The role of SOX-10 in identifying nodal metastasis in canine malignant melanoma

: Diagnosing early nodal metastases in canine malignant melanoma is challenging due to the morphological similarities between nodal melanophages and neoplastic melanocytes, and the limitations of conventional immunohistochemical markers like melan-A, which only labels a subset of tumor cells and requires tissue bleaching. This retrospective study investigated the utility of the immunohistochemical marker SOX-10, a nuclear transcription factor, in identifying metastatic cells in lymph nodes (LNs) from dogs with oral, labial, or digital malignant melanoma undergoing regional or sentinel lymphadenectomy. The analysis included 49 LNs from 27 dogs with oral (n = 10), labial (n = 9), and digital (n = 8) melanoma. Primary tumors were highly melanotic in 7 (26%) dogs, sparsely melanotic in 15 (56%), and amelanotic in 5 (19%). SOX-10 immunohistochemistry increased the detection rate of nodal metastasis from 29% (14/49 nodes) with hematoxylin and eosin staining alone and 31% (15/49 nodes) with melan-A immunohistochemistry to 33% (16/49 nodes), allowing the identification of 12 LNs with macrometastases, 2 with micrometastases, and 2 with isolated tumor cells. Compared with melan-A, SOX-10 exhibited a more uniform labeling pattern, enhancing the identification of micrometastases and isolated tumor cells. It also facilitated the distinction between neoplastic cells and melanophages, even in heavily pigmented samples, without the need for bleaching, thereby preserving tissue integrity. These findings suggest that SOX-10 is a promising diagnostic marker for canine melanoma, offering improved detection of early melanocytic metastatic lesions.