Background: The Transmissible Venereal Tumor (TVT), classified as a round cell tumor, is considered one of the oldest existing tumors. It affects dogs all over the world and has a contagious characteristic. Despite the good response to clinical treatment in most cases, it can sometimes have non-classical presentations and even different behavior. Thus, the present study aims to report 3 cases of atypical TVT treated at the Veterinary Medical Teaching Hospital of the State University of Maringá (UEM) in Umuarama, Paraná, aiming to describe the epidemiology and clinical-pathological aspects, focusing on the diagnostic method used, the treatment of choice and the clinical follow-up of each case. Cases: Case records of 3 intact male mongrel dogs with atypical Transmissible Venereal Tumor (case 1: intranasal; case 2: intra-abdominal and case 3: cutaneous with lymph node metastasis) were reviewed regarding history, clinical signs, duration of clinical signs, examination findings, results and findings of complementary exams (hematological, biochemical, radiographic, ultrasonographic and cytological), treatment, follow-up and final result. Case 1: had an ulcerated mass in the nasal plane causing significant airway obstruction and respiratory difficulty. Case 2: had a lesion in a typical location (penile mucosa in the glans area) but with a large intra-abdominal mass in the lumbar paravertebral region, causing compression of important structures. Case 3: on the other hand, had cutaneous TVT with several ulcerated plaque lesions all over the skin, in addition to popliteal lymph node enlargement due to metastasis later confirmed by microscopy. All dogs reported were mixed breed, intact males with free access to the street. Despite the fact that each patient had their own anatomical tumor characteristics, they were all diagnosed through cytological examination and all classified as lymphocytic TVT. The standard treatment for this neoplasm was instituted; the chosen chemotherapy was vincristine sulfate at a dose of 0.75 mg/m2 intravenously every 7 days for 5-7 weeks. In addition, all 3 animals needed supportive treatment due to anorexia, bacterial contamination secondary to injuries, dehydration and pain. Discussion: Usually, TVT is not considered malignant, not causing metastasis, however it is now known that its behavior has changed a lot and more and more cases of metastatic or highly infiltrative TVT have been reported. The 3 cases presented in this report had epidemiological characteristics as mentioned in the literature, but the location, macroscopic and radiographic characteristics are uncommon for this neoplasm, that is, with an atypical tumor presentation. This demonstrates the importance of a good clinical evaluation and, especially, of the cytological exam, which was essential for the definitive diagnosis for the three cases presented. It is speculated that more “aggressive” cases of TVT may be correlated with the plasmacytic cytological type, however none of the patients described here had this cytological presentation. Thus, the clinician must be aware of the risk factors associated with this neoplasia, because even in non-classical presentations, the lymphocytic cell morphological characteristic was present and the patients responded well to the classic treatment, not requiring a change in chemotherapy protocols, however a special attention must be paid to the particularities involved in each presentation of the same tumor in different patients. Keywords: TVT, canine, neoplasm, metastasis, cytology.

Serum lactate dehydrogenase (LDH) is a recognized prognostic biomarker in human lymphomas, yet its clinical significance in canine lymphoma remains insufficiently characterized. This study aimed to quantify serum LDH levels in healthy dogs and dogs with high-grade multicentric lymphoma (ML) (predominantly B-cell) and to investigate correlations between LDH levels and established clinical and laboratory prognostic indicators. Twenty-seven dogs were prospectively enrolled: healthy controls (G1, n = 7) and dogs with high-grade ML (G2, n = 20). Immunophenotyping was performed by immunohistochemistry (CD3/CD79a). LDH concentrations were measured at diagnosis (T0) and after six weeks of CHOP-based induction chemotherapy (T1). Statistical analyses included Kruskal–Wallis, Wilcoxon signed-rank, Pearson’s correlation, and mixed-effects models. Dogs with high-grade ML exhibited significantly elevated LDH levels compared to controls (median 545.5 U/L, range: 288.2–2816 U/L vs. 143 U/L, range: 66–272; p < 0.001). Dogs with thrombocytopenia had higher baseline LDH (median 746 U/L, range: 612–921; p = 0.006) and greater reductions following chemotherapy (median −1011.7 U/L, range: −159 to −2064; p = 0.004). LDH levels declined significantly after treatment (overall median reduction 50.7%; post-chemotherapy range: 60.4–752 U/L; n = 15; p = 0.013), with normalization achieved in 77.8% of dogs with complete response versus 16.7% with partial or progressive disease (p = 0.02). We confirmed that serum LDH is significantly elevated in dogs with high-grade ML and declines following effective chemotherapy, supporting its utility as a dynamic biomarker of tumor burden and treatment response. Thrombocytopenic dogs may represent a biologically distinct subset warranting further investigation.