Amelanotic Melanoma In Veterinary Pathology: A Comprehensive Review

Özet

Amelanotic melanoma (AM), an aggressive subtype of melanocytic neoplasms, is characterized by the partial or complete absence of melanin pigment despite originating from melanocytes. This pigment deficiency is not merely a phenotypic difference; it is often associated with more aggressive biological behavior, increased proliferative activity, and a poor prognosis. The greatest challenge in veterinary oncology is AM's ability to mimic other poorly differentiated neoplasms such as carcinomas, sarcomas, and round cell tumors; this frequently leads to diagnostic delays or errors in routine histopathological examinations. Therefore, AM is referred to as the "great mimicker," and the use of advanced techniques like immunohistochemistry (IHC) is an absolute necessity for a definitive diagnosis. It is common in dogs, particularly in the oral cavity, while it is rare in cats, but the amelanotic form has a worse prognosis. Although melanocytic lesions are prevalent in gray horses, non-gray horses and amelanotic variants exhibit a more aggressive course. The molecular basis of AM, unlike human cutaneous melanoma, is shaped by chromosomal abnormalities and large-scale genomic instability rather than single gene mutations such as BRAF. Specifically, focal amplification on canine chromosome 30 (CFA 30) has been directly linked to the amelanotic phenotype and a poor prognosis. Diagnosis is established through cytology, histopathology, and immunohistochemical staining with specific markers such as Melan-A and PNL-2, which confirm the melanocytic origin. Prognosis is based on a holistic evaluation of anatomical localization, clinical stage (TNM), and histopathological factors like the mitotic index