Unusual Canine Cutaneous Melanoma Presenting Parietal Bone Metastasis: A Case Report

SIMPLE SUMMARY: This report aims to investigate a case of canine cutaneous melanoma with generalized metastasis and an unusual parietal bone metastasis, never reported before to the authors’ knowledge. The primary tumour was located on the skin of the carpal region and tumoral cells migrated towards the regional lymph node, reaching the general circulation through the blood and lymphatic streams. The histopathological examination showed a mixture of spindle, epithelioid and dendritic melanocytes, with variations in terms of pigmentation, a mitotic count of 10 (calculated for 10 high-power fields) for the primary tumour and higher for the metastasis. Immunohistochemistry was used to screen the expression of matrix metalloproteinases −2, −10 and the vascular endothelial growth factor (VEGF), all considered important invasiveness factors that facilitate the modelling of the extracellular matrix to enable invasiveness and metastasis. The results showed a strong positive reaction for VEGF and MMP-10, and a moderate positivity for MMP-2. This report suggests that metastasis in canine cutaneous melanomas may be present in various locations, such as the parietal bone, and is facilitated not only by the migratory abilities of melanocytes, but also by invasiveness factor expression, such as matrix metalloproteinases, MMP-2 and MMP-10, or VEGF. ABSTRACT: Melanocytic tumour anatomic location is considered an important prognostic indicator. The cutaneous forms are generally considered benign and may show various biological behaviours. This work reports a rare case of canine cutaneous melanoma showing parietal bone metastasis. Bone invasion in melanocytic tumours is often described in oral or visceral melanomas, but not in cutaneous forms. The patient (dog, male, mixed breed, 12 years) was initially presented for the surgical removal of a cutaneous tumour located on the skin of the carpal region of the right forelimb. Four months after, the patient returned with enlarged lymph nodes and acute respiratory failure. The patient was euthanized due to a decline in physical condition. The necropsy showed metastases in the affected forelimb, regional lymph node, splanchnic organs, parietal bone and meninges. Histopathological examination of tumour tissue samples revealed a mixture of pigmented and non-pigmented spindle and epithelioid melanocytes, while according to immunohistochemistry, the tumours showed a strong immunopositivity for VEGF and MMP-10, and a moderate positivity for MMP-2 expression. This case shows that cutaneous melanocytic tumours may show an aggressive malignant form with positive immunohistochemical reactions for multiple invasiveness factors.