Follicular thyroid carcinoma in an inbred family of mongrel dogs in Trinidad & Tobago.

Thyroid tumors occur in many domestic species, but are most common in the dog, in which they are classified as follicular or medullary. During 2012-2016, we received tissue specimens or whole carcasses of 4 dogs with variable enlargement of the thyroid glands. The 2 males and 2 females were of mixed (mongrel) inbreeding, 3-4.5-y-old. All tumors had lobulated architecture forming follicular structures variably containing colloid. On immunohistochemistry of the tumors from 3 of the dogs, 2 were thyroglobulin positive, and all 3 were negative for calcitonin, confirming follicular thyroid carcinoma in 2 of the dogs. Thyroid carcinomas have not been reported previously in related mongrel dogs, to our knowledge.

Immunohistochemistry Screening of Different Tyrosine Kinase Receptors in Canine Solid Tumors-Part I: Proposal of a Receptor Panel to Predict Therapies.

The use of tyrosine kinase inhibitors (TKI) has been growing in veterinary oncology and in the past few years several TKI have been tested in dogs. However, different from human medicine, we lack strategies to select patients to be treated with each TKI. Therefore, this study aimed to screen different tumor subtypes regarding TKI target immunoexpression as a predictor strategy to personalize the canine cancer treatment. It included 18 prostatic carcinomas, 36 soft tissue sarcomas, 20 mammary gland tumors, 6 urothelial bladder carcinomas, and 7 tumors from the endocrine system. A total of 87 patients with paraffin blocks were used to perform immunohistochemistry (IHC) of human epidermal growth factor receptor 2 (HER-2), epidermal growth factor receptors 1 (EGFR1), vascular endothelial growth factor receptor 2 (VEGFR-2), platelet derived growth factor receptor beta (PDGFR-ß), c-KIT, and extracellular signal-regulated kinase 1/2 (ERK1/ERK2). The immunohistochemical screening revealed a heterogeneous protein expression among histological types with mesenchymal tumors showing the lowest expression level and carcinomas the highest expression. We have demonstrated by IHC screening that HER2, EGFR1, VEGFR-2, PDGFR-ß and ERK1/ERK2 are commonly overexpressed in dogs with different carcinomas, and KIT expression is considered relatively low in the analyzed samples.

Canine primary ureteral leiomyosarcoma treated with unilateral left ureteronephrectomy.

Background: Primary ureteral neoplasms are extremely rare in dogs, and ureteral involvement usually occurs owing to the invasion of renal and bladder tumors. Case Description: This case report describes a 12-year-old intact male mixed-breed dog referred to a private clinic with a six-month history of abdominal distention. A physical examination revealed mild abdominal pain. Hematological tests detected normocytic-normochromic anemia (hematocrit 33.6% [reference interval-RI: 37%-55%], red blood cells 4.93 M/µl [RI: 5.5-8.5 M/µl], and hemoglobin 12.4 g/dl [RI: 12-18.0 g/dl]). The results from the leukogram, thrombogram, renal, and hepatic panels were within the reference intervals for dogs. Abdominal ultrasonography revealed a cavitary mass measuring approximately 12 cm in diameter as the largest tumor in the left abdominal region over the left hepatic lobe or mesenteric site. Chest radiography did not reveal any metastasis. Therefore, the patient underwent exploratory laparotomy, during which the left ureter was found to be affected by a 12-cm mass that adhered to the left kidney. A unilateral left ureteronephrectomy was performed, and histology and immunohistochemistry (IHC) confirmed well-differentiated primary ureteral leiomyosarcoma. The patient survived for 130 days but died of lung metastasis. Conclusion: Ureteral leiomyosarcoma should be investigated and included in the list of differential diagnoses for primary ureteral neoplasms. Regardless of the therapeutic modality, the prognosis of ureteral leiomyosarcoma may be unfavorable, as shown in this report.

Evaluation of immunophenotype and inflammation in canine mammary neoplasms with solid arrangement.

Morphological and immunohistochemical studies of solid arrangement canine mammary carcinomas have shown that the different histological types may be characterized by proliferation of epithelial and/or myoepithelial cells. However, little is known about immunophenotypes and the importance of inflammation as prognostic factors in these neoplasms. The objective of the present study was to characterize the immunophenotype and degree of inflammation in the solid type of canine mammary neoplasm and to investigate their association with metastasis, Ki-67 index, tumour size, necrosis and survival. Sixty-five carcinomas with solid pattern, basaloid carcinomas, solid papillary carcinomas, malignant adenomyoepitheliomas (MAMEs) or malignant myoepitheliomas (MMEs) were investigated. Luminal A, luminal B HER2 negative and HER2 positive, HER2 overexpressed and triple negative immunophenotypes were immunolabelled as were Ki-67 protein and cyclooxygenase-2 (Cox-2). Histological peritumoural and intratumoural inflammatory infiltrates were graded (distribution × intensity) and the presence of necrosis identified. We found a statistical difference between histological types and immunophenotypes, with MME and MAME having a higher occurrence of luminal A, whereas most neoplasms had the luminal B HER-negative immunophenotype. There was no correlation between immunophenotype and degree of peri- and intratumoural inflammation, nodal metastasis, necrosis or tumour size. An increased degree of peri- and intratumoural inflammation was significantly associated with lymph node metastasis, and more severe intratumoural inflammation was associated with the presence of tumour necrosis. Tumour size, Ki-67 index and Cox-2 score were not associated with inflammation in either peri- or intratumoural regions. No difference was observed in survival in relation to immunophenotype or degree of inflammation, but the Cox regression model revealed that nodal metastasis influenced the risk of death.

Molecular and Pathobiology of Canine Mammary Tumour: Defining a Translational Model for Human Breast Cancer.

Canine mammary tumours (CMT) have histological, clinicopathological and molecular resemblances to human breast cancer (HBC), positioning them as viable models for studying the human disease. CMT initiation and progression occur spontaneously in immune-competent animals, which challenge the suggested limitations of genetically modified mice, also enabling the evaluation of immunotherapies in canine patients. Dogs have shorter life expectancy compared to humans, and cancer advances more rapidly in this species. This makes it possible to perform studies about the clinical efficacy of new therapeutic modalities in a much shorter time than in human patients. The identification of biomarkers for tumour subtypes, progression and treatment response paves the way for the development of novel therapeutic and diagnostic approaches. This review addresses the similarities between CMT and HBC and the molecular signatures identified in CMT samples that have been explored to date. We proposed a detailed molecular exploration of the CMT stroma using state-of-the-art methods in transcriptomics and proteomics. Using CMT as an analog for HBC not only helps to understand the complexities of the disease, but also to advance comparative oncology to the next level to prove the claim of dogs as a valid translational model.

Pilot study to evaluate isolation by size of circulating tumour cells in canine oral melanoma.

Liquid biopsy for circulating tumour cell (CTC) detection is generally unexplored in veterinary medicine. Dogs with highly aggressive and heterogeneous tumours, such as oral malignant melanoma (OMM), could benefit from studies involving size-based isolation methods for CTCs, as they do not depend on specific antibodies. This pilot study aimed to detect CTCs from canine OMM using Isolation by Size of Epithelial Tumor Cells (ISET), a microfiltration methodology, followed by immunocytochemistry (ICC) with Melan-A, PNL2, and S100 antibodies. Ten canine patients diagnosed by histopathology and confirmed as OMM by immunohistochemistry were enrolled, their prognostic data was assessed, and blood samples were collected for CTC analysis. Results have shown the detection of intact cells in 9/10 patients. ICC has shown 3/9 Melan-A-positive, 3/9 PNL2-positive, and 8/9 S100-positive patients, confirming the importance of opting for a multimarker assay. A significant number of negative-stained CTCs were found, suggesting their high heterogeneity in circulation. Microemboli stained with either PNL2 or S100 were found in a patient with a high isolated cell count and advanced clinical stage. Preliminary statistical analysis shows a significant difference in CTC count between patients with and without lymph node metastasis (p < .05), which may correlate with tumour metastatic potential. However, we recommend further studies with more extensive sampling to confirm this result. This pilot study is the first report of intact CTC detection in canine OMM and the first application of ISET in veterinary medicine, opening new possibilities for liquid biopsy studies in canine OMM and other tumours.

Review of the comparative pathological and immunohistochemical features of human and canine cutaneous melanocytic neoplasms.

Melanocytic neoplasms originate from melanocytes and melanoma, the malignant form, is a common canine neoplasm and the most aggressive human skin cancer. Despite many similarities between these neoplasms in both species, only a limited number of studies have approached these entities in a comparative manner. Therefore, this review compares benign and malignant melanocytic neoplasms in dogs and humans, exclusively those arising in the haired skin, with regard to their clinicopathological, immunohistochemical and molecular aspects. Shared features include spontaneous occurrence, macroscopic features and microscopic findings when comparing human skin melanoma in the advanced/invasive stage and canine cutaneous melanoma, immunohistochemical markers and several histopathological prognostic factors. Differences include the apparent absence of active mutations in the BRAF gene in canine cutaneous melanoma and less aggressive clinical behaviour in dogs than in humans. Further studies are required to elucidate the aetiology and genetic development pathways of canine cutaneous melanocytic neoplasms. Evaluation of the applicability of histopathological prognostic parameters commonly used in humans for dogs are also needed. The similarities between the species and the recent findings regarding genetic mutations in canine cutaneous melanomas suggest the potential utility of dogs as a natural model for human melanomas that are not related to ultraviolet radiation.

Bilateral testicular myxosarcoma in a cryptorchid dog.

Myxosarcoma is a rare malignant mesenchymal neoplasm of soft tissues originating from fibroblasts. This report describes a case of bilateral myxosarcoma in a three-year-old cryptorchid dog. The animal was referred to the veterinary clinic because of the absence of testicles in the scrotum. Ultrasonography revealed two masses in the abdominal cavity with testicular echotexture. Exploratory laparotomy revealed the presence of cryptorchid testicles, and orchiectomy was recommended to treat the animal. Testicles were gray and reddish in color and enlarged with firm consistency. For histopathological analysis, testis fragments were fixed in 10% formalin and stained with hematoxylin and eosin and Alcian blue. Immunohistochemistry was performed using the following primary antibodies:1A4, HHF35, desmin, glial fibrillary acidic protein, CD31, S-100, vimentin, and Ki-67. Histopathological evaluation revealed the proliferation of fusiform and round cells associated with extensive areas of myxoid matrix. Neoplasms featured multinucleated giant cells, pleomorphism, karyomegaly, nuclear hyperchromasia, anisokaryosis, mitoses, and necrosis, with coarse chromatin and prominent nucleoli. Immunohistochemical analysis of vimentin- and the Alcian blue-positive cells confirmed the diagnosis of myxosarcoma. A high mitotic count and Ki-67 proliferative index suggests this myxosarcoma had a high degree of malignancy. To the best of our knowledge, this is the first case report of bilateral testicular myxosarcoma in a cryptorchid animal.

A compared histopathological study on kidneys and eye bulbs in distinct clinical presentations of canine leishmaniasis by Leishmania infantum.

Canine leishmaniasis (CanL) caused by Leishmania infantum commonly progresses with renal and ophthalmic lesions associated with active systemic disease. As chronic inflammation related to immune complex deposits is a pathophysiological factor in the development of both glomerulonephritis and uveitis, we aimed to evaluate renal and ocular histopathological lesions and analyze whether they were related to each other and the clinical degree of the disease. For that, we evaluated 15 dogs from CanL-endemic areas. L. infantum PCR-positive dogs were studied according to disease severity into two different groups: Group-1 (G1) had data from seven dogs with mild to moderate CanL and no history of treatment, and G2 was formed with eight dogs with severe to terminal disease that had not responded to CanL treatment. Histopathological analysis of kidneys showed higher frequencies and intensities of glomerular basement membrane thickening (p = 0.026), deposits in glomeruli (p = 0.016), epithelial necrosis (p = 0.020), tubular dilatation (p = 0.003) and interstitial fibrosis (p = 0.04) in G2 dogs than in G1 dogs. Surprisingly, the histopathology of eye bulbs showed a higher frequency and intensity of retinitis (p = 0.019) in G1 dogs than in G2 dogs. The comparative analysis showed that there was no correspondence between histopathological findings in kidneys versus eyes in milder or more severe CanL. Our findings suggested that (1) clinically undetectable eye alterations can be more precocious than those in kidneys in the development of CanL, and (2) the lower frequency of eye lesions and higher frequency of renal lesions in dogs with terminal disease even after treatment indicate that therapy may have been effective in reducing CanL-associated ophthalmic disease but not proportionally in reducing kidney disease.

Case report of canine discoid lupus erythematosus in Guatemala.

An 11-year-old male Golden Retriever was presented for consultation due to a chronic progressive lesion on the nose that had started a year before. The majority of the nasal mucosa was affected, with the disruption of the normal architecture, pigment atrophy and abundant peeling on the rostral plane. Histopathology revealed a band of lichenoid infiltrate at the interface and vacuolation of the cells in the basal layer consistent with a diagnosis of canine discoid lupus erythematosus.

Encephalic meningioangiomatosis in a dog with high-grade undefined glioma.

Meningioangiomatosis (MA) is a rare proliferative meningovascular disorder that affects mainly the cerebral cortex, brainstem and spinal cord of humans and animals and can coexist with other proliferative disorders. A 7.5-year-old male Brazilian Campeiro Bulldog died after a convulsive crisis and cardiorespiratory arrest. At necropsy, a firm, white mass involving the piriform and right occipital lobes was seen. Histologically, the mass consisted of two morphologically distinct entities that collided: a congenital malformation characterized by a proliferation of meningothelial cells around blood vessels, within the perivascular spaces; and a neoplasm composed of two cell populations with astrocytic and oligodendrocytic differentiation. Meningothelial cells and neoplastic glial cells immunolabelled for vimentin. This first reported case of encephalic MA with a high-grade undefined glioma in a dog was confirmed through clinical signs, pathological and immunohistochemical findings.

Narrative operative reports inconsistently record surgical information relating to resection of soft tissue sarcomas and mast cell tumors in dogs.

OBJECTIVE: To report to what degree narrative operative reports for soft tissue sarcoma (STS) and mast cell tumor (MCT) resections met a predetermined template made up of essential elements. ANIMALS: 197 consecutive client-owned animals between May 1, 2017, and August 1, 2022. PROCEDURES: A consensus list of 9 elements made up the final synoptic operative report (SR) template. Consecutive narrative surgery reports (NRs) of dogs that underwent MCT or STS resection were then reviewed to determine how many of the SR elements were present in each NR. A score was then determined for each NR out of a maximum total of 9. RESULTS: Overall, 197 reports (99 MCT and 98 STS) were included. The median score was 5 (56% of elements reported). No report had all 9 elements, and 1 report had none of the elements reported. When MCT and STS were analyzed independently, the median score was 6 (67% of elements reported) for MCT and 5 (56% of elements reported) for STS. There was a trend of more cases with MCT that had a preoperative diagnosis, intraoperative measurements of the tumor, and surgeon margins marked compared to dogs with STS. More dogs with STS had an estimated Enneking dose compared to dogs with MCT. CLINICAL RELEVANCE: Our data show that essential elements of STS and MCT resection in dogs were inconsistently recorded and no case had all elements present. This mirrors data in people and presses the need for more standardization in reporting of cancer operations in veterinary medicine.

Cutaneous mastocytosis in 8 young dogs and review of literature.

Cutaneous mastocytosis (CM) is a rare condition in young dogs characterized by multicentric cutaneous proliferation of neoplastic mast cells. Clinical data from 8 dogs that met inclusion criteria (age of onset less than 1.5 years, greater than 3 lesions) were obtained via a standardized survey. Biopsy samples were classified by the Kiupel/Patnaik grading systems and analyzed for c-KIT mutations. The median age of onset was 6 months (range: 2-17 months). Dogs had 5 to more than 50 lesions characterized as nodules, plaques, and papules. Seven dogs were pruritic. Clinical staging in 2 dogs did not reveal visceral involvement. No dogs had systemic illnesses at diagnosis. Histologically, CM was similar to cutaneous mast cell tumor (cMCT). Two dogs had neoplasms classified as high-grade/grade II while 6 dogs had low-grade/grade II neoplasms. No dogs had mutations in c-KIT exons 8 and 11. Treatment included antihistamines (8/8), corticosteroids (7/8), lokivetmab (3/8), and toceranib (1/8). Six dogs were alive with lesions at the end of the study with a median follow-up time of 898 days, while 2 dogs were euthanized. In dogs with high-grade/grade II neoplasms, one continued to develop lesions at 1922 days post-diagnosis, while the other dog was euthanized at 56 days post-diagnosis. One dog was euthanized 621 days post-diagnosis due to rupture of a neoplasm. CM occurs in young dogs and is histologically indistinguishable from cMCT. Current histologic grading systems did not apply uniformly to the dogs of the study and further studies are needed.

Grade shifts in recurrent canine soft tissue sarcomas and mast cell tumors.

OBJECTIVE: To determine the incidence of histologic grade shift (alteration of grade relative to the original tumor) in recurrent canine soft tissue sarcoma (STS) and mast cell tumor (MCT), and to determine the level of agreement between blinded pathologist review and original histology interpretation of STS and MCT grades. ANIMALS: 15 dogs with recurrent cutaneous/subcutaneous STS and 5 dogs with recurrent cutaneous MCT. All included dogs underwent excision of both the primary and recurrent tumors and had tumor samples available for review. PROCEDURES: The medical records and histology database from a single institution were reviewed, and data were recorded and analyzed. A single board-certified veterinary pathologist performed blinded evaluation of all excisional tumor samples, including both primary and recurrent disease, and these were evaluated independently and in conjunction with initial pathologic diagnoses. RESULTS: Based on single pathologist review, 7 of 15 (46.7%) dogs with recurrent STS had grade shift characterized by a higher or lower recurrent tumor grade in 4 of 7 and 3 of 7 cases, respectively, and 1 of 5 dogs with recurrent MCT had grade shift characterized by an increased grade of the recurrent tumor. Variability in reported grade between original histology report and pathologist review occurred for 13 of 30 (43.3%) STS excisional biopsy samples and 0 of 10 MCT excisional biopsy samples. CLINICAL RELEVANCE: Grade shift has been reported in multiple tumor types in people and has the potential to alter prognosis and treatment recommendations. This is the first study to document this phenomenon in dogs. Additional large-scale studies are needed to determine factors associated with grade shift as well as prognostic significance of grade shift for recurrent canine STS and MCT.

Giant and potentially malignant bullae in a dog.

Background: Primary lung neoplasms are, frequently represented by solid, solitary, or multiple formations. However, malignant cavitary lesions may be presented as lung adenocarcinomas. Those malignant lesions differ from benignant bullae by the thickness heterogeneity of its surrounding shape. Case Description: The present clinical case reports a 14-year-old female dog, of mixed breed, with an increase in the coughs frequency, fatigue, and exercise intolerance. A chest X-ray was taken, a large emphysematous cystic area was found, with thickened and irregular walls located in the left caudal pulmonary lobe, which measured 8 × 7.5 × 3 cm, and rejected the bronchial branch corresponding to the left caudal pulmonary lobe, in addition to thickening of the bronchial walls, compatible with bronchopathy. The tomographic examination of the cavity showed an air content structure, oval to round in shape, with irregular thick hyperattenuating walls measuring approximately 0.4 cm in thickness, occupying more than 30% of the left hemithorax, and pulmonary lobectomy was chosen. Histopathology confirmed the diagnosis of bronchoalveolar adenocarcinoma, with the presence of sparse areas of necrosis and dystrophic calcification. Conclusion: The present case successfully diagnosed a malignant bulae, after a surgical remove. The tomographic finds although not confirmatory, suggest a malignant component by the shape and thickness of its wall. The tomographic exam is of great importance, because only through it, it is possible to evaluate if there is lymph node or pleural involvement or the presence of small metastasis foci. There is indication for surgery and histopathological examination of the piece for a definitive diagnosis.

Pathological Aspects and Immunohistochemical Evaluation of Troponin C in the Cardiovascular System of Dogs With Pheochromocytoma.

Functional pheochromocytomas secrete catecholamines and have been associated with cardiovascular lesions in dogs. This study aimed to describe the postmortem pathological findings in the cardiovascular system of dogs with pheochromocytoma and to evaluate the expression of cardiac troponin C in these dogs using immunohistochemical analysis. Twelve cases were identified, with a mean age of 10.6 years. The heart of all dogs was enlarged and with concentric hypertrophy of the left ventricular myocardium. Histological analysis showed cardiomyocyte necrosis and degeneration in the myocardium, with frequent bands of contraction, fibrosis, inflammation, and thickening of the medium-caliber arteries in the myocardium. There was a marked decrease or absence of immunolabeling in necrotic cardiomyocytes. We conclude that IHC for troponin C can be a useful tool for detecting myocardial necrosis in dogs with pheochromocytomas, including early cases of necrosis with only incipient cardiac changes where overt histologic abnormalities are not immediately apparent in the cardiomyocytes.

Preliminary characterization of a novel form of progressive retinal atrophy in the German Spitz dog associated with a frameshift mutation in GUCY2D.

OBJECTIVE: To describe the clinical, preliminary electroretinographic and optical coherence tomography features of a newly identified form of progressive retinal atrophy (PRA) in German Spitzes, and identify the causal gene mutation. ANIMALS: Thirty-three client-owned German Spitz dogs were included. PROCEDURES: All animals underwent a full ophthalmic examination, including vision testing. In addition, fundus photography, ERG, and OCT were performed. A DNA-marker-based association analysis was performed to screen potential candidate genes and the whole genomes of four animals were sequenced. RESULTS: Initial fundus changes were pale papilla and mild vascular attenuation. Oscillatory nystagmus was noted in 14 of 16 clinically affected puppies. Vision was impaired under both scotopic and photopic conditions. Rod-mediated ERGs were unrecordable in all affected dogs tested, reduced cone-mediated responses were present in one animal at 3 months of age and unrecordable in the other affected animals tested. Multiple small retinal bullae were observed in three clinically affected animals (two with confirmed genetic diagnosis). OCT showed that despite loss of function, retinal structure was initially well-preserved, although a slight retinal thinning developed in older animals with the ventral retina being more severely affected. Pedigree analysis supported an autosomal recessive inheritance. A mutation was identified in GUCY2D, which segregated with the disease (NM_001003207.1:c.1598_1599insT; p.(Ser534GlufsTer20)). Human subjects with GUCY2D mutations typically show an initial disconnect between loss of function and loss of structure, a feature recapitulated in the affected dogs in this study. CONCLUSION: We identified early-onset PRA in the German Spitz associated with a frameshift mutation in GUCY2D.

Microscopic Alterations of the Dental Pulp in Surgically Extracted Teeth of Dogs.

Microscopic alterations in the dental pulp of dogs have not been extensively studied. The aim of this study was to investigate microscopic alterations of the dental pulp in dogs' teeth. One hundred and ten surgically extracted teeth (20 incisors, 23 canines, 28 premolars, and 39 molars) from 74 dogs, of different ages, with a history of chronic periodontitis (66 dogs), periapical abscesses (2 dogs), pulpitis (2 dogs), oral cavity neoplasms (2 dogs), dens invaginatus (1 dog), and dental fractures (1 dog) were included. Eight-one maxillary and 29 mandibular teeth were included. Coronal, radicular, and coronal plus radicular calculus were present in 28.2%, 17.3%, and 54.5% of the teeth, respectively. In total 78 teeth (71%) had pulp alterations, including fibrosis (26%), calcification (14%), necrosis associated with the absence of odontoblasts (14%), presence of predentin and dentin inside the cavity (8%), odontoblastic hyperplasia (3%), pigmentation (3%), pulpitis (2%), and pulp stones (1%). Forty-nine (60.5%) of the maxillary teeth and all of the mandibular teeth had pulp alterations. The premolars were most affected, and the molars least affected, by pulp alterations. Pulp fibrosis, calcification, and necrosis were observed in teeth irrespective of the distribution of dental calculus.

Prevalence and anatomopathological characterization of cutaneous squamous cell carcinomas with regional and distant metastases in dogs and cats: 20 cases (1985-2020).

The aim of this study was to assess the prevalence of regional and distant metastases from cutaneous squamous cell carcinomas (SCCs) in dogs (n = 11) and cats (n = 9) in a retrospective case series over 36 years (1985-2020), as well as to characterize its macroscopic aspects (location and size), degree of differentiation (well, moderately and poorly differentiated [WD, MD and PD, respectively]) and the rate of cell proliferation, by counting the AgNORs. Immunohistochemistry (IHC) was used to identify patterns of tumour migration and invasion (islands, ribbons, cords, small aggregates, individual cells [fusiform and amoeboid]) and to evaluate the intensity of desmoplasia and the amount of myofibroblasts. The prevalence of metastatic SCCs was 4.39% (21/478), being 3.8% in dog (12/309) and 5.3% in cat (9/169). Metastases affected lymph nodes in all dogs and 66% (6/9) of cats, and less frequently distant organs. Primary tumours predominantly affected the abdominal skin in dogs and the nasal planum in cats. Among the 20 cases, 52% were MDs, 34% were WDs, and 14% were PDs. Histological lesions suggestive of exposure to chronic solar radiation were present in 57% (8/14). The main patterns of tumour migration and invasion were islands for WD SCCs and individual cells for PD SCCs. MD SCCs had a mix of patterns. In cats, individual spindle cells were restricted to PDs. A marked desmoplastic reaction was more associated with PD SCCs and often with MDs. This study highlights that the prevalence of SCC metastases in dogs and cats is predominantly regional. The IHC was essential in the identification of individual fusiform keratinocytes, whose presence in surgical margins may represent a greater risk of recurrence. Although the presence of myofibroblasts was observed in all infiltrative and metastatic tumours, further studies evaluating these cells may be important to better understand their role in the tumour microenvironment of cutaneous SCCs with metastasis in dogs and cats.