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EQUINE VETERINARY EDUCATION / AE / JANUARY 2017


Clinical Commentary


Diagnosis of equine penile and preputial masses: A clinical and pathological perspective


V. H. L. Scott* and K. Hughes* Department of Veterinary Medicine, University of Cambridge, UK. *Corresponding authors email: vhls2@cam.ac.uk; kh387@cam.ac.uk


Introduction


Masses of the prepuce and penis are common clinical presentations for the equine veterinarian, encompassing a number of differential diagnoses. In such cases, it is important to evaluate the external genitalia thoroughly and devise a methodical diagnostic and treatment plan to allow for accurate prognostication and optimal survival rates. Masses of the prepuce and penis include tumours of epithelial, mesenchymal or round cell origin such as squamous cell carcinoma (SCC), papillomas, melanocytic tumours, lymphoma, sarcoids, fibrosarcomas (Van den Top et al. 2010) and fibromas; the latter as described by De Meyer et al. (2017) in this issue. Non-neoplastic conditions that can result in masses or enlargement of the external genitalia in male horses include infection or inflammation (primary or secondary to neoplasia) leading to balanitis or balanoposthitis. Such diseases frequently lead to chronic irritation and discomfort. Some neoplastic conditions may metastasise to cause more severe sequelae.


Diagnosis Given that tumours comprise the majority of genital masses diagnosed in male horses, adoption of a thorough, standardised approach to clinical evaluation is required. Van den Top et al. (2010, 2011) have proposed a systematic assessment and classification tool for evaluating tumour type and behaviour, treatment protocol and prognosis. Visual inspection and palpation of the mass (or tumour) should occur with thorough evaluation of the external genitalia under standing sedation using an alpha-2 agonist and acepromazine to facilitate safe examination. Some clinicians advocate caution with use of acepromazine in stallions due to the possible risk of paraphimosis, priapism and penile paralysis. The mass should be assessed for size, location, mobility and degree of infiltration/involvement of the corpus cavernosum and corpus spongiosum. Palpation of enlarged superficial and deep inguinal lymph nodes can provide information about possible metastases. The superficial inguinal lymph nodes lie dorsolateral to the penis and the deep inguinal lymph nodes are located just outside the pelvis adjacent to the internal inguinal ring. The medial iliac lymph nodes (adjacent to the external iliac arteries) should also be evaluated per rectum. Lymph node palpation and fine needle aspirate biopsies can result in false positive and false negative results for metastases (Van den Top et al. 2010). It should be considered that although regional lymphadenopathy may be an indicator of metastasis, enlargement can also be due to ‘reactive’ lymph nodes, secondary to inflammation or infection associated with the


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tumour. Distant metastasis to the thoracic cavity can be evaluated by radiographic assessment; however, most tumours affecting the equine penis and prepuce metastasise locally via the regional lymph nodes, with pulmonary and skeletal metastases only in advanced disease (Cramer et al. 2011; Nelson et al. 2015). Routine use of radiography is not warranted for the majority of cases. Ultrasonography of the primary penile tumour is


commonly used in man and can provide information about the gross extent of the tumour and tissue invasion or involvement of various structures (Hyland and Church 1995). To the authors’ knowledge, there are scarce descriptions of the use of ultrasonography for evaluation of genital tumours in male horses but this modality may be a useful diagnostic adjunct. The TNM (tumour, node, metastasis) classification system is


widely used in oncology in man to aid with appropriate choices of treatment and prognosis. Recent work within the equine literature has also highlighted the importance of histological grading in penile and preputial tumours. A positive correlation between high grade SCCs and metastasis has been demonstrated and it has also been shown that tumour grading is an important prognosticator for survival in horses (van den Top et al. 2008, 2015). This information, in conjunction with a published classification system (Van den Top et al. 2011) requires a representative biopsy to be taken such that tumour grading can guide treatment protocol and provide information on prognosis. A full thickness punch or excisional biopsy is required to assess tumour architecture and depth of invasion. Expression of cell proliferation markers, such as Ki67 and


tumour suppressor genes, for example p53, may also be evaluated using immunohistochemical staining of histopathological sections (Van den Top et al. 2015). Such markers are increasingly used in assessment of numerous types of small animal neoplasms such as canine mast cell tumours (Webster et al. 2007) and feline mammary tumours (Zappulli et al. 2015), amongst many others, but their use in equine diagnostic pathology is considerably less frequent. Whilst these and similar molecular markers may provide further prognostic information for different equine penile and preputial tumour types in the future, studies to date have yet to show compelling prognostic potential (Van den Top et al. 2015). The diagnostic evaluations described (Van den Top et al.


2010, 2011; Ensink 2015) represent a ‘gold standard’ approach to penile and preputial masses but it should also be considered that many cases have attendant financial constraints, other limitations in resources, differing owner priorities, or present with additional clinical challenges such as


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