Concomitant melanoma and keratoma affecting the equine digit: clinical, pathological, and long-term follow-up findings

Abstract

Background This case report details a long-term follow-up of a hoof melanoma with dermo-epidermal activity (resembling Spreading Superfcial Melanoma (SSM)) in a bay horse with a history of a right front hoof keratoma. Melanomas involving the horse’s foot are seldom reported and usually diagnosed as anaplastic melanomas basedon signalment and post-mortem examination. The clinical-pathological characteristics of the foot melanoma in thisbay horse are consistent with SSM-like described in humans, which is considered an intermediate malignant tumour attending their biological behaviour. However, a defnitive diagnosis is limited by the single case and the lack of references in horses. Case presentation A 12-year-old bay Andalusian gelding underwent keratoma removal on the lateral aspect of the hoof wall. A partial resection of the hoof wall was performed for this purpose. Additionally, a plaque-like, hyperkeratotic pigmented lesion, 2×2X0,4 cm in size, was observed at the lateral aspect of the coronary band and was also resected for histopathological examination. Microscopically, a melanocytic tumour, characterised by small nests of large polygonal or epithelioid cells infltrating the basal and suprabasal epidermis, the dermo – epidermal junction, and the superfcial dermis, was observed. The neoplastic cells exhibited large euchromatic nuclei, prominent nucleoli, moderate pleomorphism and 4 mitotic fgures per 2,37mm2; variable amounts of dark granules (melanin) were present in the cytoplasm, as well as in numerous peritumoral macrophages. The immunophenotype of the tumour cells was PNL2+ + +, S100+ +, AE1/AE3-. A diagnosis of melanoma with dermo-epidermal junction and marked intraepidermal activity (consistent with superfcial spreading melanoma) was made. A magnetic resonance imaging (MRI) performed, revealed no further invasion into surrounding structures. Treatment was based on surgical resection and multiple local chemotherapy sessions with cisplatin were applied. The biopsies obtained after treatment showed partial regression of the tumour and diferent stages of healing. After 26 months of follow-up, there was no signs of malignant spreading into surrounding structures including the pedal bone and distal metastasis but a dark – coloured area persists over the lateral aspect of the coronary band. Conclusions. This case presents a concomitant keratoma and melanoma with dermo – epidermal activity, resembling a spreading superfcial melanoma. After a follow – up of 26 months the horse remains healthy and sound providing new information for clinicians and pathologists. Despite the poor prognosis associated with foot malignant melanocytic tumours, it is important that an early and accurate diagnosis is reached through diferent diagnostic modalities such as advanced imaging techniques and histopathology. Additionally, these fndings demonstrate that the current classifcation and prognosis for equine foot melanomas are insufcient.