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EQUINE VETERINARY EDUCATION / AE / JANUARY 2020
Discussion
In the present case a black, pigmented mass was detected in the right adrenal gland of a 7-year-old grey horse with invasion of the adjacent posterior vena cava and metastasis to the thorax. Smaller lesions were found, histologically, in the liver, spleen, lung and kidney. A diagnosis of malignant melanoma was made, based on histological and IHC examination. Erosion of neoplastic thrombus into the vena cava is a characteristic of adrenal tumours in other species (Capen 2002). Thus, in the absence of detectable lesions in the other common primary sites of melanoma in the horse, the adrenal gland was considered to be the primary site in this animal, with extension into the vena cava. A primary adrenal origin can be explained by the fact that adrenal medullary blasts and melanoblasts, similar to the nervous system and the skin, are of neuroectodermal origin. Therefore, ectopic melanocytes can exist in this location and develop metaplasia and malignant transformation, leading to the emergence of a melanoma (Carstens et al. 1984). However, primary melanoma of the adrenal gland is rare, as demonstrated by the very few cases described in the literature (Gonzalez-Saez et al. 2011), whereas metastatic spread of malignant tumours to the adrenal glands is relatively commonly seen in people with lung, breast, gastric carcinomas and malignant melanoma (Carstens et al. 1984). In the current case, expression of melanoma and
neuroendocrine markers were used to confirm the diagnosis of melanoma. Routine IHC staining procedures, in which tissue sections are bleached with melanin followed by immunostaining, could cause loss of antigen. Hongwu and Wengiao (2015) treated melanin-containing and melanin-free samples with three bleaching methods: trichloroisocyanuric acid (TCCA), potassium permanganate and potassium dichromate. Their results demonstrated that all three bleaching methods efficiently bleached melanin but also partially destroyed tissue antigen. However, according to Orchard and Calonje (1998), antigens S100, HMB-45, NKIC3, CD34 and L26 are relatively unaffected by application of a modified potassium permanganate/oxalic acid melanin bleaching procedure. In the current case, by reduction of bleaching time, the antigen degradation rate decreased to an acceptable level, so that sufficient antigens were preserved after bleaching. Equine melanocytic tumours are generally expansive in
nature and clinical signs may manifest only when the size of the tumour compromises the function of affected organs (Pasco and Summers 1981). In the present case, the clinical signs were considered to be related to the lesion in the thorax similar to those reported by Mair and Brown (1993), and the presence of the abdominalmassmay have contributed to the colic. To the author’s knowledge this is the first report of an
equine malignant melanoma potentially originating in the adrenal gland. A comprehensive post-mortem examination failed to identify an origin in other tissues commonly associated with melanoma in the horse (Capen 2002; Goldschmidt and Hendrick 2002), and therefore, the adrenal gland was considered to be the primary site.
Author’s declaration of interests No conflicts of interest have been declared.
Ethical animal research
The animal was treated in compliance with the guidelines of Animal Welfare Committee of the Research Deputy of Razi Vaccine and Serum Research Institute, Karaj, Iran.
Source of funding Razi Vaccine and Serum Research Institute, Karaj, Iran.
Acknowledgements
The author thanks Mr Ebrahim Ghanbary and Mr Mohammad Mahdi Gharagozloyan for their technical assistance. Thanks also to all staff of the Department of Pathology of Razi Vaccine and Serum Research Institute, Karaj, Iran.
Manufacturers' addresses
1Dako, Glostrup, Denmark. 2Novocastra, Newcastle upon Tyne, UK.
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