Abstract: Background Diagnosis of metastatic neoplasia is often challenging and belated. Hypothesis/Objectives To assess the success rate of ante-mortem identification of metastatic neoplasia. Animals Hospitalized equines between 2009-2024. Methods Retrospective clinical study. Inclusion criteria were the identification of a primary neoplasm with a final ante- or post-mortem diagnosis of metastatic malignancy, with at least one other site or organ affected. Clinical data, nature of tumour and diagnostic methods were recorded. Results Among the 40 cases with metastatic tumour(s), lymphoma (n=13, 32.5%) and squamous cell carcinoma (n=9, 22.5%) were the most common. Median age was 15.5-year-old (3-29). Ante-mortem diagnosis was achieved in 77.5% (31/40) of cases. Masses were observed on the body surface in 32% (13/40) and were palpated rectally in 9/40 (22.5%). A single imaging technique per anatomical site (endoscopy, ultrasound, or radiography) was required in 30% of cases (12/40), two modalities in 42.5% (17/40), three in 25% (10/40) and four for a single horse (2.5%). Neoplastic characterisation was performed using biopsy histology (21/27; 77.8%), body fluid cytology (8/33; 24.2%), blood smear cytology (3/14; 21.4%), fine-needle aspiration of the mass (8/13; 61.5%) and/or cytology of biopsy imprint (3/3; 100%). On average, 2.25 modalities were required/horse to characterise the primary neoplasm. An equivalent examination on the same site (cytology or histology) had to be repeated 2 times in 4 cases and 3 times in 1 case, before tumour cells could be identified. Conclusion-Clinical importance Combining several sampling sites and techniques considerably increases the probability of detecting and characterizing neoplastic cells.
