separately in two cases reported as 'atypical' fibrous histiocytoma (with epithelioid features), with corresponding ALK expression detectable by immunohistochemistry. The goals of this study were to determine the frequency of ALK expression by immunohistochemistry in epithelioid fibrous histiocytoma, to determine its value for the diagnosis of epithelioid fibrous histiocytoma among variants and other histologic mimics, and to evaluate ALK gene rearrangement in epithelioid fibrous histiocytoma. ALK protein expression was evaluated in whole tissue sections from 33 epithelioid fibrous histiocytomas, 41 other cases of fibrous histiocytoma (11 conventional and 10 each cellular, atypical, and aneurysmal types), 10 cutaneous syncytial myoepitheliomas, and 5 atypical fibroxanthomas, using a mouse anti-ALK monoclonal antibody. Fluorescence in situ hybridization (FISH) was performed using break-apart probes. In total, 29/33 (88%) cases of epithelioid fibrous histiocytoma showed diffuse cytoplasmic ALK expression. Staining was moderate to strong in intensity in all cases except one, which showed diffuse weak expression. All other tumor types were negative for ALK expression. FISH demonstrated ALK rearrangement in all ALK-immunoreactive cases evaluated (n=13), and not in one ALK expression-negative epithelioid fibrous histiocytoma successfully examined. In conclusion, the majority of epithelioid fibrous histiocytomas demonstrate ALK expression and ALK gene rearrangement. ALK expression is not seen in other variants of fibrous histiocytoma, providing a useful diagnostic tool to distinguish epithelioid fibrous histiocytoma from most histologic mimics. The expression of ALK suggests that epithelioid fibrous histiocytoma is a biologically distinct tumor type, unrelated to conventional fibrous histiocytoma and histologic variants.

Prevalence of Bartonella spp. in Canine Cutaneous Histiocytoma

Canine cutaneous histiocytoma (CCH) is a common, benign neoplastic proliferation of histiocytes of Langerhans cell origin that often ulcerate, become secondarily infected and regress spontaneously. Bartonella is a fastidious genus of facultative intracellular pathogens that can be transmitted through arthropod bites and epidermal animal scratches and has been identified previously in the cytoplasm of histiocytes within granulomatous lesions and in skin biopsy samples of inflammatory pustules and papules. Based on the established inflammatory and oncogenic properties of Bartonella, we hypothesized that Bartonella spp. DNA could be amplified from CCH more often than from non-lesional skin and bacteria could be localized within skin tumours using indirect immunofluorescence (IIF). Paraffin wax-embedded surgical biopsy samples from dogs with CCH and non-neoplastic skin adjacent to osteosarcomas (control group selected due to wide surgical margins) were retrieved from the archive of the pathology service of North Carolina State University College of Veterinary Medicine. DNA was extracted and regions of the 16S-23S rRNA intergenic transcribed spacer (ITS) region and the pap31 and gltA genes were amplified by polymerase chain reaction (PCR) using Bartonella-specific primers. IIF was performed using a primary Bartonella henselae monoclonal antibody to localize B. henselae in tissues of PCR-positive dogs. Bartonella vinsonii subsp. berkhoffii was amplified from 1/17 (5.8%) control tissues and B. henselae was amplified from 4/29 (13.8%) CCH tissues. The prevalence of B. vinsonii subsp. berkhoffii (P=0.37) or B. henselae (P=0.28) did not vary statistically between study groups. B. henselae could be visualized in 2/4 (50.0%) CCH tissues using IIF. Based on this study, Bartonella spp. are unlikely to cause CCH.

Diagnostic immunohistochemistry for canine cutaneous round cell tumours - retrospective analysis of 60 cases.

Canine cutaneous round cell tumours (CCRCTs) include various benign and malignant neoplastic processes. Due to their similar morphology, the diagnosis of CCRCTs based on histopathological examination alone can be challenging, often necessitating ancillary immunohistochemical (IHC) analysis. This study presents a retrospective analysis of CCRCTs. This study includes 60 cases of CCRCTs, including 55 solitary and 5 multiple tumours, evaluated immunohistochemically using a basic antibody panel (MHCII, CD18, Iba1, CD3, CD79a, CD20 and mast cell tryptase) and, when appropriate, extended antibody panel (vimentin, desmin, a-SMA, S-100, melan-A and pan-keratin). Additionally, histochemical stainings (May-Grünwald-Giemsa and methyl green pyronine) were performed. IHC analysis using a basic antibody panel revealed 27 cases of histiocytoma, one case of histiocytic sarcoma, 18 cases of cutaneous lymphoma of either T-cell (CD3+) or B-cell (CD79a+) origin, 5 cases of plas-macytoma, and 4 cases of mast cell tumours. The extended antibody panel revealed 2 cases of alveolar rhabdo-myosarcoma, 2 cases of amelanotic melanoma, and one case of glomus tumour. Both canine cutaneous histiocytoma and cutaneous lymphoma should be considered at the beginning of differential diagnosis for CCRCTs. While most poorly differentiated CCRCTs can be diagnosed immunohis-tochemically using 1-4 basic antibodies, some require a broad antibody panel, including mesenchymal, epithelial, myogenic, and melanocytic markers. The expression of Iba1 is specific for canine cutaneous histiocytic tumours, and more sensitive than CD18. The utility of CD20 in the diagnosis of CCRCTs is limited.

The vascular marker CD31 also highlights histiocytes and histiocyte-like cells within cutaneous tumors.

While useful in diagnosing angiosarcomas, CD31 can also highlight histiocytes within soft tissue tumors and lead to errors in diagnosis. We sought to determine how often CD31 highlights cutaneous histiocytomas and histiocytoma mimics. We examined eight epithelioid cell histiocytomas (ECHs), 12 xanthogranulomas (XGs), nine cases of Langerhans cell histiocytosis (LCH), eight reticulohistiocytomas, 11 xanthomas, 29 atypical fibroxanthomas, nine granular cell tumors, four cases of angiolymphoid hyperplasia with eosinophilia, nine intradermal Spitz nevi, and nine angiosarcomas with antibodies directed against CD31, CD34, CD163, and factor VIII. CD31 marked cells in three of 12 XGs, four of nine cases of LCH, one of eight reticulohistiocytomas, one of 11 xanthomas, 10 of 29 atypical fibroxanthomas, four of four cases of angiolymphoid hyperplasia with eosinophilia, nine of nine angiosarcomas, zero of nine granular cell tumors, and zero of eight ECHs. CD34 and factor VIII were negative in all nonvascular cases. Our results indicate that CD31 can mark lesional cells and imitate vascular tumors in cutaneous histiocytomas and histiocytoma mimics, an error that can be avoided by using a panel of antibodies.

Morphology and immunophenotype of canine cutaneous histiocytic tumours with particular emphasis on diagnostic application.

This study evaluated the morphology and immunohistochemistry of 85 canine cutaneous histiocytic tumours. The tumours were classified morphologically as either canine cutaneous histiocytomas (71 tumours) or canine cutaneous histiocytic sarcomas (14 tumours). The immunohistochemical analysis was conducted on paraffin sections using an antibody panel (against MHCII, CD18, CD79αcy, CD3 and E-cadherin). Histochemical staining with toluidine blue and Gomori silver impregnation was also performed. A follow-up was conducted via surveys. The histiocytic origin of the tumour cells was confirmed in 65 of the canine cutaneous histiocytomas and in 4 of the canine cutaneous histiocytic sarcomas. The tumours that had been misdiagnosed as canine cutaneous histiocytomas included plasmacytomas, epitheliotropic T-cell lymphomas and undetermined entities. The tumours misdiagnosed as canine cutaneous histiocytic sarcomas included plasmacytomas and non-epitheliotropic T-cell lymphomas, but the majority of them remained undetermined. The canine cutaneous histiocytomas showed MHCII, CD18 and E-cadherin expression, but in several of the tumours, the expression of CD18 or E-cadherin was confirmed in only a small percentage of the tumour cells. The regressing canine cutaneous histiocytomas showed increased T- and B-lymphocyte infiltration, a decreased mitotic index, transport of the MHCII molecules from the cytoplasm to the cell membrane and loss of E-cadherin expression in the tumour cells. The canine cutaneous histiocytic sarcomas showed both high morphological diversity and expression of MHCII and CD18. Two of the evaluated histiocytic sarcomas also showed expression of E-cadherin. In conclusion, immunohistochemistry, including analysis of MHCII, CD18 and the lymphocytic markers CD3 and CD79, should be performed for the diagnosis of canine cutaneous histiocytic tumours. The expression of E-cadherin in canine cutaneous histiocytic sarcomas suggests an origin of the tumour cells among Langerhans cells.