<scp>CD30</scp> Expression in Neoplastic Mast Cells and the Presence of <scp>CD30</scp> +, <scp>CD3</scp> +, and <scp>PAX</scp> ‐5+ Tumour‐Infiltrating Lymphocytes as Prognostic Markers in Canine Cutaneous Mast Cell Tumours

Canine cutaneous mast cell tumour (MCT) is the most common skin neoplasm in dogs, with histopathology serving as both the diagnostic and primary prognostic tool. However, identifying reliable biomarkers is essential for improving clinical decision-making. CD30, a member of the TNF receptor superfamily, is well-characterised in human haematopoietic malignancies. However, its role in canine MCTs remains unclear. This study aimed to evaluate CD30 expression in neoplastic mast cells and tumour-infiltrating lymphocytes (TILs), and to assess its prognostic significance in canine cutaneous MCTs. Immunohistochemical analysis of CD30, CD3, and PAX-5 was performed on 53 samples, and RNA sequencing was conducted to assess CD30 transcript levels in 17 cases. CD30 was detected in 94.6% of neoplastic mast cells, with strong staining observed in 35 cases and weak staining in 18. Strong CD30 expression (p = 0.0051), CD30+ TIL counts (p = 0.0105) and the diffuse infiltrate distribution pattern of CD3+ TILs (p = 0.0497) were associated with shorter post-surgical survival. RNA sequencing confirmed higher CD30 (TNFRSF8) gene expression levels in high-risk tumours (logFC = 2.3182; FDR = 0.0405). These findings suggest that CD30 is a promising biomarker with potential prognostic value in canine cutaneous MCTs.