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Background and Aim: Canine testicular tumors are among the most common reproductive tract tumors in male dogs and have been studied in many countries. However, to the best of our knowledge, studies with a large sample size have not been conducted in Russia. This study aimed to provide the latest information on the prevalence of canine testicular tumors in the Veterinary Oncology Scientific Center for Small Animals "Biocontrol" in Moscow, Russia, in 2010-2020 and the characteristics of the affected canine population. Materials and Methods: A retrospective review of patients and histological reports was collected and analyzed from 358 dogs with 447 testicular tumors within 11 years. Results: The mean age of the affected dogs was 10.4 years, whereas that of dogs with Sertoli cell tumors was 9.4 years p=0.009. This study includes mixed-breed dogs (18.4%), Yorkshire Terriers (8.8%), Labrador Retrievers (7.9%), Golden Retrievers (5.0%), and Fox Terriers (3.4%). The most common tumors were interstitial cell tumors (n=227, 50.8%). In contrast, 107 (23.9%) seminomas, 80 (17.9%) Sertoli cell tumors, 19 (7.4%) mixed germ cell-sex cord-stromal tumors, and 26 (7.6%) testicular tumors developed from cryptorchid testes, which included 16 (61.5%) Sertoli cell tumors, 10 (38.5%) seminomas, and no interstitial cell tumors. Conclusion: This study provides baseline information on the prevalence of canine testicular tumors in the described population, including the median age of each tumor type and overrepresented dog breeds. We further found that the most common scrotal testicular tumor was interstitial cell tumor, whereas Sertoli cell tumor was the most common in cryptorchid testicles.

The study presents an assessment of variability of histological structure, measurement, and comparison of size of the neoplasm obtained by ultrasonographic examination and pathological examination, as well as the morphometric dimensions of nuclei and cytoplasm of testicular interstitial cell tumors in dogs. The study involved 35 dogs with neoplasms of 46 testes, where 11 animals had interstitial cell tumors in both testes. Insignificant differences of the size of these neoplasms were revealed (p 0.05) using ultrasonography and pathoanatomical measurement methods. Hence, it allows using both methods to assess the size of interstitial cell tumors. In the study, interstitial cell tumor was detected in both testes at once in 50 % of cases in dogs, however, this might be due to specific characteristics of the sample, and further research is required. In the course of scientific work, a morphological study showed the presence of variability in histological structure of interstitial cell tumors, which can lead to incorrect interpretation of the morphological picture and misdiagnosis, e. g. adipocyte-like morphology of interstitial cell tumors have morphological similarity to a benign neoplasm from adipose tissue - lipoma. In addition, there was an extremely pronounced difference in size of cytoplasm (from 23.6 to 148.4 m; average 66.21 22.42 m) and nuclei (from 9 to 57.6 m; average 23.19 7.10 m) in tumor cells. It proves the presence of pronounced anisocytosis and anisokariosis, which should indicate malignancy of the neoplasm, however, testicular interstitial cell tumors extremely rarely metastasize in practice and according to numerous studies.