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

Sarcomas of synovial origin in dogs: An updated review

The purpose of this review is to clarify the terminology, possible cells of origin, and expected behavior of the most common synovial tumors in dogs. The synovial lining consists of 2 cell types, type A and type B. Type A synoviocytes are histiocytes of bone marrow origin that are immunoreactive with antibodies against typical markers of histiocyte origin, such as CD18, Iba-1, and CD204. Certain breeds and dogs with previous injury to a joint, especially cranial cruciate ligament rupture, are predisposed to synovial histiocytic sarcoma. Type B synoviocytes are mesenchymal cells that produce synovial fluid. There are no specific markers of type B synoviocytes, but based on their gross and microscopic appearance, synovial myxosarcomas (previously considered synovial myxomas) are presumed to be of type B synoviocyte origin. These can infiltrate into surrounding tissues, but are slow-growing and rarely metastasize, and then only to regional lymph nodes. Synovial histiocytic sarcomas and myxosarcomas can cause lysis in multiple bones surrounding the joint, but they have different prognoses and require histopathology and sometimes immunohistochemistry to diagnose them. Synovial sarcoma and synovial cell sarcoma are terms used in the human medical literature for a tumor that is not of synovial origin; these terms should not be used in veterinary medicine.

Immunohistochemical expression of vimentin, E-cadherin, and CD45 in natural cases of canine cutaneous round tumors

Round cell tumors are common cutaneous lesions in dogs, with increased occurrence percentages among different skin tumors. This study aimed to investigate the frequency as well as gross and pathological characteristics of round cell tumors in natural cases of tumorous dogs in relation to breed, sex, and age. Moreover, it aimed to evaluate the immunohistochemical expression of a panel of immunohistochemical stains, including vimentin, E-cadherin, and cluster of differentiation (CD45) as an adjunct technique for the differential diagnosis of cutaneous round cell neoplasm. Data were collected from 64 dogs of both sexes (36 females and 28 males), various breeds, and different ages (8 months to 7 years). The histopathological nature of neoplastic growth was reported, and neoplasm prevalence was classified using age, sex, breed, and site on the body. We observed 48 cases of transmissible venereal tumors, 12 cutaneous histiocytomas, and 4 histiocytic sarcoma. Immunohistochemical characterization revealed an intense positive immunoreactivity for vimentin in transmissible venereal tumor cells and moderate positive immunoreactivity for E-cadherin and CD45 in cutaneous histiocytoma and histiocytic sarcoma cells. In conclusion, the canine transmissible venereal tumor was the most frequent form of round cell tumor; thus, a definitive cutaneous neoplasm diagnosis should be based on histopathological morphology and immunohistochemical findings.

Pathological, histochemical, and immune-histochemical studies on some canine-skin neoplasm at Sharkia province, Egypt

Background

Cutaneous neoplastic disorders are often observed in small mammal pets, such as dogs, regardless of their gender.

Aim

An important objective of this work was to give a full account of the clinical, pathological, and immune-histochemical features of several skin tumors in dogs.

Methods

This study was a case series in the hospital clinic, Faculty of Veterinary Medicine, Zagazig University, Egypt. Twenty-five dogs (14 males and 11 females) were examined clinically during the period from March 2022 to October 2023. The skin swelling was collected from affected animals and then subjected to a detailed histopathological study to record the different gross and microscopic findings and confirm the diagnosis by immunohistochemistry.

Results

Skin neoplasia in dogs was exposed to various clinical signs, and the dogs' ages ranged between 3 and 11 years. Concerning tumor features, the majority of neoplasms were malignant (65.52%) more than benign (34.48%). The study revealed the presence of 29 cases of dogs showed neoplasia with different prevalence rates including squamous cell carcinoma (13.79%), mast cell tumor (6.89%), basal cell tumors (10.34%), histiocytoma (6.89%), trichoepithelioma (10.34%), transmissible venereal tumor (10.34%), trichilemmoma (3.44%), scalp paraganglioma (3.44%), pilomatricoma (10.34%), malignant melanomas (17.24%), and miscellaneous cases as fat necrosis (6.89%), in males and females dogs with different histopathological lesions and immunohistochemistry expressions for pan-cytokeratin (CK), melanocyte-differentiation antigens (S100 protein), and synaptophysin.

Conclusion

Malignant melanomas (17.24%) are the extremely common cutaneous tumors diagnosed in this study. Meanwhile, benign tumors such as trichilemmoma, trichoepithelioma, pilomatricoma, and paraganglioma are less frequent in dogs.

Metastatic Cutaneous Langerhans Cell Histiocytosis in a Flat-Coated Retriever Treated with Doxorubicin and Prednisone

A 2 yr old female intact flat-coated retriever dog was presented for evaluation of a histologically diagnosed cutaneous Langerhans cell histiocytosis of the muzzle with right mandibular nodal metastasis and suspected prescapular lymph node metastasis. Chemotherapy (lomustine 60 mg/m2 by mouth as a single dose) and glucocorticoid therapy (prednisone ∼20 mg/m2 by mouth every 24 hr) were initiated. Progressive disease occurred 21 days after lomustine administration. Doxorubicin (at 30 mg/m2 IV every 3 wk) was administered as a second-line therapy. Prednisone was continued at the same dose. Partial response was noted 1 wk after initiation of doxorubicin and sustained through doxorubicin #2. Complete remission was achieved following doxorubicin #3 (63 days from the start of doxorubicin rescue therapy). Progressive disease was noted after doxorubicin #5, for a total duration of response to doxorubicin of 105 days. Further rescue treatment with vinorelbine at 15 mg/m2 IV was elected. Progressive disease and clinical decline were noted 1 wk after initiation of vinorelbine. The patient was euthanized because of clinical decline 126 days after histopathologic diagnosis and 114 days after chemotherapy treatment was initiated.

Establishment and characterisation of cell lines and xenograft mouse models of canine systemic histiocytosis and disseminated histiocytic sarcoma

Canine histiocytic proliferative disorders include reactive diseases (histiocytosis) and neoplastic diseases (histiocytic sarcoma [HS]), however discrimination is challenging due to their overlapping pathological features. In the present study, novel cell lines and xenograft mouse models of systemic histiocytosis (SyH) and disseminated HS were established, and examined together with cell lines previously established from localized HS and Langerhans cell histiocytosis (LCH). The chromosomal numbers of the SyH and HS cell lines were abnormal, and their population doubling time and morphological features were comparable. Immunophenotypically, SyH and HS cells were CD204+/E-cadherin+ in vitro and in vivo, like their original tissues. Western blot analysis for E-cadherin revealed an immunopositive band of full-length E-cadherin (120 kDa) in cultured cells of localized HS and LCH but not in disseminated HS and SyH; expression level was weaker in localized HS than in LCH. An immunopositive band of fragmented E-cadherin (45 kDa) was detected in cell lines of disseminated HS and SyH. These results suggest that cultured SyH cells have features that are similar to disseminated HS, including chromosomal aberration, high proliferation activity, weak cell adhesion, and expression of fragmented E-cadherin. Fragmentation of the E-cadherin cell adhesion molecule may be associated with the loss of cell adhesion and increased abilities of invasion and migration of neoplastic cells. The established cell lines and xenograft mouse models will aid in understanding the pathogenesis and developing novel treatments of canine histiocytic proliferative disorders.

Cancer-Immunity Cycle and Therapeutic Interventions- Opportunities for Including Pet Dogs With Cancer

The tumor-immune interplay represents a dynamic series of events executed by cellular and soluble participants that either promote or inhibit successful tumor formation and growth. Throughout a tumor’s development and progression, the host organism’s immune system reacts by generating anti-cancer defenses through various incremental and combinatorial mechanisms, and this reactive orchestration is termed the cancer-immunity cycle. Success or failure of the cancer-immunity cycle dictates the fate of both host and tumor as winner or loser. Insights into how the tumor and host immune system continuously adapt to each other throughout the lifecycle of the tumor is necessary to rationally develop new effective immunotherapies. Additionally, the evolving nature of the cancer-immunity cycle necessitates therapeutic agility, requiring real-time serial assessment of immunobiologic markers that permits tailoring of therapies to the everchanging tumor immune microenvironment. In order to accelerate advances in the field of immuno-oncology, this review summarizes the steps comprising the cancer-immunity cycle, and underscores key breakpoints in the cycle that either favor cancer regression or progression, as well as shaping of the tumor microenvironment and associated immune phenotypes. Furthermore, specific large animal models of spontaneous cancers that are deemed immunogenic will be reviewed and proposed as unique resources for validating investigational immunotherapeutic protocols that are informed by the cancer-immunity cycle. Collectively, this review will provide a progressive look into the dynamic interplay between tumor and host immune responses and raise awareness for how large animal models can be included for developing combinatorial and sequenced immunotherapies to maximizing favorable treatment outcomes.

International Harmonization of Nomenclature and Diagnostic Criteria (INHAND): Nonproliferative and Proliferative Lesions of the Dog

The INHAND (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions) Project (www.toxpath.org/inhand.asp) is a joint initiative of the societies of toxicologic Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP), and North America (STP) to develop an internationally accepted nomenclature for proliferative and nonproliferative lesions in laboratory animals. The purpose of this publication is to provide a standardized nomenclature for classifying lesions observed in most tissues and organs from the dog used in nonclinical safety studies. Some of the lesions are illustrated by color photomicrographs. The standardized nomenclature presented in this document is also available electronically on the internet (http://www.goreni.org/). Sources of material included histopathology databases from government, academia, and industrial laboratories throughout the world. Content includes spontaneous lesions, lesions induced by exposure to test materials, and relevant infectious and parasitic lesions. A widely accepted and utilized international harmonization of nomenclature for lesions in laboratory animals will provide a common language among regulatory and scientific research organizations in different countries and increase and enrich international exchanges of information among toxicologists and pathologists.

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

BACKGROUND/AIM

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.

MATERIALS AND METHODS

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).

RESULTS

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.

CONCLUSION

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.

Safety analysis of ex vivo-expanded canine natural killer cells in a xenogeneic mouse model of graft-versus-host disease

Canine natural killer (NK) cells are large, granular lymphocytes that are neither B lymphocytes nor T lymphocytes. However, it has been reported that canine NK cells share some of the phenotypic characteristics of T lymphocytes, such as CD3 and CD5. Studies are needed to assess the safety of canine NK cells for immunotherapy, especially because the safety of using allogeneic NK cells as an immunotherapy for dogs has yet to be shown. In this study, the safety of cultured canine NK cells was assessed using a xenogeneic mouse model of graft-versus-host disease (GVHD). Mice were injected with either canine peripheral blood mononuclear cells (PBMCs) or cultured NK cells for 2 or 3 weeks. Data were then collected on changes in mice body weights, disease severity scores, and survival rates. Histopathological and immunohistochemical evaluations were also performed. All mice injected with canine PBMCs died within 45 days after injection. Severe clinical signs were caused by GVHD. The histopathological and immunohistochemical evaluations showed that mice injected with canine PBMCs had multiple lesions, including necrosis in their lungs, livers, kidneys, and stomachs, and the injected cells were present around the lesions. By contrast, no mice injected with cultured NK cells without removing the CD3⁺ TCR^- cells exhibited any clinical abnormalities. Moreover, they all survived the 90-day experimental period without exhibiting any histopathological changes. Accordingly, the results of this study suggest that canine NK cells do not cause significant side effects such as GVHD and allogeneic NK cells can safely be used for cancer immunotherapy in dogs.

Histopathological Classification of Canine Cutaneous Round Cell Tumors Using Deep Learning: A Multi-Center Study

Canine cutaneous round cell tumors (RCT) represent one of the routine diagnostic challenges for veterinary pathologists. Computer-aided approaches are developed to overcome these restrictions and to increase accuracy and consistency of diagnosis. These systems are also of high benefit reducing errors when a large number of cases are screened daily. In this study we describe ARCTA (Automated Round Cell Tumors Assessment), a fully automated algorithm for cutaneous RCT classification and mast cell tumors grading in canine histopathological images. ARCTA employs a deep learning strategy and was developed on 416 RCT images and 213 mast cell tumors images. In the test set, our algorithm exhibited an excellent classification performance in both RCT classification (accuracy: 91.66%) and mast cell tumors grading (accuracy: 100%). Misdiagnoses were encountered for histiocytomas in the train set and for melanomas in the test set. For mast cell tumors the reduction of a grade was observed in the train set, but not in the test set. To the best of our knowledge, the proposed model is the first fully automated algorithm in histological images specifically developed for veterinary medicine. Being very fast (average computational time 2.63 s), this algorithm paves the way for an automated and effective evaluation of canine tumors.

Comparative Evaluation of the Cytological, Histopathological and Immunohistochemical Findings of Canine Cutaneous and Subcutaneous Masses

In this study, we compared the cytological, histopathological, and immunohistochemical diagnoses of 71 canine cutaneous and subcutaneous masses. Cytological diagnoses included 56 tumors (21 mesenchymal, 15 epithelial, 16 round cell, four melanocytic), 13 inflammatory reactions, and two cysts. Of the 21 cytologically diagnosed mesenchymal tumors, three were later confirmed non-tumoral (hematoma, granulation tissue, fibroepithelial polyp). Thirteen out of 15 epithelial tumors were correctly diagnosed cytologically, whereas two cases were confirmed to be non-tumoral (fibroepithelial polyp, granulation tissue) after histopathological examination. One mast cell tumor was later confirmed as fibrous hyperplasia; diagnoses were correct in other round cell tumors. Cytological diagnoses were correct for all melanocytic tumors and cystic lesions. Five cases which had been cytologically diagnosed as inflammatory reactions were diagnosed as tumors (lymphoma, papilloma, sebaceous adenoma, and squamous cell carcinoma) after histopathological examination. Immunohistochemistry confirmed the histopathological diagnoses of all epithelial and round cell tumors, while the diagnoses of six mesenchymal tumors were changed after the immunohistochemical examination. The total accuracy of cytology in the diagnosis of tumoral/non-tumoral masses was 84.5%, and the accuracy in the determination of benign/malignant behavior was 83%. Diagnostic accordance between histopathology and immunohistochemistry was 86.6%. High success rates obtained with cytological diagnoses prove that cytology is a reliable diagnostic tool. The main diagnostic challenge remains with mesenchymal tumors and tumors accompanied by inflammatory reactions. The results suggest that immunohistochemistry is fundamental for diagnoses of most mesenchymal tumors.

Tumor Infiltrating Lymphocytes in Pet Rabbit Mammary Carcinomas: A Study with Relevance to Comparative Pathology

Simple Summary

The interaction between tumors and immune cells influences tumor fate, i.e., regression, growth, or even metastases. The evaluation of tumor infiltrating lymphocytes (TILs) in human breast cancer has prognostic value. Pet rabbits develop spontaneous mammary carcinomas and have an immune system that is comparable with that of humans, so that they have the potential to provide an animal model for human breast cancer. To further substantiate this similarity, this study examined TILs in 107 pet rabbit mammary carcinomas according to criteria established for human breast cancer. For TIL evaluation routinely stained microscopic sections were examined by light microscopy. Relevant histological and immunohistochemical tumor characteristics were obtained from a data base. Results showed that increased presence of stromal TILs was statistically associated with histological tumor features indicative of a less aggressive biological behavior, i.e., reduced tumor cell proliferation and a lower histological grade. The expression by tumor cells of calponin, a presumed tumor suppressor protein, was also associated with their reduced proliferation and a higher percentage of stromal TILs. Data suggest that higher percentages of stromal TILs may have the potential to serve as favorable prognostic indicator in rabbit mammary carcinomas and support the value of pet rabbits for comparative research.

Abstract

Tumor infiltrating lymphocytes (TILs) serve as prognostic biomarker in human breast cancer. Rabbits have the potential to act as animal model for human breast cancer, and close similarities exist between the rabbit and human immune system. The aim of this study is to characterize TILs in pet rabbit mammary carcinomas and to statistically correlate results with histological and immunohistochemical tumor characteristics. Microscopic evaluation of TILs was performed in hematoxylin and eosin stained sections of 107 rabbit mammary carcinomas according to international guidelines for human breast cancer. Data on histological features of malignancy, estrogen and progesterone receptor status and calponin expression were obtained from the data base. This study revealed a statistical association between stromal TILs in the central tumor (CT) and infiltrative margin. Higher maximal percentages of stromal TILs at the CT were statistically correlated with decreased mitotic count and lower tumor grade. An increased number of calponin positive tumor cells was statistically associated with a lower mitotic count and a higher percentage of stromal TILs. Results suggest that higher percentages of stromal TILs are useful biomarkers that may point toward a favorable prognosis in rabbit mammary carcinomas and support the concept of the use of rabbits for translational research.

FOXP3, CD208, and CD206 Expression in Canine Cutaneous Histiocytoma

Canine cutaneous histiocytoma (CCH) is a noninfectious tumor that spontaneously regresses. It is suggested that this regression is due to tumor cell maturation, which is responsible for CD8 lymphocyte activation and tumor cell destruction. Nevertheless, the possible role of the immune microenvironment in tumor regression has not been investigated to date. The aim of this study was to investigate the expression of CD208 and FoxP3 as markers of dendritic cells and regulatory T lymphocytes, respectively, and tumor cell expression of CD206 as a marker of Langerhans cell activation, and relate these parameters to the different phases of CCH and to intratumoral T cell infiltration. Formalin-fixed, paraffin-embedded samples from 31 CCH were evaluated. In each case, the mitotic count and regression phase were recorded. Within the tumor, a quantitative evaluation of immunolabeled CD208⁺ cells, FoxP3⁺ cells, and CD3⁺ lymphocytes was performed, as well as the CD206⁺ tumor cell location. Intratumoral CD208⁺ cells correlated with CD3⁺ lymphocytic infiltration. The possible role of dendritic cells in tumor regression was not confirmed since CD208 seemed to be a nonspecific marker for canine dendritic cells. FoxP3⁺ lymphocyte density was not correlated with any parameter. Neoplastic Langerhans cells presented progressive CD206 expression, from the bottom of the tumor to the epidermis, which correlated with the tumor regression phase and with intratumoral T lymphocyte infiltration. In conclusion, we confirmed a CD206 phenotype change in tumor cells in a spatial group-related pattern, supporting the hypothesis that tumoral Langerhans cells acquire a mature phenotype with tumor regression.

Genome-wide DNA copy number analysis and targeted transcriptional analysis of canine histiocytic malignancies identifies diagnostic signatures and highlights disruption of spindle assembly complex

Canine histiocytic malignancies (HM) are rare across the general dog population, but overrepresented in certain breeds, such as Bernese mountain dog and flat-coated retriever. Accurate diagnosis relies on immunohistochemical staining to rule out histologically similar cancers with different prognoses and treatment strategies (e.g., lymphoma and hemangiosarcoma). HM are generally treatment refractory with overall survival of less than 6 months. A lack of understanding regarding the mechanisms of disease development and progression hinders development of novel therapeutics. While the study of human tumors can benefit veterinary medicine, the rarity of the suggested orthologous disease (dendritic cell sarcoma) precludes this. This study aims to improve the understanding of underlying disease mechanisms using genome-wide DNA copy number and gene expression analysis of spontaneous HM across several dog breeds. Extensive DNA copy number disruption was evident, with losses of segments of chromosomes 16 and 31 detected in 93% and 72% of tumors, respectively. Droplet digital PCR (ddPCR) evaluation of these regions in numerous cancer specimens effectively discriminated HM from other common round cell tumors, including lymphoma and hemangiosarcoma, resulting in a novel, rapid diagnostic aid for veterinary medicine. Transcriptional analysis demonstrated disruption of the spindle assembly complex, which is linked to genomic instability and reduced therapeutic impact in humans. A key signature detected was up-regulation of Matrix Metalloproteinase 9 (MMP9), supported by an immunohistochemistry-based assessment of MMP9 protein levels. Since MMP9 has been linked with rapid metastasis and tumor aggression in humans, the data in this study offer a possible mechanism of aggression in HM.

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.

Matrix metalloproteinases expression in spontaneous canine histiocytic sarcomas and its xenograft model

Canine histiocytic sarcoma (HS) represents a malignant neoplastic disorder often with a rapid and progressive clinical course. A better understanding of the interaction between tumor cells and the local microenvironment may provide new insights into mechanisms of tumor growth and metastasis. The influence of matrix metalloproteinases (MMPs) and their inhibitors (TIMPs) on tumor angiogenesis, invasion and metastasis has been detailed in previous studies. In addition, inflammatory cells infiltrating neoplasms especially tumor associated macrophages (TAM) may contribute significantly to tumor progression. Due to the high variability of spontaneously occurring canine HS, standardized models are highly required to investigate tumor progression and interaction with its microenvironment. Therefore, the present study comparatively characterized the intratumoral macrophage infiltration as well as the expression of MMP-2, MMP-9, MMP-14 and TIMP-1 in spontaneous canine HS and its murine model. In spontaneous canine HS, scattered MAC 387-positive macrophages were randomly found in tumor center and periphery, whereas tumor cells were negative for this marker. Interestingly, quantitative analysis revealed that MMPs and TIMP-1 were mainly expressed at the invasive front while tumor centers exhibited significantly reduced immunoreactivity. Similar findings were obtained in xenotransplanted HS. Interestingly, murine tumor associated macrophages (TAM), characterized by Mac3 expression (CD107b/LAMP2), which was not present in xenotransplanted histiocytic sarcoma cells, strongly express MMPs and TIMP-1. In addition, MMPs are known to regulate angiogenesis and a positive correlation between MMP-14 expression and microvessel density was demonstrated in xenotransplanted histiocytic sarcomas. Summarized similar findings with respect to MMP and TIMP distribution and the role of macrophages in spontaneously-occurring and xenotransplanted HS indicate the high suitability of this murine model to further investigate HS under standardized conditions. Moreover results indicate that MMP expression contributes to tumor progression and invasion and TAMs seem to be major players in the interaction between neoplastic cells, the microenvironment and vessel formation indicating that therapeutic approaches modulating TAM associated molecules might represent promising future treatment options.

Comparisons among MRI signs, apparent diffusion coefficient, and fractional anisotropy in dogs with a solitary intracranial meningioma or histiocytic sarcoma

Although MRI has become widely used in small animal practice, little is known about the validity of advanced MRI techniques such as diffusion-weighted imaging and diffusion tensor imaging. The aim of this retrospective analytical observational study was to investigate the characteristics of diffusion parameters, that is the apparent diffusion coefficient and fractional anisotropy, in dogs with a solitary intracranial meningioma or histiocytic sarcoma. Dogs were included based on the performance of diffusion MRI and histological confirmation. Statistical analyses were performed to compare apparent diffusion coefficient and fractional anisotropy for the two types of tumor in the intra- and peritumoral regions. Eleven cases with meningioma and six with histiocytic sarcoma satisfied the inclusion criteria. Significant differences in apparent diffusion coefficient value (× 10^-3 mm² /s) between meningioma vs. histiocytic sarcoma were recognized in intratumoral small (1.07 vs. 0.76) and large (1.04 vs. 0.77) regions of interest, in the peritumoral margin (0.93 vs. 1.08), and in the T2 high region (1.21 vs. 1.41). Significant differences in fractional anisotropy values were found in the peritumoral margin (0.29 vs. 0.24) and the T2 high region (0.24 vs. 0.17). The current study identified differences in measurements of apparent diffusion coefficient and fractional anisotropy for meningioma and histiocytic sarcoma in a small sample of dogs. In addition, we observed that all cases of intracranial histiocytic sarcoma showed leptomeningeal enhancement and/or mass formation invading into the sulci in the contrast study. Future studies are needed to determine the sensitivity of these imaging characteristics for differentiating between these tumor types.

Canine Histiocytic Malignancies-Challenges and Opportunities

Canine histiocytic malignancies (HM) are aggressive tumors that occur with particularly high frequency in certain breeds including Bernese mountain dogs and flat-coated retrievers. Robust diagnosis of HM commonly utilizes immunohistochemical stains that are broadly ineffective on formalin-fixed tissues; thus the diagnosis is often one of exclusion. Clinical outcomes are generally poor, with frequent metastasis and therapeutic failure lowering overall survival at time of diagnosis to an average of less than two months in the majority of published work. The limited understanding of the molecular mechanisms underlying HM has hindered the development of more effective diagnostic modalities and the identification of therapeutic targets. A potential avenue exists for advancing clinical management of canine cancers through extrapolation from a close counterpart in human medicine. Historically, HM have been compared to the rare and understudied subset of human cancers involving the dendritic lineage, such as dendritic cell sarcoma or Langerhans cell sarcoma. Recent data have now thrown into question the cellular origin of HM, suggesting that the disease may originate from the macrophage lineage. This review summarizes existing knowledge of HM from the clinical, histologic and molecular perspectives, and highlights avenues for future research that may aid the development of novel diagnostic and therapeutic approaches. In turn, a more advanced appreciation of the mechanisms underlying HM should clarify their cellular origin and identify appropriate opportunities for synergistic extrapolation between related canine and human cancers.