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A 13-year old male Poodle dog was presented with a considerable disparity in the size of testes. Palpation demonstrated enlargement of the right testicle; smooth surface without any sign of nodular hyperplasia was detected. Both testes were removed, the enlarged one was sent to a histopathology laboratory. Microscopic examination revealed massive neoplastic proliferation of the testicular germ cell tumour (seminoma) and an accompanying smaller tumour originating from the interstitial Leydig cells (Leydigoma), which was confirmed by immunohistochemistry. Simultaneous occurrence of two different types of tumours in testes is possible, representing a multiple primary malignancy case, which is a rare phenomenon in veterinary practice. Canine, testis, multiple primary malignancies, seminoma, interstitial cell tumour, Leydig cell tumour Testicular tumours are second in terms of frequency of occurrence in dogs and constitute a serious oncological problem in this species. Dogs are most frequently affected among domestic animals. A longer life span of dogs and less frequent castration in comparison with other species significantly affect the high incidence of the tumours in this species. Testicular tumours occur in older dogs, usually over 10 years of age (7–12 years), mostly unilaterally, though a bilateral location is not unusual (Kennedy et al. 1998). The aetiology, as in almost every type of a spontaneous tumour, is not unequivocally established, but the off-scrotum positioning of testes certainly has a major effect on tumour growth (Peters and Sluijs 1996; Thonneau et al. 2003; Maiolino et al. 2004; Reuter 2005). It is estimated that the risk of neoplasia in the retained testes increases several times (Romagnoli 1991; Thonneau et al. 2003). Isolated cases of testicular tumours are also described in dogs with sex development disorders (Dzimira et al. 2015). In humans it was proved that the increased risk of testicular tumour development is observed in syndromes associated with testicular dysgenesis, such as testicular feminization syndrome, or Klinefelter’s syndrome in men (Oosterhuis and Looijenga 2005). There are a number of histological types that behave differently, the type of tumour depending on specific histological structures. Morphological differences influence the rate of growth of tumours and any potential metastases, and thus also their malignancy and the observed clinical symptoms. The latter are related not only to the size and location of the tumour but also to its potential hormonal activity. Primary testicular tumours are most often derived from seminiferous epithelial cells (testicular seminomas seminomata), Sertoli supporting cells and Leydig interstitial cells. Testicular germinal cell tumours develop from primitive cells that can further differentiate into gonads, or can convert into the populations of pluripotent cells. Pluripotent cells can remain at the stage of undifferentiated cells (embryonal carcinoma carcinoma embryonale), differentiate into extra-embryonic tissues (endodermal sinus tumour yolk sac tumour, malignant chorionic epithelioma choriocarcinoma) or somatic tissues (teratoma). ACTA VET. BRNO 2017, 86: 373–378; https://doi.org/10.2754/avb201786040373 Address for correspondence: Dr Stanisław Dzimira Department of Pathology, Faculty of Veterinary Medicine University of Environmental and Life Sciences Norwida 31, 50-375 Wrocław, Poland Phone: +48 71 320 52 56 E-mail: [email protected] http://actavet.vfu.cz/ According to most authors metastatic testicular tumours occur very rarely in dogs. Dissemination of seminomas occurs primarily through the lymphatic vessels and the metastases are localized mostly in the retroperitoneal lymph nodes. Seminomas metastasize by both the lymphatic vessels and blood vessels primarily to the lungs, liver, bone, and brain. Less common forms of malignant tumour of supporting and interstitial cells metastasize mainly to the inguinal and lumbar lymph nodes (Restucci et al. 2003). It should be noted that hormonally active tumour metastases may also produce hormones (Kennedy et al. 1998; Grieco et al. 2008). In the initial stage of the disease, testicular tumours manifest themselves most often by enlargement of the organ or a small nodule. Pain usually accompanies rapid growth, bleeding into the nodule or cryptorchidism. Azoospermia and/or oligospermia are often observed. Infertility and dermatitis (acanthosis, seborrhea) are often reported as well. Sertoli supporting cell tumours and seminomas may secrete androgens or oestrogens (Patnaik and Mostofi 1993).